lix/lix/libexpr/eval.cc

#include "lix/libexpr/eval.hh"
#include "lix/libexpr/eval-settings.hh"
#include "lix/libstore/path.hh"
#include "lix/libutil/archive.hh"
#include "lix/libutil/ansicolor.hh"
#include "lix/libutil/async.hh"
#include "lix/libutil/current-process.hh"
#include "lix/libutil/deprecated-features.hh"
#include "lix/libutil/error.hh"
#include "lix/libutil/english.hh"
#include "lix/libutil/fmt.hh"
#include "lix/libexpr/primops.hh"
#include "lix/libexpr/print-options.hh"
#include "lix/libmain/shared.hh"
#include "lix/libutil/logging.hh"
#include "lix/libutil/suggestions.hh"
#include "lix/libutil/types.hh"
#include "lix/libstore/store-api.hh"
#include "lix/libstore/derivations.hh"
#include "lix/libexpr/gc-alloc.hh"
#include "lix/libstore/filetransfer.hh"
#include "lix/libexpr/function-trace.hh"
#include "lix/libstore/profiles.hh"
#include "lix/libexpr/print.hh"
#include "lix/libexpr/gc-small-vector.hh"
#include "lix/libfetchers/fetch-to-store.hh"
#include "lix/libexpr/flake/flakeref.hh"
#include "lix/libutil/exit.hh"
#include "lix/libutil/json.hh"
#include "symbol-table.hh"
#include "value.hh"

#include <algorithm>
#include <iostream>
#include <ostream>
#include <sstream>
#include <cstring>
#include <optional>
#include <string>
#include <unistd.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <fstream>
#include <functional>
#include <ranges>

#include <sys/resource.h>
#include <boost/container/small_vector.hpp>

// Ignore all internal signals boehm uses for parallel marking
// FIXME: Find out how to do this with LLDB for macOS!
#ifndef __APPLE__
[[gnu::section(".debug_gdb_scripts"), gnu::used, gnu::aligned(1)]]
// TODO: We should use `SECTION_SCRIPT_ID_PYTHON_TEXT` from
// `<gdb/section-scripts.h>` instead of hardcoding 4.
// But why isn't this header exported by GDB?
static const char printer_script[] =
    "\4"
    R"(lix-ignore-boehm-signals
import gdb
gdb.execute("handle SIGPWR SIGXCPU ignore")
)";
#endif

#if HAVE_BOEHMGC

#define GC_INCLUDE_NEW

#include <gc/gc.h>
#include <gc/gc_cpp.h>

#endif

namespace nix {

RootValue allocRootValue(Value v)
{
    return std::allocate_shared<Value>(TraceableAllocator<Value>(), v);
}

// Pretty print types for assertion errors
std::ostream & operator << (std::ostream & os, const ValueType t) {
    os << showType(t);
    return os;
}

std::string printValue(EvalState & state, Value & v)
{
    std::ostringstream out;
    v.print(state, out);
    return out.str();
}

std::string_view showType(ValueType type, bool withArticle)
{
    #define WA(a, w) withArticle ? a " " w : w
    switch (type) {
        case nInt: return WA("an", "integer");
        case nBool: return WA("a", "Boolean");
        case nString: return WA("a", "string");
        case nPath: return WA("a", "path");
        case nNull: return "null";
        case nAttrs: return WA("a", "set");
        case nList: return WA("a", "list");
        case nFunction: return WA("a", "function");
        case nExternal: return WA("an", "external value");
        case nFloat: return WA("a", "float");
        case nThunk: return WA("a", "thunk");
    }
    abort();
}


std::string showType(const Value & v)
{
    // Allow selecting a subset of enum values
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wswitch-enum"
    switch (v.internalType()) {
    case tString:
        if (v.string().isPath()) {
            return "a path";
        } else {
            return v.string().context ? "a string with context" : "a string";
        }
    case tAuxiliary:
#pragma GCC diagnostic push
#pragma GCC diagnostic error "-Wswitch-enum"
        switch (v.auxiliary()->type()) {
        case Value::Acb::tExternal:
            return v.external()->showType();
        case Value::Acb::tFloat:
        case Value::Acb::tNull:
        case Value::Acb::tLambda:
        case Value::Acb::tInt:
            return std::string(showType(v.type()));
        case Value::Acb::tPrimOp:
            return fmt("the built-in function '%s'", v.primOp()->name);
        }
#pragma GCC diagnostic pop
    case tThunk:
        return v.isBlackhole() ? "a black hole" : "a thunk";
    case tApp:
        if (v.isPrimOpApp()) {
            return fmt(
                "the partially applied built-in function '%s'", v.app().target().primOp()->name
            );
        } else {
            return "a function application";
        }
    default:
        return std::string(showType(v.type()));
    }
#pragma GCC diagnostic pop
}


#if HAVE_BOEHMGC
/* Called when the Boehm GC runs out of memory. */
static void * oomHandler(size_t requested)
{
    /* Convert this to a proper C++ exception. */
    throw std::bad_alloc();
}

#endif

Symbol getName(const AttrName & name, EvalState & state, Env & env)
{
    if (name.symbol) {
        return name.symbol;
    } else {
        Value nameValue = name.expr->eval(state, env);
        state.forceStringNoCtx(nameValue, name.expr->getPos(), "while evaluating an attribute name");
        return state.ctx.symbols.create(nameValue.str());
    }
}

static bool libexprInitialised = false;

void initLibExpr()
{
    if (libexprInitialised) return;

#if HAVE_BOEHMGC
    /* Initialise the Boehm garbage collector. */

    /* Don't look for interior pointers. This reduces the odds of
       misdetection a bit. */
    GC_set_all_interior_pointers(0);
    for (int i = 1; i < 8; i++) {
        GC_REGISTER_DISPLACEMENT(i);
    }

    /* We don't have any roots in data segments, so don't scan from
       there. */
    GC_set_no_dls(1);

    GC_INIT();

    // Enable parallel marking
    GC_start_mark_threads();

    GC_set_oom_fn(oomHandler);

    /* Set the initial heap size to something fairly big (25% of
       physical RAM, up to a maximum of 384 MiB) so that in most cases
       we don't need to garbage collect at all.  (Collection has a
       fairly significant overhead.)  The heap size can be overridden
       through libgc's GC_INITIAL_HEAP_SIZE environment variable.  We
       should probably also provide a nix.conf setting for this.  Note
       that GC_expand_hp() causes a lot of virtual, but not physical
       (resident) memory to be allocated.  This might be a problem on
       systems that don't overcommit. */
    if (!getEnv("GC_INITIAL_HEAP_SIZE")) {
        int64_t size = 32l * 1024 * 1024;
#if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
        int64_t maxSize = 384l * 1024 * 1024;
        int64_t pageSize = sysconf(_SC_PAGESIZE);
        int64_t pages = sysconf(_SC_PHYS_PAGES);
        if (pageSize != -1) {
            size = (pageSize * pages) / 4; // 25% of RAM
        }
        if (size > maxSize) size = maxSize;
#endif
        debug("setting initial heap size to %1% bytes", size);
        GC_expand_hp(size);
    }

#endif

    fetchers::initLibFetchers();

    libexprInitialised = true;
}

EvalMemory::EvalMemory()
{
    assert(libexprInitialised);
#if HAVE_BOEHMGC
    GC_add_roots(static_cast<void *>(gcCache), static_cast<void *>(gcCache + CACHES));
#endif
}

EvalMemory::~EvalMemory()
{
#if HAVE_BOEHMGC
    GC_remove_roots(static_cast<void *>(gcCache), static_cast<void *>(gcCache + CACHES));
#endif
}

EvalBuiltins::EvalBuiltins(
    EvalMemory & mem,
    SymbolTable & symbols,
    const SearchPath & searchPath,
    const Path & storeDir,
    size_t size
)
    : mem(mem)
    , symbols(symbols)
    , env(mem.allocEnv(size))
    , staticEnv{std::make_shared<StaticEnv>(nullptr, nullptr)}
{
    createBaseEnv(searchPath, storeDir);
}

EvalPaths::EvalPaths(
    AsyncIoRoot & aio,
    const ref<Store> & store,
    SearchPath searchPath,
    EvalErrorContext & errors
)
    : store(store)
    , searchPath_(std::move(searchPath))
    , errors(errors)
{
    if (evalSettings.restrictEval || evalSettings.pureEval) {
        allowedPaths = AllowedPath{.allowAllChildren = false};

        for (auto & i : searchPath_.elements) {
            auto r = aio.blockOn(resolveSearchPathPath(i.path));
            if (!r) continue;

            auto path = std::move(*r);

            if (store->isInStore(path)) {
                try {
                    StorePathSet closure;
                    aio.blockOn(store->computeFSClosure(store->toStorePath(path).first, closure));
                    for (auto & path : closure)
                        allowPath(path);
                } catch (InvalidPath &) {
                    allowPath(path);
                }
            } else
                allowPath(path);
        }
    }

#if LIX_MAJOR >= 3 || (LIX_MAJOR == 2 && LIX_MINOR >= 98)
#warning \
    "The feature nix-path-shadow was deprecated in 2.95 with a warning, error in 2.96, and we should consider removing the bypass in 2.98"
#endif

    if (!featureSettings.isEnabled(DeprecatedFeature::NixPathShadow)) {
        for (auto & [prefix, path] : searchPath_.elements) {
            // Match on the 'nix' prefix
            if (prefix.s == "nix") {
                throw EvalError(HintFmt(
                    "The prefix '%s' is reserved for internal use by Lix in the Nix search "
                    "path, its usage is deprecated and will be forbidden in the future.\n"
                    "Use %s to silence this error.\n"
                    "This is due to adding '%s=%s' in the Nix search path, either through the "
                    "environment variable '%s' or by passing the flag %s to the nix "
                    "invocation.",
                    "nix",
                    "--extra-deprecated-features nix-path-shadow",
                    prefix.s,
                    path.s,
                    "NIX_PATH",
                    "-I"
                ));
            } else
                // Match prefixless paths that contain a `nix` directory
                if (auto res =
                        prefix.suffixIfPotentialMatch("nix").and_then([&](std::string_view s) {
                            return aio.blockOn(resolveSearchPathPath(path))
                                .and_then([&](std::string r) {
                                    Path res = s.length() ? concatStrings(r, "/", s): r;

                                    return pathExists(res) ? std::optional(res) : std::nullopt;
                                });
                        }))
                {
                    throw EvalError(HintFmt(
                        "Shadowing '%s' by configuring the nix-path is deprecated and "
                        "will be forbidden in the future.\n"
                        "Use %s to silence this error.\n"
                        "This is due to adding '%s' to the nix-path without a prefix, "
                        "either by passing the flag '-I %s' to the nix invocation or by "
                        "adding this path to the environment variable '%s'.",
                        "<nix/...>",
                        "--extra-deprecated-features nix-path-shadow",
                        path.s,
                        path.s,
                        "NIX_PATH"
                    ));
                }
        }
    }
}

Evaluator::Evaluator(
    AsyncIoRoot & aio,
    const SearchPath & _searchPath,
    ref<Store> store,
    std::shared_ptr<Store> buildStore,
    std::function<ReplExitStatus(EvalState & es, ValMap const & extraEnv)> debugRepl
)
    : paths(aio, store, [&] {
        SearchPath searchPath;
        if (!evalSettings.pureEval) {
            for (auto & i : _searchPath.elements)
                searchPath.elements.emplace_back(SearchPath::Elem {i});
            for (auto & i : evalSettings.nixPath.get())
                searchPath.elements.emplace_back(SearchPath::Elem::parse(i));
        }
        return searchPath;
    }(), errors)
    , builtins(mem, symbols, paths.searchPath(), store->config().storeDir)
    , repair(NoRepair)
    , store(store)
    , buildStore(buildStore ? ref<Store>::unsafeFromPtr(buildStore) : store)
    , debug{
          debugRepl ? std::make_unique<DebugState>(
              positions,
              symbols,
              [this, debugRepl](const ValMap & extraEnv, NeverAsync) {
                  return activeEval
                      ? debugRepl(*activeEval, extraEnv)
                      : ReplExitStatus::Continue;
              })
                    : nullptr
      }
    , errors{positions, debug.get()}
{
    stats.countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";

    static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
}

box_ptr<EvalState> Evaluator::begin(AsyncIoRoot & aio)
{
    assert(!activeEval);

    // Increase the default stack size for the evaluator and for
    // libstdc++'s std::regex.
    ensureStackSizeAtLeast(64ul * 1024 * 1024);

    return box_ptr<EvalState>::unsafeFromNonnull(
        std::unique_ptr<EvalState>(new EvalState(aio, *this))
    );
}

EvalState::EvalState(AsyncIoRoot & aio, Evaluator & ctx) : ctx(ctx), aio(aio)
{
    ctx.activeEval = this;
}

EvalState::~EvalState()
{
    ctx.activeEval = nullptr;
}


void EvalPaths::allowPath(const Path & path)
{
    if (!allowedPaths) {
        return;
    }

    CanonPath p(path);
    auto * level = &*allowedPaths;
    for (const auto & entry : p) {
        level = &level->children.emplace(std::piecewise_construct, std::tuple(entry), std::tuple())
                     .first->second;
    }
    level->allowAllChildren = true;
}

void EvalPaths::allowPath(const StorePath & storePath)
{
    if (allowedPaths)
        allowPath(store->toRealPath(storePath));
}

Value EvalPaths::allowAndSetStorePathString(const StorePath & storePath)
{
    allowPath(storePath);

    return mkStorePathString(storePath);
}

CheckedSourcePath EvalPaths::checkSourcePath(const SourcePath & path_)
{
    if (!allowedPaths) return auto(path_).unsafeIntoChecked();

    auto i = resolvedPaths.find(path_.canonical().abs());
    if (i != resolvedPaths.end())
        return i->second;

    /* First canonicalize the path without symlinks, so we make sure an
     * attacker can't append ../../... to a path that would be in allowedPaths
     * and thus leak symlink targets.
     */
    const CanonPath abspath{path_.canonical().abs()};

    if (abspath.abs().starts_with(corepkgsPrefix)) {
        return SourcePath(std::move(abspath)).unsafeIntoChecked();
    }

    /* Resolve symlinks. This is mostly restricted copy of canonPath with
       resolveSymlinks=true, because we need access to intermediat paths. */
    debug("checking access to '%s'", abspath);

    /* Count the number of times we follow a symlink and stop at some
       arbitrary (but high) limit to prevent infinite loops. */
    unsigned int followCount = 0, maxFollow = 1024;

    std::optional<CanonPath> componentsBacking;
    std::vector<std::string_view> components(abspath.begin(), abspath.end());

retry:

    if (++followCount >= maxFollow) {
        throw Error("infinite symlink recursion in path '%1%'", path_);
    }

    // TODO: tests for this stuff
    const auto * level = &*allowedPaths;
    CheckedSourcePath current = SourcePath(CanonPath::root).unsafeIntoChecked();
    for (auto ct = components.begin(); ct != components.end(); ct++) {
        auto & p = *ct;
        // an empty level means all subpaths are allowed, propagate this forwards
        // by setting level=nullptr for the subsequent checks. a symlink will set
        // level to the "VFS" root and restart the check with a the resolved path
        if (level) {
            if (level->allowAllChildren) {
                level = nullptr;
            } else if (auto it = level->children.find(p); it != level->children.end()) {
                level = &it->second;
            } else {
                goto failed;
            }
        }
        auto next = (current + p).unsafeIntoChecked();
        auto st = next.maybeLstat();
        // resolve symlinks, treating nonexistant components like regular directories.
        // this mirrors canonPath behavior and is necessary for `builtins.pathExists`.
        if (st && st->type == InputAccessor::tSymlink) {
            auto target = next.readLink();
            auto levelResolved = target.starts_with("/")
                ? CanonPath(target)
                : CanonPath(current.canonical().abs() + "/" + target);
            for (ct++; ct != components.end(); ct++) {
                levelResolved.push(*ct);
            }
            components = {levelResolved.begin(), levelResolved.end()};
            componentsBacking = std::move(levelResolved);
            followCount += 1;
            goto retry;
        }
        current = std::move(next);
    }
    // Downstream users (e.g. `builtins.readDir` or `builtins.path`) will want to descend.
    if (level && !level->allowAllChildren) {
        goto failed;
    }

    resolvedPaths.insert_or_assign(path_.canonical().abs(), current);
    return current;

failed:
    auto modeInformation = evalSettings.pureEval
        ? "in pure eval mode (use '--impure' to override)"
        : "in restricted mode";
    throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", abspath, modeInformation);
}


void EvalPaths::checkURI(const std::string & uri)
{
    if (!evalSettings.restrictEval) return;

    /* 'uri' should be equal to a prefix, or in a subdirectory of a
       prefix. Thus, the prefix https://github.co does not permit
       access to https://github.com. Note: this allows 'http://' and
       'https://' as prefixes for any http/https URI. */
    for (auto & prefix : evalSettings.allowedUris.get())
        if (uri == prefix ||
            (uri.size() > prefix.size()
            && prefix.size() > 0
            && uri.starts_with(prefix)
            && (prefix[prefix.size() - 1] == '/' || uri[prefix.size()] == '/')))
            return;

    /* If the URI is a path, then check it against allowedPaths as
       well. */
    if (uri.starts_with("/")) {
        checkSourcePath(CanonPath(uri));
        return;
    }

    if (uri.starts_with("file://")) {
        checkSourcePath(CanonPath(std::string(uri, 7)));
        return;
    }

    throw RestrictedPathError("access to URI '%s' is forbidden in restricted mode", uri);
}


Path EvalPaths::toRealPath(const Path & path, const NixStringContext & context)
{
    // FIXME: check whether 'path' is in 'context'.
    return
        !context.empty() && store->isInStore(path)
        ? store->toRealPath(path)
        : path;
}

void EvalBuiltins::addConstant(const std::string & name, const Value & v, Constant info)
{
    auto name2 = name.substr(0, 2) == "__" ? name.substr(2) : name;

    constantInfos.push_back({name2, info});

    if (!(evalSettings.pureEval && info.impureOnly)) {
        /* Check the type, if possible.

           We might know the type of a thunk in advance, so be allowed
           to just write it down in that case. */
        if (auto gotType = v.type(true); gotType != nThunk) {
            assert(info.type == gotType);
        }

        /* Install value the base environment. */
        staticEnv->vars.insert_or_assign(symbols.create(name), baseEnvDispl);
        env.values[baseEnvDispl++] = v;
        env.values[0].attrs()->push_back(Attr(symbols.create(name2), v));
    }
}

std::ostream & operator<<(std::ostream & output, const PrimOp & primOp)
{
    output << "primop " << primOp.name;
    return output;
}

void EvalBuiltins::addPrimOp(PrimOpDetails primOp)
{
    /* Hack to make constants lazy: turn them into a application of
       the primop to a dummy value. */
    if (primOp.arity == 0) {
        primOp.arity = 1;
        Value vPrimOp{NewValueAs::primop, *new PrimOp(primOp)};
        Value v{NewValueAs::app, mem, vPrimOp, vPrimOp};
        addConstant(
            vPrimOp.primOp()->name,
            v,
            {
                .type = nFunction,
                .doc = vPrimOp.primOp()->doc,
            }
        );
    }

    auto envName = symbols.create(primOp.name);
    if (primOp.name.starts_with("__"))
        primOp.name = primOp.name.substr(2);

    Value v{NewValueAs::primop, *new PrimOp(std::move(primOp))};
    staticEnv->vars.insert_or_assign(auto(envName), baseEnvDispl);
    env.values[baseEnvDispl++] = v;
    env.values[0].attrs()->push_back(Attr(symbols.create(v.primOp()->name), v));
}


Value & EvalBuiltins::get(const std::string & name)
{
    return env.values[0].attrs()->get(symbols.create(name))->value;
}


std::optional<EvalBuiltins::Doc> EvalBuiltins::getDoc(Value & v)
{
    if (v.isPrimOp()) {
        auto v2 = &v;
        if (auto * doc = v2->primOp()->doc)
            return Doc {
                .pos = {},
                .name = v2->primOp()->name,
                .arity = v2->primOp()->arity,
                .args = v2->primOp()->args,
                .doc = doc,
            };
    }
    return {};
}


static std::set<std::string_view> sortedBindingNames(const SymbolTable & st, const StaticEnv & se)
{
    std::set<std::string_view> bindings;
    for (auto [symbol, displ] : se.vars)
        bindings.emplace(st[symbol]);
    return bindings;
}


// just for the current level of StaticEnv, not the whole chain.
void printStaticEnvBindings(const SymbolTable & st, const StaticEnv & se)
{
    std::cout << ANSI_MAGENTA;
    for (auto & i : sortedBindingNames(st, se))
        std::cout << i << " ";
    std::cout << ANSI_NORMAL;
    std::cout << std::endl;
}

// just for the current level of Env, not the whole chain.
void printWithBindings(const SymbolTable & st, const Env & env)
{
    if (env.values[0].type() == nAttrs) {
        std::set<std::string_view> bindings;
        for (const auto & attr : *env.values[0].attrs()) {
            bindings.emplace(st[attr.name]);
        }

        std::cout << "with: ";
        std::cout << ANSI_MAGENTA;
        for (auto & i : bindings)
            std::cout << i << " ";
        std::cout << ANSI_NORMAL;
        std::cout << std::endl;
    }
}

void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, int lvl)
{
    std::cout << "Env level " << lvl << std::endl;

    if (se.up && env.up) {
        std::cout << "static: ";
        printStaticEnvBindings(st, se);
        if (se.isWith)
            printWithBindings(st, env);
        std::cout << std::endl;
        printEnvBindings(st, *se.up, *env.up, ++lvl);
    } else {
        std::cout << ANSI_MAGENTA;
        // for the top level, don't print the double underscore ones;
        // they are in builtins.
        for (auto & i : sortedBindingNames(st, se))
            if (!i.starts_with("__"))
                std::cout << i << " ";
        std::cout << ANSI_NORMAL;
        std::cout << std::endl;
        if (se.isWith)
            printWithBindings(st, env);  // probably nothing there for the top level.
        std::cout << std::endl;

    }
}

void printEnvBindings(const EvalState &es, const Expr & expr, const Env & env)
{
    // just print the names for now
    auto se = es.ctx.debug->staticEnvFor(expr);
    if (se)
        printEnvBindings(es.ctx.symbols, *se, env, 0);
}

void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, ValMap & vm)
{
    // add bindings for the next level up first, so that the bindings for this level
    // override the higher levels.
    // The top level bindings (builtins) are skipped since they are added for us by initEnv()
    if (env.up && se.up) {
        mapStaticEnvBindings(st, *se.up, *env.up, vm);

        if (se.isWith && env.values[0].type() == nAttrs) {
            // add 'with' bindings.
            Bindings::iterator j = env.values[0].attrs()->begin();
            while (j != env.values[0].attrs()->end()) {
                vm.insert_or_assign(std::string(st[j->name]), j->value);
                ++j;
            }
        } else {
            // iterate through staticenv bindings and add them.
            for (auto & i : se.vars)
                vm.insert_or_assign(std::string(st[i.first]), env.values[i.second]);
        }
    }
}

std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env)
{
    auto vm = std::make_unique<ValMap>();
    mapStaticEnvBindings(st, se, env, *vm);
    return vm;
}

/**
 * Sets `inDebugger` to true on construction and false on destruction.
 */
class DebuggerGuard {
    bool & inDebugger;
public:
    DebuggerGuard(bool & inDebugger) : inDebugger(inDebugger) {
        inDebugger = true;
    }
    ~DebuggerGuard() {
        inDebugger = false;
    }
};

void DebugState::onEvalError(
    const EvalError * error, const Env & env, const Expr & expr, NeverAsync
)
{
    // Make sure we have a debugger to run and we're not already in a debugger.
    if (inDebugger)
        return;

    auto dts =
        error && expr.getPos()
        ? addTrace(DebugTrace {
            .pos = error->info().pos ? error->info().pos : positions[expr.getPos()],
            .expr = expr,
            .env = env,
            .hint = error->info().msg,
            .isError = true
        })
        : nullptr;

    if (error)
    {
        printError("%s\n", Uncolored(error->what()));

        if (trylevel > 0 && error->info().level != lvlInfo) {
            if (evalSettings.ignoreExceptionsDuringTry) {
                printError("This exception occurred in a 'tryEval' call, " ANSI_RED "despite the use of " ANSI_GREEN "--ignore-try" ANSI_RED " to attempt to skip these" ANSI_NORMAL ". This is probably a bug. We would appreciate if you report it along with what caused it at https://git.lix.systems/lix-project/lix/issues.\n");
            } else {
                printError("This exception occurred in a 'tryEval' call. Use " ANSI_GREEN "--ignore-try" ANSI_NORMAL " to skip these.\n");
            }
        }
    }

    auto se = staticEnvFor(expr);
    if (se) {
        auto vm = mapStaticEnvBindings(symbols, *se.get(), env);
        DebuggerGuard _guard(inDebugger);
        auto exitStatus = errorCallback(*vm, {});
        switch (exitStatus) {
            case ReplExitStatus::QuitAll:
                if (error)
                    throw *error;
                throw Exit(0);
            case ReplExitStatus::Continue:
                break;
            default:
                abort();
        }
    }
}

DebugState::TraceFrame DebugState::addTrace(DebugTrace t)
{
    struct UnlinkDebugTrace
    {
        DebugState * state;
        void operator()(DebugTrace * trace)
        {
            state->latestTrace = trace->parent;
            delete trace;
        }
    };

    t.parent = latestTrace.lock();
    std::unique_ptr<DebugTrace, UnlinkDebugTrace> trace(
        new auto(std::move(t)), UnlinkDebugTrace{.state = this}
    );
    std::shared_ptr<DebugTrace> entry(std::move(trace));
    latestTrace = entry;
    return TraceFrame{entry};
}

Value::List * EvalMemory::newList(size_t size)
{
    auto list =
        reinterpret_cast<Value::List *>(allocBytes(sizeof(Value::List) + size * sizeof(Value *)));
    list->size = size;
    stats.nrListElems += size;
    return list;
}

Value Evaluator::evalLazily(Expr & e)
{
    return e.makeThunk(*this, builtins.env);
}

Value EvalState::mkPos(PosIdx p)
{
    auto origin = ctx.positions.originOf(p);
    if (auto path = std::get_if<CheckedSourcePath>(&origin)) {
        auto attrs = ctx.buildBindings(3);
        attrs.insert(ctx.symbols.sym_file, {NewValueAs::string, path->to_string()});
        auto [line, col] = makePositionThunks(*this, p);
        attrs.insert(ctx.symbols.sym_line, line);
        attrs.insert(ctx.symbols.sym_column, col);
        return {NewValueAs::attrs, attrs};
    } else
        return Value::VNULL;
}

Value EvalPaths::mkStorePathString(const StorePath & p)
{
    return {
        NewValueAs::string,
        store->printStorePath(p),
        NixStringContext{
            NixStringContextElem::Opaque{.path = p},
        }
    };
}


std::string EvalState::mkOutputStringRaw(
    const StorePath & staticOutputPath)
{
    return ctx.store->printStorePath(staticOutputPath);
}

Value EvalState::mkOutputString(const SingleDerivedPath::Built & b, const StorePath & staticOutputPath)
{
    return {NewValueAs::string, mkOutputStringRaw(staticOutputPath), NixStringContext{b}};
}


std::string EvalState::mkSingleDerivedPathStringRaw(
    const SingleDerivedPath & p)
{
    return std::visit(overloaded {
        [&](const SingleDerivedPath::Opaque & o) {
            return ctx.store->printStorePath(o.path);
        },
        [&](const SingleDerivedPath::Built & b) {
            auto drv = aio.blockOn(ctx.store->readDerivation(b.drvPath.path));
            auto i = drv.outputs.find(b.output);
            if (i == drv.outputs.end())
                throw Error("derivation '%s' does not have output '%s'", b.drvPath.to_string(*ctx.store), b.output);
            auto staticOutputPath = i->second.path(*ctx.store, drv.name, b.output);
            return mkOutputStringRaw(staticOutputPath);
        }
    }, p.raw());
}


struct CachedEvalFile
{
    Value result;
    explicit CachedEvalFile(Value result): result(result) {}
};

Value EvalState::evalFile(const SourcePath & path_)
{
    auto path = ctx.paths.checkSourcePath(path_);

    if (auto i = ctx.caches.fileEval.find(path); i != ctx.caches.fileEval.end()) {
        return i->second->result;
    }

    auto resolvedPath = ctx.paths.resolveExprPath(path);
    if (auto i = ctx.caches.fileEval.find(resolvedPath); i != ctx.caches.fileEval.end()) {
        return i->second->result;
    }

    debug("evaluating file '%1%'", resolvedPath);
    Expr & e = ctx.parseExprFromFile(ctx.paths.checkSourcePath(resolvedPath));

    try {
        auto dts = ctx.debug
            ? makeDebugTraceStacker(
                *this,
                e,
                ctx.builtins.env,
                e.getPos() ? std::make_shared<Pos>(ctx.positions[e.getPos()]) : nullptr,
                "while evaluating the file '%1%':", resolvedPath.to_string())
            : nullptr;

        Value v = eval(e);

        auto cache = std::allocate_shared<CachedEvalFile>(TraceableAllocator<CachedEvalFile>(), v);
        ctx.caches.fileEval[resolvedPath] = cache;
        if (path != resolvedPath) {
            ctx.caches.fileEval[path] = cache;
        }

        return v;
    } catch (Error & e) {
        e.addTrace(nullptr, "while evaluating the file '%1%':", resolvedPath.to_string());
        throw;
    }
}


void EvalState::resetFileCache()
{
    ctx.caches.fileEval.clear();
}

Value EvalState::eval(Expr & e)
{
    return e.eval(*this, ctx.builtins.env);
}

std::string showAttrPath(EvalState & state, Env & env, const AttrPath & attrPath)
{
    std::ostringstream out;
    bool first = true;
    for (auto & i : attrPath) {
        if (!first) out << '.'; else first = false;
        try {
            out << state.ctx.symbols[getName(i, state, env)];
        } catch (Error & e) {
            assert(!i.symbol);
            out << "\"${...}\"";
        }
    }
    return out.str();
}

struct FormalsMatch
{
    std::vector<SymbolStr> missing;
    std::vector<SymbolStr> unexpected;
    std::set<std::string> unused;
};

/** Matchup an attribute argument set to a lambda's formal arguments,
 * or return what arguments were required but not given, or given but not allowed.
 */
FormalsMatch matchupLambdaAttrs(EvalState & state, Env & env, Displacement & displ, AttrsPattern const & pattern, Bindings & attrs, SymbolTable & symbols)
{
    size_t attrsUsed = 0;

    FormalsMatch result;

    for (auto const & formal : pattern.formals) {

        // The attribute whose name matches the name of the formal we're matching up, if it exists.
        Attr const * matchingArg = attrs.get(formal.name);
        if (matchingArg) {
            attrsUsed += 1;
            env.values[displ] = matchingArg->value;
            displ += 1;

            // We're done here. Move on to the next formal.
            continue;
        }

        // The argument for this formal wasn't given.
        result.unused.emplace(symbols[formal.name]);
        // If the formal has a default, use it.
        if (formal.def) {
            env.values[displ] = formal.def->maybeThunk(state, env);
            displ += 1;
        } else {
            // Otherwise, let our caller know what was missing.
            result.missing.push_back(symbols[formal.name]);
        }
    }

    // Check for unexpected extra arguments.
    if (!pattern.ellipsis && attrsUsed != attrs.size()) {
        // Return the first unexpected argument.
        for (Attr const & attr : attrs) {
            if (!pattern.has(attr.name)) {
                result.unexpected.push_back(symbols[attr.name]);
            }
        }
    }

    return result;
}

Env & SimplePattern::match(
    ExprLambda & lambda, EvalState & state, Env & up, Value & arg, const PosIdx pos
)
{
    Env & env2(state.ctx.mem.allocEnv(1));
    env2.up = &up;
    env2.values[0] = arg;
    return env2;
}

Env & AttrsPattern::match(
    ExprLambda & lambda, EvalState & state, Env & up, Value & arg, const PosIdx pos
)
{
    auto & ctx = state.ctx;

    Env & env2(ctx.mem.allocEnv(formals.size() + (name ? 1 : 0)));
    env2.up = &up;
    Displacement displ = 0;

    try {
        state.forceAttrs(
            arg, lambda.pos, "while evaluating the value passed for the lambda argument"
        );
    } catch (Error & e) {
        if (pos) e.addTrace(ctx.positions[pos], "from call site");
        throw;
    }

    if (name) {
        env2.values[displ++] = arg;
    }

    ///* For each formal argument, get the actual argument.  If
    //   there is no matching actual argument but the formal
    //   argument has a default, use the default. */
    auto const formalsMatch =
        matchupLambdaAttrs(state, env2, displ, *this, *arg.attrs(), ctx.symbols);

    if (!formalsMatch.unexpected.empty() || !formalsMatch.missing.empty()) {
        Suggestions sug; // empty suggestions -> no suggestions
        if (!formalsMatch.unexpected.empty()) {
            // suggestions only for the first unexpected argument
            // TODO: suggestions for all unexpected arguments
            sug = Suggestions::bestMatches(formalsMatch.unused, formalsMatch.unexpected.front());
        }

        auto argFmt = [](const SymbolStr & argument) { return HintFmt("'%s'", argument); };

        [&](){
            if (formalsMatch.unexpected.empty() && !formalsMatch.missing.empty()) {
                return ctx.errors.make<TypeError>(
                    "function '%s' called without required argument%s %s", lambda.getName(ctx.symbols),
                    Uncolored((formalsMatch.missing.size() == 1) ? "" : "s"),
                    Uncolored(concatStringsCommaAnd(argFmt, formalsMatch.missing))
                );
            } else if (!formalsMatch.unexpected.empty() && formalsMatch.missing.empty()) {
                return ctx.errors.make<TypeError>(
                    "function '%s' called with unexpected argument%s %s", lambda.getName(ctx.symbols),
                    Uncolored((formalsMatch.unexpected.size() == 1) ? "" : "s"),
                    Uncolored(concatStringsCommaAnd(argFmt, formalsMatch.unexpected))
                );
            } else {
                return ctx.errors.make<TypeError>(
                    "function '%s' called without required argument%s %s and with unexpected argument%s %s", lambda.getName(ctx.symbols),
                    Uncolored((formalsMatch.missing.size() == 1) ? "" : "s"),
                    Uncolored(concatStringsCommaAnd(argFmt, formalsMatch.missing)),
                    Uncolored((formalsMatch.unexpected.size() == 1) ? "" : "s"),
                    Uncolored(concatStringsCommaAnd(argFmt, formalsMatch.unexpected))
                );
            }
        }().atPos(lambda.pos)
            .withTrace(pos, "from call site")
            .withSuggestions(sug)
            .withFrame(up, lambda)
            .debugThrow();
    }

    return env2;
}

Value EvalState::callFunction(Value & fun, std::span<Value> args, const PosIdx pos)
{
    if (callDepth > evalSettings.maxCallDepth)
        ctx.errors.make<EvalError>("stack overflow; max-call-depth exceeded").atPos(pos).debugThrow();
    MaintainCount _level(callDepth);

    auto trace = evalSettings.traceFunctionCalls
        ? std::make_unique<FunctionCallTrace>(ctx.positions[pos])
        : nullptr;

    forceValue(fun, pos);

    Value vCur(fun);

    auto makeAppChain = [&]() -> Value {
        if (vCur.isApp()) {
            auto & app = vCur.app();
            return {NewValueAs::app, ctx.mem, app.left(), app.args(), args};
        } else {
            return {NewValueAs::app, ctx.mem, vCur, args};
        }
    };

    const Attr * functor;

    while (args.size() > 0) {

        if (vCur.isLambda()) {

            ExprLambda & lambda(*vCur.lambda().fun);

            Env & env2 = lambda.pattern->match(lambda, *this, *vCur.lambda().env(), args[0], pos);

            ctx.stats.nrFunctionCalls++;
            if (ctx.stats.countCalls) ctx.stats.addCall(lambda);

            /* Evaluate the body. */
            try {
                vCur = lambda.body->eval(*this, env2);
            } catch (Error & e) {
                if (loggerSettings.showTrace.get()) {
                    e.addTrace(
                        ctx.positions[lambda.pos],
                        "while calling %s",
                        lambda.getQuotedName(ctx.symbols));
                    if (pos) e.addTrace(ctx.positions[pos], "from call site");
                }
                throw;
            }

            args = args.subspan(1);
        }

        else if (vCur.isPrimOp()) {

            size_t argsLeft = vCur.primOp()->arity;

            if (args.size() < argsLeft) {
                /* We don't have enough arguments, so create a tPrimOpApp chain. */
                return makeAppChain();
            } else {
                /* We have all the arguments, so call the primop. */
                auto * fn = vCur.primOp();

                ctx.stats.nrPrimOpCalls++;
                if (ctx.stats.countCalls) ctx.stats.primOpCalls[fn->name]++;

                try {
                    SmallVector<Value *, 4> pargs(argsLeft);
                    for (unsigned i = 0; i < argsLeft; i++) {
                        pargs[i] = &args[i];
                    }
                    vCur = fn->fun(*this, pargs.data());
                } catch (ThrownError & e) {
                    // Distinguish between an error that simply happened while "throw"
                    // was being evaluated and an explicit thrown error.
                    if (fn->name == "throw" && !e.hasTrace()) {
                        e.addTrace(ctx.positions[pos], "caused by explicit %s", "throw");
                    } else {
                        e.addTrace(ctx.positions[pos], "while calling the '%s' builtin", fn->name);
                    }
                    throw;
                } catch (Error & e) {
                    e.addTrace(ctx.positions[pos], "while calling the '%1%' builtin", fn->name);
                    throw;
                }

                args = args.subspan(argsLeft);
            }
        }

        else if (vCur.isPrimOpApp()) {
            auto & app = vCur.app();
            auto prevArgs = app.args();

            assert(!vCur.app().left().isApp());

            /* Figure out the number of arguments still needed. */
            Value primOp = app.target();
            auto arity = primOp.primOp()->arity;

            if (args.size() < arity - prevArgs.size()) {
                /* We still don't have enough arguments, so extend the tPrimOpApp chain. */
                return makeAppChain();
            } else {
                /* We have all the arguments, so call the primop with
                   the previous and new arguments. */

                // max arity as of writing is 3. even 4 seems excessive though.
                SmallVector<Value *, 4> vArgs;
                for (auto & arg : prevArgs) {
                    vArgs.push_back(&arg);
                }
                while (vArgs.size() < arity) {
                    vArgs.push_back(&args[0]);
                    args = args.subspan(1);
                }

                auto fn = primOp.primOp();
                ctx.stats.nrPrimOpCalls++;
                if (ctx.stats.countCalls) ctx.stats.primOpCalls[fn->name]++;

                try {
                    // TODO:
                    // 1. Unify this and above code. Heavily redundant.
                    // 2. Create a fake env (arg1, arg2, etc.) and a fake expr (arg1: arg2: etc: builtins.name arg1 arg2 etc)
                    //    so the debugger allows to inspect the wrong parameters passed to the builtin.
                    vCur = fn->fun(*this, vArgs.data());
                } catch (Error & e) {
                    e.addTrace(ctx.positions[pos], "while calling the '%1%' builtin", fn->name);
                    throw;
                }
            }
        }

        else if (vCur.type() == nAttrs && (functor = vCur.attrs()->get(ctx.symbols.sym___functor)))
        {
            /* 'vCur' may be allocated on the stack of the calling
               function, but for functors we may keep a reference, so
               heap-allocate a copy and use that instead. */
            Value args2[] = {vCur, args[0]};
            try {
                vCur = callFunction(functor->value, args2, functor->pos);
            } catch (Error & e) {
                e.addTrace(ctx.positions[pos], "while calling a functor (an attribute set with a '__functor' attribute)");
                throw;
            }
            args = args.subspan(1);
        }

        else {
            ctx.errors
                .make<TypeError>(
                    "attempt to call something which is not a function but %1%: %2%",
                    showType(vCur),
                    ValuePrinter(*this, vCur, errorPrintOptions)
                )
                .atPos(pos)
                .debugThrow();
        }
    }

    return vCur;
}

// Lifted out of callFunction() because it creates a temporary that
// prevents tail-call optimisation.
void EvalStatistics::addCall(ExprLambda & fun)
{
    functionCalls[&fun]++;
}

Value EvalState::autoCallFunction(Bindings & args, Value & fun, PosIdx pos)
{
    forceValue(fun, pos);

    if (fun.type() == nAttrs) {
        auto found = fun.attrs()->get(ctx.symbols.sym___functor);
        if (found) {
            Value v = callFunction(found->value, fun, pos);
            forceValue(v, pos);
            return autoCallFunction(args, v, pos);
        }
    }

    if (!fun.isLambda()) {
        return fun;
    }
    auto pattern = dynamic_cast<AttrsPattern *>(fun.lambda().fun->pattern.get());
    if (!pattern) {
        return fun;
    }

    auto attrs = ctx.buildBindings(std::max(static_cast<uint32_t>(pattern->formals.size()), args.size()));

    if (pattern->ellipsis) {
        // If the formals have an ellipsis (eg the function accepts extra args) pass
        // all available automatic arguments (which includes arguments specified on
        // the command line via --arg/--argstr)
        for (auto & v : args)
            attrs.insert(v);
    } else {
        // Otherwise, only pass the arguments that the function accepts
        for (auto & i : pattern->formals) {
            auto j = args.get(i.name);
            if (j) {
                attrs.insert(*j);
            } else if (!i.def) {
                ctx.errors
                    .make<MissingArgumentError>(
                        R"(cannot evaluate a function that has an argument without a value ('%1%')
Lix attempted to evaluate a function as a top level expression; in
this case it must have its arguments supplied either by default
values, or passed explicitly with '--arg' or '--argstr'. See
https://docs.lix.systems/manual/lix/stable/language/constructs.html#functions)",
                        ctx.symbols[i.name]
                    )
                    .atPos(i.pos)
                    .withFrame(*fun.lambda().env(), *fun.lambda().fun)
                    .debugThrow();
            }
        }
    }

    Value vAttrs{NewValueAs::attrs, attrs.finish()};
    return callFunction(fun, vAttrs, pos);
}

Value EvalState::updateAttrs(const Value & v1, const Value & v2)
{
    ctx.stats.nrOpUpdates++;

    if (v1.attrs()->size() == 0) {
        return v2;
    }
    if (v2.attrs()->size() == 0) {
        return v1;
    }

    auto attrs = ctx.buildBindings(v1.attrs()->size() + v2.attrs()->size());

    /* Merge the sets, preferring values from the second set.  Make
       sure to keep the resulting vector in sorted order. */
    Bindings::iterator i = v1.attrs()->begin();
    Bindings::iterator j = v2.attrs()->begin();

    while (i != v1.attrs()->end() && j != v2.attrs()->end()) {
        if (i->name == j->name) {
            attrs.insert(*j);
            ++i;
            ++j;
        } else if (i->name < j->name) {
            attrs.insert(*i++);
        } else {
            attrs.insert(*j++);
        }
    }

    while (i != v1.attrs()->end()) {
        attrs.insert(*i++);
    }
    while (j != v2.attrs()->end()) {
        attrs.insert(*j++);
    }

    Value v = {NewValueAs::attrs, attrs.alreadySorted()};
    ctx.stats.nrOpUpdateValuesCopied += v.attrs()->size();
    return v;
}

Value EvalState::concatLists(std::span<Value> lists, const PosIdx pos, std::string_view errorCtx)
{
    ctx.stats.nrListConcats++;

    Value * nonEmpty = 0;
    size_t len = 0;
    for (size_t n = 0; n < lists.size(); ++n) {
        forceList(lists[n], pos, errorCtx);
        auto l = lists[n].listSize();
        len += l;
        if (l) {
            nonEmpty = &lists[n];
        }
    }

    if (nonEmpty && len == nonEmpty->listSize()) {
        return *nonEmpty;
    }

    auto list = ctx.mem.newList(len);
    auto out = list->elems;
    for (size_t n = 0, pos = 0; n < lists.size(); ++n) {
        auto l = lists[n].listSize();
        if (l) {
            std::copy(lists[n].listItems().begin(), lists[n].listItems().end(), out + pos);
        }
        pos += l;
    }
    return {NewValueAs::list, list};
}

// always force this to be separate, otherwise forceValue may inline it and take
// a massive perf hit
[[gnu::noinline]]
void EvalState::tryFixupBlackHolePos(Value & v, PosIdx pos)
{
    if (!v.isBlackhole())
        return;
    auto e = std::current_exception();
    try {
        std::rethrow_exception(e);
    } catch (InfiniteRecursionError & e) {
        e.atPos(ctx.positions[pos]);
    } catch (...) {
    }
}


void EvalState::forceValueDeep(Value & v)
{
    std::set<const Value *> seen;

    std::function<void(Value & v)> recurse;

    recurse = [&](Value & v) {
        if (!seen.insert(&v).second) return;

        forceValue(v, noPos);

        if (v.type() == nAttrs) {
            for (auto & i : *v.attrs())
                try {
                    // If the value is a thunk, we're evaling. Otherwise no trace necessary.
                    auto dts = ctx.debug && i.value.isThunk()
                        ? makeDebugTraceStacker(
                              *this,
                              *i.value.thunk().expr,
                              *i.value.thunk().env(),
                              ctx.positions[i.pos],
                              "while evaluating the attribute '%1%'",
                              ctx.symbols[i.name]
                          )
                        : nullptr;

                    recurse(i.value);
                } catch (Error & e) {
                    e.addTrace(ctx.positions[i.pos], "while evaluating the attribute '%1%'", ctx.symbols[i.name]);
                    throw;
                }
        }

        else if (v.isList()) {
            for (auto & v2 : v.listItems()) {
                recurse(v2);
            }
        }
    };

    recurse(v);
}


NixInt EvalState::forceInt(Value & v, const PosIdx pos, std::string_view errorCtx)
{
    try {
        forceValue(v, pos);
        return checkInt(v);
    } catch (Error & e) {
        e.addTrace(ctx.positions[pos], errorCtx);
        throw;
    }
}


NixFloat EvalState::forceFloat(Value & v, const PosIdx pos, std::string_view errorCtx)
{
    try {
        forceValue(v, pos);
        return checkFloat(v);
    } catch (Error & e) {
        e.addTrace(ctx.positions[pos], errorCtx);
        throw;
    }
}


bool EvalState::forceBool(Value & v, const PosIdx pos, std::string_view errorCtx)
{
    try {
        forceValue(v, pos);
        return checkBool(v);
    } catch (Error & e) {
        e.addTrace(ctx.positions[pos], errorCtx);
        throw;
    }
}


bool EvalState::isFunctor(Value & fun)
{
    return fun.type() == nAttrs && fun.attrs()->get(ctx.symbols.sym___functor);
}


void EvalState::forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx)
{
    try {
        forceValue(v, pos);
        if (v.type() != nFunction && !isFunctor(v))
            ctx.errors.make<TypeError>(
                "expected a function but found %1%: %2%",
                showType(v),
                ValuePrinter(*this, v, errorPrintOptions)
            ).atPos(pos).debugThrow();
    } catch (Error & e) {
        e.addTrace(ctx.positions[pos], errorCtx);
        throw;
    }
}


std::string_view EvalState::forceString(Value & v, const PosIdx pos, std::string_view errorCtx)
{
    try {
        forceValue(v, pos);
        if (v.type() != nString)
            ctx.errors.make<TypeError>(
                "expected a string but found %1%: %2%",
                showType(v),
                ValuePrinter(*this, v, errorPrintOptions)
            ).atPos(pos).debugThrow();
        return v.str();
    } catch (Error & e) {
        e.addTrace(ctx.positions[pos], errorCtx);
        throw;
    }
}


void copyContext(const Value & v, NixStringContext & context)
{
    if (v.string().context)
        for (const char * * p = v.string().context; *p; ++p)
            context.insert(NixStringContextElem::parse(*p));
}


std::string_view EvalState::forceString(Value & v, NixStringContext & context, const PosIdx pos, std::string_view errorCtx)
{
    auto s = forceString(v, pos, errorCtx);
    copyContext(v, context);
    return s;
}


std::string_view EvalState::forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx)
{
    auto s = forceString(v, pos, errorCtx);
    if (v.string().context) {
        ctx.errors
            .make<EvalError>(
                "the string '%1%' is not allowed to refer to a store path (such as '%2%')",
                v.str(),
                v.string().context[0]
            )
            .withTrace(pos, errorCtx)
            .debugThrow();
    }
    return s;
}


bool EvalState::isDerivation(Value & v)
{
    if (v.type() != nAttrs) return false;
    auto i = v.attrs()->get(ctx.symbols.sym_type);
    if (!i) {
        return false;
    }
    forceValue(i->value, i->pos);
    if (i->value.type() != nString) {
        return false;
    }
    return i->value.str() == "derivation";
}


std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value & v,
    NixStringContext & context, StringCoercionMode mode, bool copyToStore)
{
    auto i = v.attrs()->get(ctx.symbols.sym___toString);
    if (i) {
        try {
            Value v1 = callFunction(i->value, v, i->pos);
            return coerceToString(pos, v1, context,
                    "while evaluating the result of the `__toString` attribute",
                    mode, copyToStore).toOwned();
        } catch (EvalError & e) {
            e.addTrace(ctx.positions[pos], "while converting a set to string");
            throw;
        }
    }

    return {};
}

BackedStringView EvalState::coerceToString(
    const PosIdx pos,
    Value & v,
    NixStringContext & context,
    std::string_view errorCtx,
    StringCoercionMode mode,
    bool copyToStore,
    bool canonicalizePath)
{
    forceValue(v, pos);

    if (v.type() == nString) {
        copyContext(v, context);
        return v.str();
    }

    if (v.type() == nPath) {
        return !canonicalizePath && !copyToStore
            // FIXME: hack to preserve path literals that end in a slash, as in /foo/${x}.
            ? std::string(v.string().content->str())
            : (copyToStore
                   ? ctx.store->printStorePath(
                         aio.blockOn(ctx.paths.copyPathToStore(context, v.path(), ctx.repair))
                             .unwrap()
                     )
                   : v.path().to_string());
    }

    if (v.type() == nAttrs) {
        auto maybeString = tryAttrsToString(pos, v, context, mode, copyToStore);
        if (maybeString)
            return std::move(*maybeString);
        auto i = v.attrs()->get(ctx.symbols.sym_outPath);
        if (!i) {
            ctx.errors.make<TypeError>(
                "cannot coerce %1% to a string: %2%",
                showType(v),
                ValuePrinter(*this, v, errorPrintOptions)
            )
                .withTrace(pos, errorCtx)
                .debugThrow();
        }
        return coerceToString(
            pos, i->value, context, errorCtx, mode, copyToStore, canonicalizePath
        );
    }

    if (v.type() == nExternal) {
        try {
            return v.external()->coerceToString(*this, pos, context, mode, copyToStore);
        } catch (Error & e) {
            e.addTrace(nullptr, errorCtx);
            throw;
        }
    }

    /* Raito: Any addition to this mode is subject to extra scrutiny
     * until we have better formatting tools. */
    if (mode >= StringCoercionMode::Interpolation) {
        if (v.type() == nInt) {
            return std::to_string(v.integer().value);
        }
    }

    if (mode >= StringCoercionMode::ToString) {
        /* Note that `false' is represented as an empty string for
           shell scripting convenience, just like `null'. */
        if (v.type() == nBool && v.boolean()) {
            return "1";
        }
        if (v.type() == nBool && !v.boolean()) {
            return "";
        }
        if (v.type() == nFloat) return std::to_string(v.fpoint());
        if (v.type() == nNull) return "";

        if (v.isList()) {
            std::string result;
            for (auto [n, v2] : enumerate(v.listItems())) {
                try {
                    result += *coerceToString(
                        pos,
                        v2,
                        context,
                        "while evaluating one element of the list",
                        mode,
                        copyToStore,
                        canonicalizePath
                    );
                } catch (Error & e) {
                    e.addTrace(ctx.positions[pos], errorCtx);
                    throw;
                }
                if (n < v.listSize() - 1
                    /* !!! not quite correct */
                    && (!v2.isList() || v2.listSize() != 0))
                {
                    result += " ";
                }
            }
            return result;
        }
    }

    ctx.errors.make<TypeError>("cannot coerce %1% to a string: %2%",
        showType(v),
        ValuePrinter(*this, v, errorPrintOptions)
    )
        .withTrace(pos, errorCtx)
        .debugThrow();
}


kj::Promise<Result<EvalPaths::PathResult<StorePath, EvalError>>>
EvalPaths::copyPathToStore(NixStringContext & context, const SourcePath & path, RepairFlag repair)
try {
    if (nix::isDerivation(path.canonical().abs()))
        co_return errors.make<EvalError>("file names are not allowed to end in '%1%'", drvExtension);

    auto i = srcToStore.find(path);

    auto dstPath = i != srcToStore.end()
        ? i->second
        : ({
            auto dstPath = TRY_AWAIT(fetchToStoreRecursive(
                *store,
                *prepareDump(checkSourcePath(path).canonical().abs()),
                path.baseName(),
                repair
            ));
            allowPath(dstPath);
            srcToStore.insert_or_assign(path, dstPath);
            printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
            std::move(dstPath);
        });

    context.insert(NixStringContextElem::Opaque {
        .path = dstPath
    });
    co_return dstPath;
} catch (...) {
    co_return result::current_exception();
}


SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx)
{
    auto path = coerceToString(pos, v, context, errorCtx, StringCoercionMode::Strict, false, true).toOwned();
    if (path == "" || path[0] != '/')
        ctx.errors.make<EvalError>("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow();
    return CanonPath(path);
}


StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx)
{
    auto path = coerceToString(pos, v, context, errorCtx, StringCoercionMode::Strict, false, true).toOwned();
    if (auto storePath = ctx.store->maybeParseStorePath(path))
        return *storePath;
    ctx.errors.make<EvalError>("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow();
}


std::pair<SingleDerivedPath, std::string_view> EvalState::coerceToSingleDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx)
{
    NixStringContext context;
    auto s = forceString(v, context, pos, errorCtx);
    auto csize = context.size();
    if (csize != 1)
        ctx.errors.make<EvalError>(
            "string '%s' has %d entries in its context. It should only have exactly one entry",
            s, csize)
            .withTrace(pos, errorCtx).debugThrow();
    auto derivedPath = std::visit(overloaded {
        [&](NixStringContextElem::Opaque && o) -> SingleDerivedPath {
            return std::move(o);
        },
        [&](NixStringContextElem::DrvDeep &&) -> SingleDerivedPath {
            ctx.errors.make<EvalError>(
                "string '%s' has a context which refers to a complete source and binary closure. This is not supported at this time",
                s).withTrace(pos, errorCtx).debugThrow(always_progresses);
        },
        [&](NixStringContextElem::Built && b) -> SingleDerivedPath {
            return std::move(b);
        },
    }, ((NixStringContextElem &&) *context.begin()).raw);
    return {
        std::move(derivedPath),
        std::move(s),
    };
}


SingleDerivedPath EvalState::coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx)
{
    auto [derivedPath, s_] = coerceToSingleDerivedPathUnchecked(pos, v, errorCtx);
    auto s = s_;
    auto sExpected = mkSingleDerivedPathStringRaw(derivedPath);
    if (s != sExpected) {
        /* `std::visit` is used here just to provide a more precise
           error message. */
        std::visit(overloaded {
            [&](const SingleDerivedPath::Opaque & o) {
                ctx.errors.make<EvalError>(
                    "path string '%s' has context with the different path '%s'",
                    s, sExpected)
                    .withTrace(pos, errorCtx).debugThrow(always_progresses);
            },
            [&](const SingleDerivedPath::Built & b) {
                ctx.errors.make<EvalError>(
                    "string '%s' has context with the output '%s' from derivation '%s', but the string is not the right placeholder for this derivation output. It should be '%s'",
                    s, b.output, b.drvPath.to_string(*ctx.store), sExpected)
                    .withTrace(pos, errorCtx).debugThrow(always_progresses);
            }
        }, derivedPath.raw());
    }
    return derivedPath;
}


bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_view errorCtx)
{
    forceValue(v1, pos);
    forceValue(v2, pos);

    // Special case type-compatibility between float and int
    if (v1.type() == nInt && v2.type() == nFloat) {
        return v1.integer().value == v2.fpoint();
    }
    if (v1.type() == nFloat && v2.type() == nInt) {
        return v1.fpoint() == v2.integer().value;
    }

    // All other types are not compatible with each other.
    if (v1.type() != v2.type()) return false;

    /* !!! Hack to support some old broken code that relies on pointer
       equality tests between sets.  (Specifically, builderDefs calls
       uniqList on a list of sets.)  Will remove this eventually. */
    auto pointerEq = [&] { return v1.pointerEqProxy() == v2.pointerEqProxy(); };

    switch (v1.type()) {
        case nInt:
            return v1.integer() == v2.integer();

        case nBool:
            return v1.boolean() == v2.boolean();

        case nString:
            return v1.str() == v2.str();

        case nPath:
            return v1.string().content->str() == v2.string().content->str();

        case nNull:
            return true;

        case nList:
            if (pointerEq()) return true;
            if (v1.listSize() != v2.listSize()) return false;
            for (size_t n = 0; n < v1.listSize(); ++n) {
                if (!eqValues(v1.listElems()[n], v2.listElems()[n], pos, errorCtx)) {
                    return false;
                }
            }
            return true;

        case nAttrs: {
            if (pointerEq()) return true;
            /* If both sets denote a derivation (type = "derivation"),
               then compare their outPaths. */
            if (isDerivation(v1) && isDerivation(v2)) {
                auto i = v1.attrs()->get(ctx.symbols.sym_outPath);
                auto j = v2.attrs()->get(ctx.symbols.sym_outPath);
                if (i && j) {
                    return eqValues(i->value, j->value, pos, errorCtx);
                }
            }

            if (v1.attrs()->size() != v2.attrs()->size()) return false;

            /* Otherwise, compare the attributes one by one. */
            Bindings::iterator i, j;
            for (i = v1.attrs()->begin(), j = v2.attrs()->begin(); i != v1.attrs()->end(); ++i, ++j)
            {
                if (i->name != j->name || !eqValues(i->value, j->value, pos, errorCtx)) {
                    return false;
                }
            }

            return true;
        }

        /* Functions are incomparable, except for identity (see note above about this nonsense). */
        case nFunction:
            return pointerEq();

        case nExternal:
            if (pointerEq()) return true;
            return *v1.external() == *v2.external();

        case nFloat:
            return v1.fpoint() == v2.fpoint();

        case nThunk: // Must not be left by forceValue
        default:
            ctx.errors.make<EvalError>("cannot compare %1% with %2%", showType(v1), showType(v2)).withTrace(pos, errorCtx).debugThrow();
    }
}

bool Evaluator::fullGC() {
#if HAVE_BOEHMGC
    GC_gcollect();
    // Check that it ran. We might replace this with a version that uses more
    // of the boehm API to get this reliably, at a maintenance cost.
    // We use a 1K margin because technically this has a race condtion, but we
    // probably won't encounter it in practice, because the CLI isn't concurrent
    // like that.
    return GC_get_bytes_since_gc() < 1024;
#else
    return false;
#endif
}

void Evaluator::maybePrintStats()
{
    bool showStats = getEnv("NIX_SHOW_STATS").value_or("0") != "0";

    if (showStats) {
        // Make the final heap size more deterministic.
#if HAVE_BOEHMGC
        if (!fullGC()) {
            printTaggedWarning("failed to perform a full GC before reporting stats");
        }
#endif
        printStatistics();
    }
}

void Evaluator::printStatistics()
{
    struct rusage buf;
    getrusage(RUSAGE_SELF, &buf);
    float cpuTime = buf.ru_utime.tv_sec + ((float) buf.ru_utime.tv_usec / 1000000);
    auto mem = this->mem.getStats();

    uint64_t bEnvs = mem.nrEnvs * sizeof(Env) + mem.nrValuesInEnvs * sizeof(Value *);
    uint64_t bLists = mem.nrListElems * sizeof(Value *);
    uint64_t bAttrsets = mem.nrAttrsets * sizeof(Bindings) + mem.nrAttrsInAttrsets * sizeof(Attr);

#if HAVE_BOEHMGC
    GC_word heapSize, totalBytes;
    GC_get_heap_usage_safe(&heapSize, 0, 0, 0, &totalBytes);
#endif

    auto outPath = getEnv("NIX_SHOW_STATS_PATH").value_or("-");
    std::fstream fs;
    if (outPath != "-")
        fs.open(outPath, std::fstream::out);
    JSON topObj = JSON::object();
    topObj["cpuTime"] = cpuTime;
    topObj["envs"] = {
        {"number", mem.nrEnvs},
        {"elements", mem.nrValuesInEnvs},
        {"bytes", bEnvs},
    };
    topObj["list"] = {
        {"elements", mem.nrListElems},
        {"bytes", bLists},
        {"concats", stats.nrListConcats},
    };
    // reported for compatibility, even though we no longer allocate these on the heap
    topObj["values"] = {
        {"number", 0},
        {"bytes", 0},
    };
    topObj["symbols"] = {
        {"number", symbols.size()},
        {"bytes", symbols.totalSize()},
    };
    topObj["sets"] = {
        {"number", mem.nrAttrsets},
        {"bytes", bAttrsets},
        {"elements", mem.nrAttrsInAttrsets},
    };
    topObj["sizes"] = {
        {"Env", sizeof(Env)},
        {"Value", sizeof(Value)},
        {"Bindings", sizeof(Bindings)},
        {"Attr", sizeof(Attr)},
    };
    topObj["nrOpUpdates"] = stats.nrOpUpdates;
    topObj["nrOpUpdateValuesCopied"] = stats.nrOpUpdateValuesCopied;
    topObj["nrThunks"] = stats.nrThunks;
    topObj["nrAvoided"] = stats.nrAvoided;
    topObj["nrLookups"] = stats.nrLookups;
    topObj["nrPrimOpCalls"] = stats.nrPrimOpCalls;
    topObj["nrFunctionCalls"] = stats.nrFunctionCalls;
#if HAVE_BOEHMGC
    topObj["gc"] = {
        {"heapSize", heapSize},
        {"totalBytes", totalBytes},
    };
#endif

    if (stats.countCalls) {
        topObj["primops"] = stats.primOpCalls;
        {
            auto& list = topObj["functions"];
            list = JSON::array();
            for (auto & [fun, count] : stats.functionCalls) {
                JSON obj = JSON::object();
                if (fun->name)
                    obj["name"] = (std::string_view) symbols[fun->name];
                else
                    obj["name"] = nullptr;
                if (auto pos = positions[fun->pos]) {
                    if (auto path = std::get_if<CheckedSourcePath>(&pos.origin))
                        obj["file"] = path->to_string();
                    obj["line"] = pos.line;
                    obj["column"] = pos.column;
                }
                obj["count"] = count;
                list.push_back(obj);
            }
        }
        {
            auto list = topObj["attributes"];
            list = JSON::array();
            for (auto & i : stats.attrSelects) {
                JSON obj = JSON::object();
                if (auto pos = positions[i.first]) {
                    if (auto path = std::get_if<CheckedSourcePath>(&pos.origin))
                        obj["file"] = path->to_string();
                    obj["line"] = pos.line;
                    obj["column"] = pos.column;
                }
                obj["count"] = i.second;
                list.push_back(obj);
            }
        }
    }

    if (getEnv("NIX_SHOW_SYMBOLS").value_or("0") != "0") {
        // XXX: overrides earlier assignment
        topObj["symbols"] = JSON::array();
        auto &list = topObj["symbols"];
        symbols.dump([&](std::string_view s) { list.emplace_back(s); });
    }
    if (outPath == "-") {
        std::cerr << topObj.dump(2) << std::endl;
    } else {
        fs << topObj.dump(2) << std::endl;
    }
}


CheckedSourcePath EvalPaths::resolveExprPath(SourcePath path_)
{
    auto path = checkSourcePath(path_);
    unsigned int followCount = 0, maxFollow = 1024;

    /* If `path' is a symlink, follow it.  This is so that relative
       path references work. */
    while (true) {
        // Basic cycle/depth limit to avoid infinite loops.
        if (++followCount >= maxFollow)
            throw Error("too many symbolic links encountered while traversing the path '%s'", path);
        if (path.lstat().type != InputAccessor::tSymlink) break;
        path = checkSourcePath(
            CanonPath(path.readLink(), path.canonical().parent().value_or(CanonPath::root))
        );
    }

    /* If `path' refers to a directory, append `/default.nix'. */
    if (path.lstat().type == InputAccessor::tDirectory)
        return checkSourcePath(path + "default.nix");

    return path;
}


Expr & Evaluator::parseExprFromFile(const CheckedSourcePath & path)
{
    return parseExprFromFile(path, builtins.staticEnv);
}


Expr & Evaluator::parseExprFromFile(const CheckedSourcePath & path, std::shared_ptr<StaticEnv> & staticEnv)
{
    auto buffer = path.readFile();
    return *parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
}


Expr & Evaluator::parseExprFromString(
    std::string s_,
    const SourcePath & basePath,
    std::shared_ptr<StaticEnv> & staticEnv,
    const FeatureSettings & featureSettings
)
{
    auto s = make_ref<std::string>(std::move(s_));
    return *parse(s->data(), s->size(), Pos::String{.source = s}, basePath, staticEnv, featureSettings);
}


Expr & Evaluator::parseExprFromString(
    std::string s,
    const SourcePath & basePath,
    const FeatureSettings & featureSettings
)
{
    return parseExprFromString(std::move(s), basePath, builtins.staticEnv, featureSettings);
}

std::variant<std::unique_ptr<Expr>, ExprReplBindings>
Evaluator::parseReplInput(
    std::string s_,
    const SourcePath & basePath,
    std::shared_ptr<StaticEnv> & staticEnv,
    const FeatureSettings & featureSettings
)
{
    auto s = make_ref<std::string>(std::move(s_));
    return parse_repl(s->data(), s->size(), Pos::String{.source = s}, basePath, staticEnv, featureSettings);
}

Expr & Evaluator::parseStdin()
{
    //Activity act(*logger, lvlTalkative, "parsing standard input");
    auto s = make_ref<std::string>(drainFD(0));
    return *parse(
        s->data(), s->size(), Pos::Stdin{.source = s}, CanonPath::fromCwd(), builtins.staticEnv
    );
}

std::optional<DebugTrace const *> Evaluator::nextDebugTrace() const
{
    if (!debug) {
        return std::nullopt;
    }

    return debug->traces().next();
}


kj::Promise<Result<EvalPaths::PathResult<SourcePath, ThrownError>>>
EvalPaths::findFile(const std::string_view path)
{
    return findFile(searchPath_, path);
}


kj::Promise<Result<EvalPaths::PathResult<SourcePath, ThrownError>>>
EvalPaths::findFile(const SearchPath & searchPath, const std::string_view path, const PosIdx pos)
try {
    for (auto & i : searchPath.elements) {
        auto suffixOpt = i.prefix.suffixIfPotentialMatch(path);

        if (!suffixOpt) continue;
        auto suffix = *suffixOpt;

        auto rOpt = TRY_AWAIT(resolveSearchPathPath(i.path));
        if (!rOpt) continue;
        auto r = *rOpt;

        Path res = suffix == "" ? r : concatStrings(r, "/", suffix);
        if (pathExists(res)) co_return SourcePath(CanonPath(canonPath(res)));
    }

    if (path.starts_with("nix/"))
        co_return SourcePath(CanonPath(concatStrings(corepkgsPrefix, path.substr(4))));

    co_return errors.make<ThrownError>(
        evalSettings.pureEval
            ? "cannot look up '<%s>' in pure evaluation mode (use '--impure' to override)"
            : "file '%s' was not found in the Nix search path (add it using $NIX_PATH or -I)",
        path
    ).atPos(pos);
} catch (...) {
    co_return result::current_exception();
}


kj::Promise<Result<std::optional<std::string>>>
EvalPaths::resolveSearchPathPath(const SearchPath::Path & value0)
try {
    auto & value = value0.s;
    auto i = searchPathResolved.find(value);
    if (i != searchPathResolved.end()) co_return i->second;

    std::optional<std::string> res;

    if (EvalSettings::isPseudoUrl(value)) {
        try {
            auto storePath = TRY_AWAIT(fetchers::downloadTarball(
                store, EvalSettings::resolvePseudoUrl(value), "source", false)).tree.storePath;
            res = { store->toRealPath(storePath) };
        } catch (FileTransferError & e) {
            e.addTrace(nullptr, "while downloading %s to satisfy NIX_PATH lookup, ignoring search path entry", value);
            logWarning(e.info());
            res = std::nullopt;
        }
    }

    else if (value.starts_with("flake:")) {
        experimentalFeatureSettings.require(Xp::Flakes);
        auto flakeRef = parseFlakeRef(value.substr(6), {}, true, false);
        debug("fetching flake search path element '%s''", value);
        auto storePath =
            TRY_AWAIT(TRY_AWAIT(flakeRef.resolve(store)).fetchTree(store)).first.storePath;
        res = {store->toRealPath(storePath)};
    }

    else {
        auto path = absPath(value);
        if (pathExists(path))
            res = { path };
        else {
            logWarning({
                .msg = HintFmt("Nix search path entry '%1%' does not exist, ignoring", value)
            });
            res = std::nullopt;
        }
    }

    if (res)
        debug("resolved search path element '%s' to '%s'", value, *res);
    else
        debug("failed to resolve search path element '%s'", value);

    searchPathResolved[value] = res;
    co_return res;
} catch (...) {
    co_return result::current_exception();
}


std::string ExternalValueBase::coerceToString(EvalState & state, const PosIdx & pos, NixStringContext & context, StringCoercionMode mode, bool copyToStore) const
{
    state.ctx.errors.make<TypeError>(
        "cannot coerce %1% to a string: %2%", showType(), *this
    ).atPos(pos).debugThrow();
}


bool ExternalValueBase::operator==(const ExternalValueBase & b) const
{
    return false;
}


std::ostream & operator << (std::ostream & str, const ExternalValueBase & v) {
    return v.print(str);
}


}