libexpr: fix app chain extension

during the value rewrite we accidentally broke extension of incomplete
primop application. this only shows up when binding on incomplete call
to a primop to a name, binding an incomplete call to *that* to another
name, and then finally calling the second binding with enough args for
a complete primop application. since this only shows up when calling a
primop with three or more args it took a while to surface. we have few
builtins that match this: foldl', replaceStrings, and substring. these
are not used incompletely in this manner very often, so this lingered.

fixes #1102

Change-Id: I218dffc14ae876efc86a86c7eb6c895e2405201c

lix/libexpr/eval-inline.hh

@@ -20,11 +20,21 @@ inline Value::Value(app_t, EvalMemory & mem, Value & lhs, Value & rhs)
 }
 
 inline Value::Value(app_t, EvalMemory & mem, Value & lhs, std::span<Value> args)
+    : Value(app_t{}, mem, lhs, args, {})
 {
-    auto app = static_cast<Value::App *>(mem.allocBytes(sizeof(Value::App) + args.size_bytes()));
+}
+
+inline Value::Value(
+    app_t, EvalMemory & mem, const Value & lhs, std::span<Value> baseArgs, std::span<Value> moreArgs
+)
+{
+    auto app = static_cast<Value::App *>(
+        mem.allocBytes(sizeof(Value::App) + baseArgs.size_bytes() + moreArgs.size_bytes())
+    );
     app->_left = lhs;
-    app->_n = args.size();
-    std::copy(args.begin(), args.end(), app->_args);
+    app->_n = baseArgs.size() + moreArgs.size();
+    std::copy(baseArgs.begin(), baseArgs.end(), app->_args);
+    std::copy(moreArgs.begin(), moreArgs.end(), app->_args + baseArgs.size());
     raw = tag(tApp, app);
 }
 

lix/libexpr/eval.cc

@@ -1138,7 +1138,14 @@ void EvalState::callFunction(Value & fun, std::span<Value> args, Value & vRes, c
 
     Value vCur(fun);
 
-    auto makeAppChain = [&]() { vRes = {NewValueAs::app, ctx.mem, vCur, args}; };
+    auto makeAppChain = [&]() {
+        if (vCur.isApp()) {
+            auto & app = vCur.app();
+            vRes = {NewValueAs::app, ctx.mem, app.left(), app.args(), args};
+        } else {
+            vRes = {NewValueAs::app, ctx.mem, vCur, args};
+        }
+    };
 
     const Attr * functor;
 

lix/libexpr/value.hh

@@ -544,6 +544,10 @@ public:
     /// lazy and/or partial application of a function.
     Value(app_t, EvalMemory & mem, Value & lhs, std::span<Value> args);
 
+    /// Constructs a nix language value of type "lambda", which represents a
+    /// lazy and/or partial application of a function.
+    Value(app_t, EvalMemory & mem, const Value & lhs, std::span<Value> baseArgs, std::span<Value> moreArgs);
+
     /// Constructs a nix language value of type "external", which is only used
     /// by plugins. Do any existing plugins even use this mechanism?
     Value(external_t, ExternalValueBase & external)

tests/functional2/lang/call-primop/eval-okay-app-extension.out.exp

@@ -0,0 +1 @@
+"234"

tests/functional2/lang/call-primop/in-app-extension.nix

@@ -0,0 +1,5 @@
+let
+  a = builtins.substring 1;
+  b = a 3;
+in
+b "1234567890"