feat(nix-instantiate): add --raw flag

The experimental `nix eval` command already supports a `--raw` flag.
This commit implements the same flag for the stable nix-instantiate command.

Until now instructions and scripts that didn't want to rely on experimental
features had to use workarounds such as:

    nix-instantiate --eval <something> | tr -d \"

(which also undesirably also removes double quotation marks within the string), or

    nix-instantiate --eval <something> | jq -j

(which undesirably depends on another package).

Co-authored-by: Raito Bezarius <raito@lix.systems>
Co-authored-by: Silvan Mosberger <silvan.mosberger@tweag.io>
Change-Id: Iced9a80ee7edd60af2385c5193485f1774175339

doc/manual/change-authors.yml

@@ -101,6 +101,9 @@ ian-h-chamberlain:
   forgejo: ian-h-chamberlain
   github: ian-h-chamberlain
 
+infinisil:
+  github: infinisil
+
 isabelroses:
   forgejo: isabelroses
   github: isabelroses
@@ -158,6 +161,10 @@ midnightveil:
 ncfavier:
   github: ncfavier
 
+not-my-profile:
+  display_name: Martin Fischer
+  github: not-my-profile
+
 p-e-meunier:
   display_name: Pierre-Etienne Meunier
   github: P-E-Meunier

doc/manual/rl-next/nix-instantiate-raw.md

@@ -0,0 +1,12 @@
+---
+synopsis: "Add --raw flag to `nix-instantiate --eval` for unescaped output"
+issues: []
+prs: [gh#12119]
+cls: [2886]
+category: Improvements
+credits: [not-my-profile, infinisil, raito]
+---
+
+The `nix-instantiate --eval` command now supports a `--raw` flag. When used,
+the result must be coercible to a string (as with `${...}`) and is printed
+verbatim, without quotes or escaping.

doc/manual/src/command-ref/nix-instantiate.md

@@ -5,7 +5,7 @@
 # Synopsis
 
 `nix-instantiate`
-  [`--parse` | `--eval` [`--strict`] [`--json`] [`--xml`] ]
+  [`--parse` | `--eval` [`--strict`] [`--raw`] [`--json`] [`--xml`] ]
   [`--read-write-mode`]
   [`--arg` *name* *value*]
   [{`--attr`| `-A`} *attrPath*]
@@ -107,6 +107,11 @@ See that section for complete details (`nix-build --help`), but in summary, a pa
     > This option can cause non-termination, because lazy data
     > structures can be infinitely large.
 
+  - `--raw`
+    When used with `--eval`, the result must be coercible to a string, i.e.,
+    something that can be converted using `${...}`. The output is
+    printed exactly as-is, with no quotes, escaping, or trailing newline.
+
   - `--json`\
     When used with `--eval`, print the resulting value as an JSON
     representation of the abstract syntax tree rather than as a Nix expression.

lix/legacy/nix-instantiate.cc

@@ -22,7 +22,7 @@ static Path gcRoot;
 static int rootNr = 0;
 
 
-enum OutputKind { okPlain, okXML, okJSON };
+enum OutputKind { okPlain, okRaw, okXML, okJSON };
 
 void processExpr(EvalState & state, const Strings & attrPaths,
     bool parseOnly, bool strict, Bindings & autoArgs,
@@ -48,7 +48,11 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                 vRes = v;
             else
                 state.autoCallFunction(autoArgs, v, vRes, noPos);
-            if (output == okXML)
+            if (output == okRaw)
+                std::cout << *state.coerceToString(noPos, vRes, context, "while generating the nix-instantiate output", StringCoercionMode::Strict);
+                // We intentionally don't output a newline here. The default PS1 for Bash in NixOS starts with a newline
+                // and other interactive shells like Zsh are smart enough to print a missing newline before the prompt.
+            else if (output == okXML)
                 printValueAsXML(state, strict, location, vRes, std::cout, context, noPos);
             else if (output == okJSON) {
                 printValueAsJSON(state, strict, vRes, noPos, std::cout, context);
@@ -130,6 +134,8 @@ static int main_nix_instantiate(AsyncIoRoot & aio, std::string programName, Stri
                 gcRoot = getArg(*arg, arg, end);
             else if (*arg == "--indirect")
                 ;
+            else if (*arg == "--raw")
+                outputKind = okRaw;
             else if (*arg == "--xml")
                 outputKind = okXML;
             else if (*arg == "--json")

tests/functional/eval.nix

@@ -1,5 +1,5 @@
 {
   int = 123;
-  str = "foo";
+  str = "foo\nbar";
   attr.foo = "bar";
 }

tests/functional/eval.sh

@@ -15,13 +15,17 @@ nix eval --expr 'assert 1 + 2 == 3; true'
 nix eval -E 'assert 1 + 2 == 3; true'
 
 [[ $(nix eval int -f "./eval.nix") == 123 ]]
-[[ $(nix eval str -f "./eval.nix") == '"foo"' ]]
-[[ $(nix eval str --raw -f "./eval.nix") == 'foo' ]]
+[[ $(nix eval str -f "./eval.nix") == '"foo\nbar"' ]]
+[[ $(nix eval str --raw -f "./eval.nix") == $'foo\nbar' ]]
 [[ "$(nix eval attr -f "./eval.nix")" == '{ foo = "bar"; }' ]]
 [[ $(nix eval attr --json -f "./eval.nix") == '{"foo":"bar"}' ]]
 [[ $(nix eval int -f - < "./eval.nix") == 123 ]]
 [[ "$(nix eval --expr '{"assert"=1;bar=2;}')" == '{ "assert" = 1; bar = 2; }' ]]
 
+# Non-coercible values throws errors under `--raw`
+topLevelInteger="$(expectStderr 1 nix eval int --raw -f "./eval.nix")"
+[[ "$topLevelInteger" =~ "error: cannot coerce an integer to a string: 123" ]]
+
 # Top-level eval errors should be printed to stderr with a traceback.
 topLevelThrow="$(expectStderr 1 nix eval --expr 'throw "a sample throw message"')"
 [[ "$topLevelThrow" =~ "a sample throw message" ]]
@@ -36,12 +40,17 @@ outputOfNestedThrow="$(nix eval --expr '{ throws = throw "a sample throw message
 
 nix-instantiate --eval -E 'assert 1 + 2 == 3; true'
 [[ $(nix-instantiate -A int --eval "./eval.nix") == 123 ]]
-[[ $(nix-instantiate -A str --eval "./eval.nix") == '"foo"' ]]
+[[ $(nix-instantiate -A str --eval "./eval.nix") == '"foo\nbar"' ]]
+[[ $(nix-instantiate -A str --raw --eval "./eval.nix") == $'foo\nbar' ]]
 [[ "$(nix-instantiate -A attr --eval "./eval.nix")" == '{ foo = "bar"; }' ]]
 [[ $(nix-instantiate -A attr --eval --json "./eval.nix") == '{"foo":"bar"}' ]]
 [[ $(nix-instantiate -A int --eval - < "./eval.nix") == 123 ]]
 [[ "$(nix-instantiate --eval -E '{"assert"=1;bar=2;}')" == '{ "assert" = 1; bar = 2; }' ]]
 
+# Non-coercible values throws errors under `--raw`
+topLevelInteger="$(expectStderr 1 nix-instantiate -A int --raw "./eval.nix")"
+[[ "$topLevelInteger" =~ "error: expression was expected to be a derivation or collection of derivations, but instead was an integer" ]]
+
 # Check that symlink cycles don't cause a hang.
 ln -sfn cycle.nix $TEST_ROOT/cycle.nix
 (! nix eval --file $TEST_ROOT/cycle.nix)