Add warn-short-path-literals setting

Add a new setting to warn about path literals that don't start with "." or "/". When enabled,
expressions like `foo/bar` will emit a warning suggesting to use `./foo/bar` instead.

A functional test is included.

The setting defaults to false for backward compatibility but could eventually default to true in
the future.

Closes: #13374

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libexpr/include/nix/expr/eval-settings.hh

@@ -327,6 +327,21 @@ struct EvalSettings : Config
 
           This option can be enabled by setting `NIX_ABORT_ON_WARN=1` in the environment.
         )"};
+
+    Setting<bool> warnShortPathLiterals{
+        this,
+        false,
+        "warn-short-path-literals",
+        R"(
+          If set to true, the Nix evaluator will warn when encountering relative path literals
+          that don't start with `./` or `../`.
+
+          For example, with this setting enabled, `foo/bar` would emit a warning
+          suggesting to use `./foo/bar` instead.
+
+          This is useful for improving code readability and making path literals
+          more explicit.
+    )"};
 };
 
 /**

src/libexpr/parser.y

@@ -365,6 +365,15 @@ string_parts_interpolated
 path_start
   : PATH {
     std::string_view literal({$1.p, $1.l});
+
+    /* check for short path literals */
+    if (state->settings.warnShortPathLiterals && literal.front() != '/' && literal.front() != '.') {
+        logWarning({
+            .msg = HintFmt("relative path literal '%s' should be prefixed with '.' for clarity: './%s'. (" ANSI_BOLD "warn-short-path-literals" ANSI_NORMAL " = true)", literal, literal),
+            .pos = state->positions[CUR_POS]
+        });
+    }
+
     Path path(absPath(literal, state->basePath.path.abs()));
     /* add back in the trailing '/' to the first segment */
     if (literal.size() > 1 && literal.back() == '/')

tests/functional/meson.build

@@ -112,6 +112,7 @@ suites = [
       'impure-eval.sh',
       'pure-eval.sh',
       'eval.sh',
+      'short-path-literals.sh',
       'repl.sh',
       'binary-cache-build-remote.sh',
       'search.sh',

tests/functional/short-path-literals.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+clearStoreIfPossible
+
+# Test 1: Without the setting (default), no warnings should be produced
+nix eval --expr 'test/subdir' 2>"$TEST_ROOT"/stderr
+grepQuietInverse < "$TEST_ROOT/stderr" -E "relative path|path literal" || fail "Should not produce warnings by default"
+
+# Test 2: With the setting enabled, warnings should be produced for short path literals
+nix eval --warn-short-path-literals --expr 'test/subdir' 2>"$TEST_ROOT"/stderr
+grepQuiet "relative path literal 'test/subdir' should be prefixed with '.' for clarity: './test/subdir'" "$TEST_ROOT/stderr"
+
+# Test 3: Different short path literals should all produce warnings
+nix eval --warn-short-path-literals --expr 'foo/bar' 2>"$TEST_ROOT"/stderr
+grepQuiet "relative path literal 'foo/bar' should be prefixed with '.' for clarity: './foo/bar'" "$TEST_ROOT/stderr"
+
+nix eval --warn-short-path-literals --expr 'a/b/c/d' 2>"$TEST_ROOT"/stderr
+grepQuiet "relative path literal 'a/b/c/d' should be prefixed with '.' for clarity: './a/b/c/d'" "$TEST_ROOT/stderr"
+
+# Test 4: Paths starting with ./ should NOT produce warnings
+nix eval --warn-short-path-literals --expr './test/subdir' 2>"$TEST_ROOT"/stderr
+grepQuietInverse "relative path literal" "$TEST_ROOT/stderr"
+
+# Test 5: Paths starting with ../ should NOT produce warnings
+nix eval --warn-short-path-literals --expr '../test/subdir' 2>"$TEST_ROOT"/stderr
+grepQuietInverse "relative path literal" "$TEST_ROOT/stderr"
+
+# Test 6: Absolute paths should NOT produce warnings
+nix eval --warn-short-path-literals --expr '/absolute/path' 2>"$TEST_ROOT"/stderr
+grepQuietInverse "relative path literal" "$TEST_ROOT/stderr"
+
+# Test 7: Test that the warning is at the correct position
+nix eval --warn-short-path-literals --expr 'foo/bar' 2>"$TEST_ROOT"/stderr
+grepQuiet "at «string»:1:1:" "$TEST_ROOT/stderr"
+
+# Test 8: Test that evaluation still works correctly despite the warning
+result=$(nix eval --warn-short-path-literals --expr 'test/subdir' 2>/dev/null)
+expected="$PWD/test/subdir"
+[[ "$result" == "$expected" ]] || fail "Evaluation result should be correct despite warning"
+
+# Test 9: Test with nix-instantiate as well
+nix-instantiate --warn-short-path-literals --eval -E 'foo/bar' 2>"$TEST_ROOT"/stderr
+grepQuiet "relative path literal 'foo/bar' should be prefixed" "$TEST_ROOT/stderr"
+
+# Test 10: Test that the setting can be set via configuration
+NIX_CONFIG='warn-short-path-literals = true' nix eval --expr 'test/file' 2>"$TEST_ROOT"/stderr
+grepQuiet "relative path literal 'test/file' should be prefixed" "$TEST_ROOT/stderr"
+
+# Test 11: Test that command line flag overrides config
+NIX_CONFIG='warn-short-path-literals = true' nix eval --no-warn-short-path-literals --expr 'test/file' 2>"$TEST_ROOT"/stderr
+grepQuietInverse "relative path literal" "$TEST_ROOT/stderr"
+
+echo "short-path-literals test passed!"