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() == '/')