libexpr: Warn on floating point literals without leading or trailing zero

Co-authored-by: Commentator2.0 <lix@crystal-cavern.systems>
Change-Id: I0b58531ad091b22dc59d5717f5d1c8c814b4d2ea

doc/manual/rl-next/deprecated-features.md

@@ -1,7 +1,7 @@
 ---
 synopsis: 'more deprecated features'
 issues: []
-cls: [2092, 2310]
+cls: [2092, 2310, 2311]
 category: Breaking Changes
 credits: [piegames, commentator2.0]
 ---
@@ -10,3 +10,4 @@ You can opt in into the old behavior with `--extra-deprecated-features` or any e
 
 - `broken-string-indentation` indented strings (those starting with  `''`) might produce unintended results due to how the whitespace stripping is done. Those cases will now warn the user.
 - `broken-string-escape` "escaped" characters without a properly defined escape sequence evaluate to "themselves". This is in most cases unintended behaviour, both for writing regexes, and using legacy or uncommon escape sequences like `\f`. The user will now be warned, if those are present.
+- `floating-without-zero` so far, one was able to declare a float using something like `.123`. This can cause confusion about accessing attributes. Floating point numbers must now always include the leading zero, i.e. `0.123`

lix/libexpr/parser/grammar.hh

@@ -163,10 +163,14 @@ struct integer : seq<
     not_at<_extend_as_path>
 > {};
 
-struct floating : seq<
+struct _floating {
+    struct no_leading_zero : seq<one<'.'>, plus<digit>> {};
+};
+struct floating : _floating, seq<
     sor<
         seq<range<'1', '9'>, star<digit>, one<'.'>, star<digit>>,
-        seq<opt<one<'0'>>, one<'.'>, plus<digit>>
+        seq<one<'0'>, one<'.'>, plus<digit>>,
+        _floating::no_leading_zero
     >,
     opt<one<'E', 'e'>, opt<one<'+', '-'>>, plus<digit>>,
     not_at<_extend_as_path>

lix/libexpr/parser/parser-impl1.inc.cc

@@ -517,6 +517,21 @@ template<> struct BuildAST<grammar::v1::expr::int_> {
     }
 };
 
+template<> struct BuildAST<grammar::v1::t::floating::no_leading_zero> {
+    static void apply(const auto & in, ExprState & s, State & ps) {
+        if (!ps.featureSettings.isEnabled(Dep::FloatingWithoutZero)) {
+            logWarning(
+                {.msg = HintFmt(
+                     "Found floating point literal without leading zero. To fix this "
+                     "warning, add a zero before the dot. Use %s to silence this warning",
+                     "--extra-deprecated-feature floating-without-zero"
+                 ),
+                 .pos = ps.positions[ps.at(in)]}
+            );
+        }
+    }
+};
+
 template<> struct BuildAST<grammar::v1::expr::float_> {
     static void apply(const auto & in, ExprState & s, State & ps) {
         // copy the input into a temporary string so we can call stod.

lix/libutil/deprecated-features/floating-without-zero.md

@@ -0,0 +1,14 @@
+---
+name: floating-without-zero
+internalName: FloatingWithoutZero
+timeline:
+  - date: 2026-01-30
+    release: 2.95.0
+    cls: [2311]
+    message: Introduced as warning.
+---
+The short-hand notation for floating point numbers `.123` instead of `0.123` is deprecated.
+Floating point literals are seldomly used, and saving one character adds little benefit.
+This deprecation will free the syntax for possible future new language feature (See NixOS RFC 181).
+
+To fix this, add a zero before the dot.

lix/libutil/meson.build

@@ -178,6 +178,7 @@ deprecated_feature_definitions = files(
   'deprecated-features/broken-string-escape.md',
   'deprecated-features/broken-string-indentation.md',
   'deprecated-features/cr-line-endings.md',
+  'deprecated-features/floating-without-zero.md',
   'deprecated-features/nix-path-shadow.md',
   'deprecated-features/nul-bytes.md',
   'deprecated-features/rec-set-overrides.md',

tests/functional2/lang/float/allowed-depr-syntax.out.exp

@@ -0,0 +1 @@
+{ a = 0.1; }

tests/functional2/lang/float/in-depr-syntax.nix

@@ -0,0 +1,3 @@
+rec {
+    a = .1;
+}

tests/functional2/lang/float/test.toml

@@ -0,0 +1,14 @@
+[[test]]
+runner = "eval-okay"
+in = "in.nix"
+
+[[test]]
+name = "warn"
+runner = "eval-okay"
+in = "in-depr-syntax.nix"
+
+[[test]]
+name = "allowed"
+runner = "eval-okay"
+in = "in-depr-syntax.nix"
+flags = ["--extra-deprecated-features", "floating-without-zero"]

tests/functional2/lang/float/warn-depr-syntax.err.exp

@@ -0,0 +1,6 @@
+warning: Found floating point literal without leading zero. To fix this warning, add a zero before the dot. Use --extra-deprecated-feature floating-without-zero to silence this warning
+         at /pwd/in.nix:2:9:
+              1| rec {
+              2|     a = .1;
+               |         ^
+              3| }

tests/functional2/lang/float/warn-depr-syntax.out.exp

@@ -0,0 +1 @@
+{ a = 0.1; }