libexpr/nix2: Make floats require leading digit

Forbid literals like `.3`

Change-Id: Ia05b3729bf9f23d64a2cdb797458aad630d6a639

src/libexpr/parser/grammar2.hh

@@ -163,18 +163,15 @@ struct identifier : _not_at_any_keyword<
 
 // floats may extend ints, thus these rules are very similar.
 struct integer : seq<
-    sor<
-        seq<range<'1', '9'>, star<digit>, not_at<one<'.'>>>,
-        seq<one<'0'>, not_at<one<'.'>, digit>, star<digit>>
-    >,
+    plus<digit>,
+    not_at<one<'.'>>,
     not_at<_extend_as_path>
 > {};
 
 struct floating : seq<
-    sor<
-        seq<range<'1', '9'>, star<digit>, one<'.'>, star<digit>>,
-        seq<opt<one<'0'>>, one<'.'>, plus<digit>>
-    >,
+    plus<digit>,
+    one<'.'>,
+    star<digit>,
     opt<one<'E', 'e'>, opt<one<'+', '-'>>, plus<digit>>,
     not_at<_extend_as_path>
 > {};

src/libutil/experimental-features.cc

@@ -183,6 +183,10 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             Switch the parser and evaluator to a newer, experimental version of the language.
             See https://wiki.lix.systems/books/lix-contributors/page/nix-lang-v2
             This is the playground for new language improvements.
+
+            Notable changes done compared to the stable Nix language:
+
+            - Floating point literals must start with a digit, literals like `.3` are forbidden
         )",
     },
     {

tests/unit/libexpr/trivial.cc

@@ -62,6 +62,11 @@ namespace nix {
     TEST_F(TrivialExpressionTest, pointfloat) {
         auto v = eval(".234");
         ASSERT_THAT(v, IsFloatEq(0.234));
+
+        FeatureSettings mockFeatureSettings;
+        mockFeatureSettings.set("experimental-features", "nix-lang2");
+
+        ASSERT_THROW(eval(".234", true, mockFeatureSettings), Error);
     }
 
     TEST_F(TrivialExpressionTest, updateAttrs) {