rollup merge of #20310: fhahn/issue-15877-model-lexer-range

This patch for #15877 uses Antlr's input lookahead (`_input.LA(1) != '.'`) to solve the conflict between the LIT_FLOAT and the range syntax.

Note that in order to execute the grammar tests, #20245 should land first.

src/grammar/RustLexer.g4

@@ -112,7 +112,8 @@ LIT_INTEGER
   ;
 
 LIT_FLOAT
-  : [0-9][0-9_]* ('.' | ('.' [0-9][0-9_]*)? ([eE] [-+]? [0-9][0-9_]*)? SUFFIX?)
+  : [0-9][0-9_]* ( '.' {_input.LA(1) != '.'}?
+                 | ('.' [0-9][0-9_]*)? ([eE] [-+]? [0-9][0-9_]*)? SUFFIX?)
   ;
 
 LIT_STR