[gcc r14-7928] gccrs: fix lexer exponent output on tuple indices

[Arthur Cohen]

https://gcc.gnu.org/g:a7eacd8879e422485a0044be37d03a05d3ed262e

commit r14-7928-ga7eacd8879e422485a0044be37d03a05d3ed262e
Author: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>
Date:   Thu Aug 3 15:28:40 2023 +0200

    gccrs: fix lexer exponent output on tuple indices
    
    The lexer did output a literal for values such as 42.e wich are invalid
    in rust.
    
    gcc/rust/ChangeLog:
    
            * lex/rust-lex.cc (Lexer::parse_decimal_int_or_float): Only
            accept digits after a dot instead of accepting any float
            member.
    
    Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

Diff:
---
 gcc/rust/lex/rust-lex.cc | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/gcc/rust/lex/rust-lex.cc b/gcc/rust/lex/rust-lex.cc
index 53895c14199..8142aa0cf78 100644
--- a/gcc/rust/lex/rust-lex.cc
+++ b/gcc/rust/lex/rust-lex.cc
@@ -2295,7 +2295,17 @@ Lexer::parse_decimal_int_or_float (location_t loc)
   length += std::get<1> (initial_decimal);
 
   // detect float literal
-  if (current_char == '.' && is_float_digit (peek_input (1).value))
+  //
+  // Note:
+  //
+  // We should not use is_float_digit () for this verification but instead
+  // directly ISDIGIT because rust does not support non digit values right after
+  // a dot.
+  // The following value is not legal in rust:
+  // let a = 3.e1;
+  // A `0` should be put between the dot and the exponent to be valid
+  // (eg. 3.0e1).
+  if (current_char == '.' && ISDIGIT (peek_input (1).value))
     {
       // float with a '.', parse another decimal into it