[gcc r14-8576] gccrs: Allow const and async specifiers in functions

[Arthur Cohen <cohenarthur@gcc.gnu.org>]

https://gcc.gnu.org/g:68990dbeadc5f4b90c0aa14387cb80d0f411aa43

commit r14-8576-g68990dbeadc5f4b90c0aa14387cb80d0f411aa43
Author: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>
Date:   Wed Nov 22 10:35:44 2023 +0100

    gccrs: Allow const and async specifiers in functions
    
    We need to account for const specifiers in async parsing as const
    can be used in the syntax before the async keyword.
    
    gcc/rust/ChangeLog:
    
            * parse/rust-parse-impl.h (Parser::parse_vis_item): Allow parsing async
            items in const.
            (Parser::parse_async_item): Account for const offset during async
            lookahead.
    
    Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

Diff:
---
 gcc/rust/parse/rust-parse-impl.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/gcc/rust/parse/rust-parse-impl.h b/gcc/rust/parse/rust-parse-impl.h
index a1ad4f11993a..acceec302a2d 100644
--- a/gcc/rust/parse/rust-parse-impl.h
+++ b/gcc/rust/parse/rust-parse-impl.h
@@ -1383,6 +1383,8 @@ Parser<ManagedTokenSource>::parse_vis_item (AST::AttrVec outer_attrs)
 	  return parse_const_item (std::move (vis), std::move (outer_attrs));
 	case UNSAFE:
 	case EXTERN_KW:
+	case ASYNC:
+	  return parse_async_item (std::move (vis), std::move (outer_attrs));
 	case FN_KW:
 	  return parse_function (std::move (vis), std::move (outer_attrs));
 	default:
@@ -1445,7 +1447,9 @@ std::unique_ptr<AST::Function>
 Parser<ManagedTokenSource>::parse_async_item (AST::Visibility vis,
 					      AST::AttrVec outer_attrs)
 {
-  const_TokenPtr t = lexer.peek_token ();
+  auto offset = (lexer.peek_token ()->get_id () == CONST) ? 1 : 0;
+  const_TokenPtr t = lexer.peek_token (offset);
+
   if (Session::get_instance ().options.get_edition ()
       == CompileOptions::Edition::E2015)
     {
@@ -1456,7 +1460,7 @@ Parser<ManagedTokenSource>::parse_async_item (AST::Visibility vis,
 		     "to use %<async fn%>, switch to Rust 2018 or later"));
     }
 
-  t = lexer.peek_token (1);
+  t = lexer.peek_token (offset + 1);
 
   switch (t->get_id ())
     {