Added support to Parse ASYNC function

Fixes issue Rust-GCC#2650
The parser now parses ASYNC functions. Added ASYNC case to parse_item
Added a new function parse_async_item which is called in
parse_vis_item to handle the ASYNC case. Parse_async_item
also checks the current Rust edition and generates an error if the
edition is 2015

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_item): Likewise.
	(Parser::parse_vis_item): Likewise.
	(Parser::parse_async_item): Likewise.
	* parse/rust-parse.h: Made declaration for parse_async_item.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2650-1.rs: New test.(edition=2018)
	* rust/compile/issue-2650-2.rs: New test.(edition=2015)

Signed-off-by: M V V S Manoj Kumar <mvvsmanojkumar@gmail.com>

gcc/rust/parse/rust-parse-impl.h

@@ -30,6 +30,7 @@
 #include "rust-dir-owner.h"
 #include "rust-attribute-values.h"
 #include "rust-keyword-values.h"
+#include "rust-session-manager.h"
 
 #include "optional.h"
 
@@ -1112,6 +1113,8 @@ Parser<ManagedTokenSource>::parse_item (bool called_from_statement)
 	  add_error (std::move (error));
 	}
       return nullptr;
+
+    case ASYNC:
     case PUB:
     case MOD:
     case EXTERN_TOK:
@@ -1388,6 +1391,10 @@ Parser<ManagedTokenSource>::parse_vis_item (AST::AttrVec outer_attrs)
 	  lexer.skip_token (1); // TODO: is this right thing to do?
 	  return nullptr;
 	}
+    // for async functions
+    case ASYNC:
+      return parse_async_item (std::move (vis), std::move (outer_attrs));
+
     case STATIC_TOK:
       return parse_static_item (std::move (vis), std::move (outer_attrs));
     case AUTO:
@@ -1428,6 +1435,39 @@ Parser<ManagedTokenSource>::parse_vis_item (AST::AttrVec outer_attrs)
   return nullptr;
 }
 
+template <typename ManagedTokenSource>
+std::unique_ptr<AST::Function>
+Parser<ManagedTokenSource>::parse_async_item (AST::Visibility vis,
+					      AST::AttrVec outer_attrs)
+{
+  const_TokenPtr t = lexer.peek_token ();
+  if (Session::get_instance ().options.get_edition ()
+      == CompileOptions::Edition::E2015)
+    {
+      add_error (Error (t->get_locus (), ErrorCode::E0670,
+			"%<async fn%> is not permitted in Rust 2015"));
+      add_error (
+	Error::Hint (t->get_locus (),
+		     "to use %<async fn%>, switch to Rust 2018 or later"));
+    }
+
+  t = lexer.peek_token (1);
+
+  switch (t->get_id ())
+    {
+    case UNSAFE:
+    case FN_TOK:
+      return parse_function (std::move (vis), std::move (outer_attrs));
+
+    default:
+      add_error (
+	Error (t->get_locus (), "expected item, found keyword %<async%>"));
+
+      lexer.skip_token (1);
+      return nullptr;
+    }
+}
+
 // Parses a macro rules definition syntax extension whatever thing.
 template <typename ManagedTokenSource>
 std::unique_ptr<AST::MacroRulesDefinition>

gcc/rust/parse/rust-parse.h

@@ -357,6 +357,8 @@ template <typename ManagedTokenSource> class Parser
   std::unique_ptr<AST::ExternBlock>
   parse_extern_block (AST::Visibility vis, AST::AttrVec outer_attrs);
   std::unique_ptr<AST::Function> parse_method ();
+  std::unique_ptr<AST::Function> parse_async_item (AST::Visibility vis,
+						   AST::AttrVec outer_attrs);
 
   // Expression-related (Pratt parsed)
   std::unique_ptr<AST::Expr>

gcc/testsuite/rust/compile/issue-2650-1.rs

@@ -0,0 +1,5 @@
+// { dg-additional-options "-frust-edition=2018" }
+
+pub async fn a() -> u32 {
+    1
+}

gcc/testsuite/rust/compile/issue-2650-2.rs

@@ -0,0 +1,5 @@
+// { dg-additional-options "-frust-edition=2015" }
+
+pub async fn a() -> u32 { // { dg-error "'async fn' is not permitted in Rust 2015" }
+    1
+}