Expect identifier subpatterns to be non-alt

Fixes #4071.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_identifier_pattern):
	Use parse_pattern_no_alt to parse identifier pattern
	subpatterns.
	(Parser::parse_ident_leading_pattern): Likewise.

gcc/testsuite/ChangeLog:

	* rust/compile/parse_closure_bind.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>

Author: powerboat9

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

@@ -11113,7 +11113,7 @@ Parser<ManagedTokenSource>::parse_identifier_pattern ()
       lexer.skip_token ();
 
       // parse required pattern to bind
-      bind_pattern = parse_pattern ();
+      bind_pattern = parse_pattern_no_alt ();
       if (bind_pattern == nullptr)
 	{
 	  Error error (lexer.peek_token ()->get_locus (),
@@ -11240,7 +11240,8 @@ Parser<ManagedTokenSource>::parse_ident_leading_pattern ()
 	    // identifier with pattern bind
 	    lexer.skip_token ();
 
-	    std::unique_ptr<AST::Pattern> bind_pattern = parse_pattern ();
+	    std::unique_ptr<AST::Pattern> bind_pattern
+	      = parse_pattern_no_alt ();
 	    if (bind_pattern == nullptr)
 	      {
 		Error error (

gcc/testsuite/rust/compile/parse_closure_bind.rs

@@ -0,0 +1,19 @@
+// { dg-additional-options "-frust-compile-until=typecheck" }
+// TODO: this should typecheck
+
+#[lang = "sized"]
+trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    /// The returned type after the call operator is used.
+    #[lang = "fn_once_output"]
+    type Output;
+
+    /// Performs the call operation.
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+pub fn foo() {
+    (|_a @ _b| {}) (1)
+}