Fix false positive for top-level AltPattern

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-pattern.cc
	(ASTLoweringPattern::visit):
	Reset is_let_top_level while visiting GroupedPattern.

gcc/testsuite/ChangeLog:

	* rust/compile/let_alt.rs: Check for false positive.

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

gcc/rust/hir/rust-ast-lower-pattern.cc

@@ -276,6 +276,7 @@ ASTLoweringPattern::visit (AST::RangePattern &pattern)
 void
 ASTLoweringPattern::visit (AST::GroupedPattern &pattern)
 {
+  is_let_top_level = false;
   pattern.get_pattern_in_parens ()->accept_vis (*this);
 }
 

gcc/testsuite/rust/compile/let_alt.rs

@@ -1,4 +1,5 @@
 fn main() {
     let _a | _a = 12;
     // { dg-error "top level or-patterns are not allowed for .let. bindings" "" { target *-*-* } .-1 }
+    let (_b | _b) = 12;
 }