parser: Fix while let expr parsing

While let expr return unit but are valid construct in rust, they should
therefore be included in the parsing code. Also add a new test to check
parsing of while let expressions.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_while_let_loop_expr):
	Prevent hard error on token skip.
	(Parser::null_denotation): Fix parser for while let expressions.

gcc/testsuite/ChangeLog:

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

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

2 files changed, 24 insertions, 4 deletions

diff --git a/gcc/rust/parse/rust-parse-impl.h b/gcc/rust/parse/rust-parse-impl.h
index b17744f..4670411 100644
--- a/gcc/rust/parse/rust-parse-impl.h
+++ b/gcc/rust/parse/rust-parse-impl.h
@@ -8372,7 +8372,7 @@ Parser<ManagedTokenSource>::parse_while_let_loop_expr (AST::AttrVec outer_attrs,
     locus = lexer.peek_token ()->get_locus ();
   else
     locus = label.get_locus ();
-  skip_token (WHILE);
+  maybe_skip_token (WHILE);
 
   /* check for possible accidental recognition of a while loop as a while let
    * loop */
@@ -13114,9 +13114,16 @@ Parser<ManagedTokenSource>::null_denotation (const_TokenPtr tok,
       return parse_loop_expr (std::move (outer_attrs), AST::LoopLabel::error (),
 			      tok->get_locus ());
     case WHILE:
-      return parse_while_loop_expr (std::move (outer_attrs),
-				    AST::LoopLabel::error (),
-				    tok->get_locus ());
+      if (lexer.peek_token ()->get_id () == LET)
+	{
+	  return parse_while_let_loop_expr (std::move (outer_attrs));
+	}
+      else
+	{
+	  return parse_while_loop_expr (std::move (outer_attrs),
+					AST::LoopLabel::error (),
+					tok->get_locus ());
+	}
     case MATCH_TOK:
       // also an expression with block
       return parse_match_expr (std::move (outer_attrs), tok->get_locus ());
diff --git a/gcc/testsuite/rust/compile/while_let_expr.rs b/gcc/testsuite/rust/compile/while_let_expr.rs
new file mode 100644
index 0000000..113ad74
--- /dev/null
+++ b/gcc/testsuite/rust/compile/while_let_expr.rs
@@ -0,0 +1,13 @@
+// { dg-options "-fsyntax-only" }
+
+pub enum Option<T> {
+    None,
+    Some(T),
+}
+
+fn main() {
+    let mut x = Option::Some(3);
+    let a = while let Option::Some(1) = x {
+        x = Option::None;
+    };
+}