Use tl::expected for parse_block_expr results

gcc/rust/ChangeLog:

	* parse/rust-parse-error.h (struct BlockExpr): Add BlockExpr error type
	* parse/rust-parse-impl-expr.hxx: Update return types.
	* parse/rust-parse-impl.hxx: Likewise.
	* parse/rust-parse.h: Update function prototypes.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

Author: P-E-P

gcc/rust/parse/rust-parse-error.h

@@ -385,6 +385,23 @@ struct Self
   Self (Kind kind) : kind (kind) {}
 };
 
+struct BlockExpr
+{
+  static tl::expected<std::unique_ptr<AST::BlockExpr>, BlockExpr>
+  make_malformed ()
+  {
+    return tl::unexpected<BlockExpr> (BlockExpr (Kind::MALFORMED));
+  }
+
+  enum class Kind
+  {
+    MALFORMED,
+  } kind;
+
+private:
+  BlockExpr (Kind kind) : kind (kind) {}
+};
+
 } // namespace Error
 } // namespace Parse
 } // namespace Rust