Add ast::ExprKind::Dummy

compiler/rustc_parse/src/parser/expr.rs

@@ -3949,7 +3949,8 @@ impl MutVisitor for CondChecker<'_> {
             | ExprKind::Become(_)
             | ExprKind::IncludedBytes(_)
             | ExprKind::FormatArgs(_)
-            | ExprKind::Err => {
+            | ExprKind::Err
+            | ExprKind::Dummy => {
                 // These would forbid any let expressions they contain already.
             }
         }

compiler/rustc_parse/src/parser/pat.rs

@@ -388,7 +388,7 @@ impl<'a> Parser<'a> {
         // Parse `?`, `.f`, `(arg0, arg1, ...)` or `[expr]` until they've all been eaten.
         if let Ok(expr) = snapshot
             .parse_expr_dot_or_call_with(
-                self.mk_expr_err(pat_span), // equivalent to transforming the parsed pattern into an `Expr`
+                self.mk_expr(pat_span, ExprKind::Dummy), // equivalent to transforming the parsed pattern into an `Expr`
                 pat_span,
                 AttrVec::new(),
             )