Auto merge of #31915 - nagisa:mir-unpretty-fix, r=arielb1

Fixes https://github.com/rust-lang/rust/issues/31913

src/test/compile-fail/mir-unpretty.rs

@@ -0,0 +1,15 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z unstable-options --unpretty=mir
+
+fn main() {
+    let x: () = 0; //~ ERROR: mismatched types
+}