Auto merge of #37095 - petrochenkov:metactor, r=alexcrichton

Temporary fix for metadata decoding for struct constructors

Same as https://github.com/rust-lang/rust/pull/37078, but for nightly.
Ideally, metadata lookup functions should "just work" for constructor ids, but this fixes the issue as well.

Fixes https://github.com/rust-lang/rust/issues/37026
r? @alexcrichton

src/test/compile-fail/issue-37026.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+// aux-build:empty-struct.rs
+
+extern crate empty_struct;
+
+fn main() {
+    let empty_struct::XEmpty2 = (); //~ ERROR mismatched types
+    let empty_struct::XEmpty6(..) = (); //~ ERROR mismatched types
+}