Fix a typecheck regression with constant borrowed pointers in patterns

Change the eqtype relationship to be a suptype relationship instead.

Fixes #18350.
Fixes #18352.

Author: Jakub Bukaj

src/librustc/middle/typeck/check/_match.rs

@@ -74,7 +74,7 @@ pub fn check_pat(pcx: &pat_ctxt, pat: &ast::Pat, expected: ty::t) {
             let const_did = tcx.def_map.borrow().get_copy(&pat.id).def_id();
             let const_pty = ty::lookup_item_type(tcx, const_did);
             fcx.write_ty(pat.id, const_pty.ty);
-            demand::eqtype(fcx, pat.span, expected, const_pty.ty);
+            demand::suptype(fcx, pat.span, expected, const_pty.ty);
         }
         ast::PatIdent(bm, ref path, ref sub) if pat_is_binding(&tcx.def_map, pat) => {
             let typ = fcx.local_ty(pat.span, pat.id);

src/test/run-pass/issue-18352.rs

@@ -0,0 +1,22 @@
+// Copyright 2014 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.
+
+const X: &'static str = "12345";
+
+fn test(s: String) -> bool {
+    match s.as_slice() {
+        X => true,
+        _ => false
+    }
+}
+
+fn main() {
+    assert!(test("12345".to_string()));
+}