Fix misoptimizations when matching against strings/slices

When matching against strings/slices, we call the comparison function
for strings, which takes two string slices by value. The slices are
passed in memory, and currently we just pass in a pointer to the
original slice. That can cause misoptimizations because we emit a call
to llvm.lifetime.end for all by-value arguments at the end of a
function, which in this case marks the original slice as dead.

So we need to properly create copies of the slices to pass them to the
comparison function.

Fixes #22008

src/librustc_trans/trans/_match.rs

@@ -828,7 +828,19 @@ fn compare_values<'blk, 'tcx>(cx: Block<'blk, 'tcx>,
                            &format!("comparison of `{}`",
                                    cx.ty_to_string(rhs_t))[],
                            StrEqFnLangItem);
-        callee::trans_lang_call(cx, did, &[lhs, rhs], None, debug_loc)
+        let t = ty::mk_str_slice(cx.tcx(), cx.tcx().mk_region(ty::ReStatic), ast::MutImmutable);
+        // The comparison function gets the slices by value, so we have to make copies here. Even
+        // if the function doesn't write through the pointer, things like lifetime intrinsics
+        // require that we do this properly
+        let lhs_arg = alloc_ty(cx, t, "lhs");
+        let rhs_arg = alloc_ty(cx, t, "rhs");
+        memcpy_ty(cx, lhs_arg, lhs, t);
+        memcpy_ty(cx, rhs_arg, rhs, t);
+        let res = callee::trans_lang_call(cx, did, &[lhs_arg, rhs_arg], None, debug_loc);
+        call_lifetime_end(res.bcx, lhs_arg);
+        call_lifetime_end(res.bcx, rhs_arg);
+
+        res
     }
 
     let _icx = push_ctxt("compare_values");

src/test/run-pass/issue22008.rs

@@ -0,0 +1,18 @@
+// 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.
+
+pub fn main() {
+    let command = "a";
+
+    match command {
+        "foo" => println!("foo"),
+        _     => println!("{}", command),
+    }
+}