rustc: Add `_imp_` symbols later in compilation

On MSVC targets rustc will add symbols prefixed with `_imp_` to LLVM modules to
"emulate" dllexported statics as that workaround is still in place after #27438
hasn't been solved otherwise. These statics, however, were getting gc'd by
ThinLTO accidentally which later would cause linking failures.

This commit updates the location we add such symbols to happen just before
codegen to ensure that (a) they're not eliminated by the optimizer and (b) the
optimizer doesn't even worry about them.

Closes #45347

1 files changed, 21 insertions, 0 deletions

diff --git a/src/test/run-pass/thinlto/auxiliary/msvc-imp-present.rs b/src/test/run-pass/thinlto/auxiliary/msvc-imp-present.rs
new file mode 100644
index 00000000000..eff7802a245
--- /dev/null
+++ b/src/test/run-pass/thinlto/auxiliary/msvc-imp-present.rs
@@ -0,0 +1,21 @@
+// Copyright 2017 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.
+
+// no-prefer-dynamic
+// compile-flags: -Z thinlto -C codegen-units=8 -C prefer-dynamic
+
+#![crate_type = "rlib"]
+#![crate_type = "dylib"]
+
+pub static A: u32 = 43;
+
+pub mod a {
+    pub static A: u32 = 43;
+}