Create _imp__ symbols also when doing ThinLTO

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -2165,8 +2165,14 @@ fn msvc_imps_needed(tcx: TyCtxt<'_>) -> bool {
             && tcx.sess.opts.cg.prefer_dynamic)
     );
 
+    // We need to generate _imp__ symbol if we are generating an rlib or we include one
+    // indirectly from ThinLTO. In theory these are not needed as ThinLTO could resolve
+    // these, but it currently does not do so.
+    let can_have_static_objects =
+        tcx.sess.lto() == Lto::Thin || tcx.crate_types().iter().any(|ct| *ct == CrateType::Rlib);
+
     tcx.sess.target.is_like_windows &&
-        tcx.crate_types().iter().any(|ct| *ct == CrateType::Rlib) &&
+    can_have_static_objects   &&
     // ThinLTO can't handle this workaround in all cases, so we don't
     // emit the `__imp_` symbols. Instead we make them unnecessary by disallowing
     // dynamic linking when linker plugin LTO is enabled.

tests/run-make/msvc-lld-thinlto-imp-symbols/issue_81408.rs

@@ -0,0 +1,13 @@
+use std::sync::atomic::{AtomicPtr, Ordering};
+
+#[inline(always)]
+pub fn memrchr() {
+    fn detect() {}
+
+    static CROSS_CRATE_STATIC_ITEM: AtomicPtr<()> = AtomicPtr::new(detect as *mut ());
+
+    unsafe {
+        let fun = CROSS_CRATE_STATIC_ITEM.load(Ordering::SeqCst);
+        std::mem::transmute::<*mut (), fn()>(fun)()
+    }
+}

tests/run-make/msvc-lld-thinlto-imp-symbols/main.rs

@@ -0,0 +1,5 @@
+extern crate issue_81408;
+
+fn main() {
+    issue_81408::memrchr();
+}

tests/run-make/msvc-lld-thinlto-imp-symbols/rmake.rs

@@ -0,0 +1,33 @@
+// This is a non-regression test for issue #81408 involving an lld bug and ThinLTO, on windows.
+// MSVC's link.exe doesn't need any workarounds in rustc, but lld does, so we'll check that the
+// binary runs successfully instead of using a codegen test.
+
+//@ only-x86_64-pc-windows-msvc
+//@ needs-rust-lld
+//@ ignore-cross-compile: the built binary is executed
+
+use run_make_support::{run, rustc};
+
+fn test_with_linker(linker: &str) {
+    rustc().input("issue_81408.rs").crate_name("issue_81408").crate_type("lib").opt().run();
+    rustc()
+        .input("main.rs")
+        .crate_type("bin")
+        .arg("-Clto=thin")
+        .opt()
+        .arg(&format!("-Clinker={linker}"))
+        .extern_("issue_81408", "libissue_81408.rlib")
+        .run();
+
+    // To make possible failures clearer, print an intro that will only be shown if the test does
+    // fail when running the binary.
+    eprint!("Running binary linked with {linker}... ");
+    run("main");
+    eprintln!("ok");
+}
+
+fn main() {
+    // We want the reproducer to work when linked with both linkers.
+    test_with_linker("link");
+    test_with_linker("rust-lld");
+}