Fix linking for symbols starting with ? on i686-pc-windows-msvc

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -711,6 +711,9 @@ pub(crate) fn linking_symbol_name_for_instance_in_crate<'tcx>(
     }
 
     let prefix = match &target.arch[..] {
+        "x86" | "x86_64" if target.is_like_msvc && undecorated.starts_with("?") => {
+            return undecorated;
+        }
         "x86" => Some('_'),
         "x86_64" => None,
         // Only functions are decorated for arm64ec.

tests/ui/symbol-names/symbol-with-question-mark-links.rs

@@ -0,0 +1,25 @@
+// This test ensures functions with an exported name beginning with a question mark
+// successfully compile and link.
+//
+// Regression test for <https://github.com/rust-lang/rust/issues/44282>
+
+//@ build-pass
+//@ only-windows
+//@ only-x86
+// Reason: This test regards a linker issue which only applies to Windows.
+// Specifically, it only occurs due to Windows x86 name decoration, combined with
+// a mismatch between LLVM's decoration logic and Rust's (for `lib.def` generation)
+
+#![crate_type = "cdylib"]
+
+#[no_mangle]
+pub extern "C" fn decorated(a: i32, b: i32) -> i32 {
+    1
+}
+
+// This isn't just `?undecorated` because MSVC's linker fails if the decorated
+// symbol is not valid.
+#[export_name = "?undecorated@@YAXXZ"]
+pub extern "C" fn undecorated(a: i32, b: i32) -> i32 {
+    2
+}