Significant test improvements

Author: checkraisefold

tests/run-make/issue-44282-questionmark-mangle/main.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
+}