Rollup merge of rust-lang#140681 - jieyouxu:test_helpers, r=petrochenkov

Don't ignore compiler stderr in `lib-defaults.rs`

Closes rust-lang#54222.

- Don't ignore compiler stderr.
- Document test intent.
- Move under `tests/ui/link-native-libs/` instead.

This was previously discussed on https://web.archive.org/web/20181028094402/https://botbot.me/mozilla/rust-tools/2017-02-21/?page=1.

try-job: armhf-gnu
try-job: test-various
try-job: x86_64-msvc-1
try-job: i686-msvc-1
try-job: x86_64-mingw-1
try-job: aarch64-apple
try-job: x86_64-apple-1

Author: GuillaumeGomez

tests/ui/abi/lib-defaults.rs

@@ -0,0 +1,17 @@
+//! By default, `-l NAME` without an explicit kind will default to dylib. However, if there's also
+//! an `#[link(name = NAME, kind = KIND)]` attribute with an explicit `KIND`, it should override the
+//! CLI flag. In particular, this should not result in any duplicate flag warnings from the linker.
+
+//@ run-pass
+//@ compile-flags: -lrust_test_helpers
+
+#[link(name = "rust_test_helpers", kind = "static")]
+extern "C" {
+    pub fn rust_dbg_extern_identity_u32(x: u32) -> u32;
+}
+
+fn main() {
+    unsafe {
+        rust_dbg_extern_identity_u32(42);
+    }
+}