Add a run-make test that LLD is not being used by default on the x64 beta/stable channel

tests/run-make/rust-lld-by-default-beta-stable/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/rust-lld-by-default-beta-stable/rmake.rs

@@ -0,0 +1,29 @@
+// Ensure that rust-lld is *not* used as the default linker on `x86_64-unknown-linux-gnu` on stable
+// or beta.
+
+//@ ignore-nightly
+//@ only-x86_64-unknown-linux-gnu
+
+use run_make_support::regex::Regex;
+use run_make_support::rustc;
+use std::process::Output;
+
+fn main() {
+    // A regular compilation should not use rust-lld by default. We'll check that by asking the
+    // linker to display its version number with a link-arg.
+    let output = rustc()
+        .env("RUSTC_LOG", "rustc_codegen_ssa::back::link=info")
+        .link_arg("-Wl,-v")
+        .input("main.rs")
+        .run();
+    assert!(
+        !find_lld_version_in_logs(output.stderr_utf8()),
+        "the LLD version string should not be present in the output logs:\n{}",
+        output.stderr_utf8()
+    );
+}
+
+fn find_lld_version_in_logs(stderr: String) -> bool {
+    let lld_version_re = Regex::new(r"^LLD [0-9]+\.[0-9]+\.[0-9]+").unwrap();
+    stderr.lines().any(|line| lld_version_re.is_match(line.trim()))
+}

tests/run-make/rust-lld-by-default/rmake.rs → tests/run-make/rust-lld-by-default-nightly/rmake.rs

@@ -1,5 +1,5 @@
-// Ensure that rust-lld is used as the default linker on `x86_64-unknown-linux-gnu`, and that it can
-// also be turned off with a CLI flag.
+// Ensure that rust-lld is used as the default linker on `x86_64-unknown-linux-gnu` on the nightly
+// channel, and that it can also be turned off with a CLI flag.
 
 //@ needs-rust-lld
 //@ ignore-beta