tests: migrate libs-through-symlink to rmake.rs

- Document test intent, backlink to rust-lang#13890 and fix PR rust-lang#13903.
- Fix the test logic: the `Makefile` version seems to not actually be
  exercising the "library search traverses symlink" logic, because the
  actual symlinked-to-library is present under the directory tree when
  `bar.rs` is compiled, because the `$(RUSTC)` invocation has an
  implicit `-L $(TMPDIR)`. The symlink itself was actually broken, i.e.
  it should've been `ln -nsf $(TMPDIR)/outdir/$(NAME) $(TMPDIR)` but it
  used `ln -nsf outdir/$(NAME) $(TMPDIR)`.

Co-authored-by: Oneirical <manchot@videotron.ca>

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -2,7 +2,6 @@ run-make/branch-protection-check-IBT/Makefile
 run-make/cat-and-grep-sanity-check/Makefile
 run-make/extern-fn-reachable/Makefile
 run-make/jobserver-error/Makefile
-run-make/libs-through-symlinks/Makefile
 run-make/split-debuginfo/Makefile
 run-make/symbol-mangling-hashed/Makefile
 run-make/translation/Makefile

tests/run-make/libs-through-symlinks/rmake.rs

@@ -0,0 +1,34 @@
+//! Regression test for [rustc doesn't handle relative symlinks to libraries
+//! #13890](https://github.com/rust-lang/rust/issues/13890).
+//!
+//! The rust compiler searches by default for libraries in its current directory, but used to have
+//! difficulty following symlinks leading to required libraries if the real ones were located not
+//! under the same directory tree. After this was fixed in #13903, this test checks that compilation
+//! succeeds through use of the symlink.
+
+//@ ignore-cross-compile
+
+use run_make_support::{bare_rustc, cwd, path, rfs, rust_lib_name};
+
+fn main() {
+    let actual_lib_dir = path("actual_lib_dir");
+    let symlink_lib_dir = path("symlink_lib_dir");
+    rfs::create_dir_all(&actual_lib_dir);
+    rfs::create_dir_all(&symlink_lib_dir);
+
+    // NOTE: `bare_rustc` is used because it does not introduce an implicit `-L .` library search
+    // flag.
+    bare_rustc().input("foo.rs").output(actual_lib_dir.join(rust_lib_name("foo"))).run();
+
+    rfs::symlink_file(
+        actual_lib_dir.join(rust_lib_name("foo")),
+        symlink_lib_dir.join(rust_lib_name("foo")),
+    );
+
+    // Make rustc's $CWD be in the directory containing the symlink-to-lib.
+    bare_rustc()
+        .current_dir(&symlink_lib_dir)
+        .library_search_path(".")
+        .input(cwd().join("bar.rs"))
+        .run();
+}