Add failing regression test for #[link="dl"]

(cherry picked from commit 5c95f8b)

Author: JonathanBrouwer

tests/ui/attributes/link-dl.default_fcw.stderr

@@ -0,0 +1,26 @@
+error[E0539]: malformed `link` attribute input
+  --> $DIR/link-dl.rs:14:1
+   |
+LL | #[link="dl"]
+   | ^^^^^^^^^^^^ expected this to be a list
+   |
+   = note: for more information, visit <https://doc.rust-lang.org/reference/items/external-blocks.html#the-link-attribute>
+help: try changing it to one of the following valid forms of the attribute
+   |
+LL - #[link="dl"]
+LL + #[link(name = "...")]
+   |
+LL - #[link="dl"]
+LL + #[link(name = "...", import_name_type = "decorated|noprefix|undecorated")]
+   |
+LL - #[link="dl"]
+LL + #[link(name = "...", kind = "dylib|static|...")]
+   |
+LL - #[link="dl"]
+LL + #[link(name = "...", kind = "dylib|static|...", wasm_import_module = "...", import_name_type = "decorated|noprefix|undecorated")]
+   |
+   = and 1 other candidate
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0539`.

tests/ui/attributes/link-dl.rs

@@ -0,0 +1,19 @@
+// Regression test for an issue discovered in https://github.com/rust-lang/rust/pull/143193/files and rediscovered in https://github.com/rust-lang/rust/issues/147254#event-20049906781
+// Malformed #[link] attribute was supposed to be deny-by-default report-in-deps FCW,
+// but accidentally was landed as a hard error.
+//
+// revision `default_fcw` tests that with `ill_formed_attribute_input` (the default) denied,
+// the attribute produces an FCW
+// revision `allowed` tests that with `ill_formed_attribute_input` allowed the test passes
+
+//@ revisions: default_fcw allowed
+//@[allowed] check-pass
+
+#[cfg_attr(allowed, allow(ill_formed_attribute_input))]
+
+#[link="dl"]
+//[default_fcw]~^ ERROR valid forms for the attribute are
+//[default_fcw]~| WARN previously accepted
+extern "C" { }
+
+fn main() {}