Rollup merge of #62047 - Centril:cfg-attr-empty-lint, r=estebank

Trigger `unused_attribute` lint on `#[cfg_attr($pred,)]`

Lint on `#[cfg_attr($pred,)]` as decided in https://github.com/rust-lang/rust/issues/54881#issuecomment-441442173.

Closes https://github.com/rust-lang/rust/issues/54881.

r? @estebank

2 files changed, 34 insertions, 0 deletions

diff --git a/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs
new file mode 100644
index 00000000000..4c96d6e7ca1
--- /dev/null
+++ b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs
@@ -0,0 +1,13 @@
+// Check that `#[cfg_attr($PREDICATE,)]` triggers the `unused_attribute` lint.
+
+// compile-flags: --cfg TRUE
+
+#![deny(unused)]
+
+#[cfg_attr(FALSE,)] //~ ERROR unused attribute
+fn _f() {}
+
+#[cfg_attr(TRUE,)] //~ ERROR unused attribute
+fn _g() {}
+
+fn main() {}
diff --git a/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr
new file mode 100644
index 00000000000..cd3563e66c7
--- /dev/null
+++ b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr
@@ -0,0 +1,21 @@
+error: unused attribute
+  --> $DIR/cfg-attr-empty-is-unused.rs:7:1
+   |
+LL | #[cfg_attr(FALSE,)]
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/cfg-attr-empty-is-unused.rs:5:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: #[deny(unused_attributes)] implied by #[deny(unused)]
+
+error: unused attribute
+  --> $DIR/cfg-attr-empty-is-unused.rs:10:1
+   |
+LL | #[cfg_attr(TRUE,)]
+   | ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+