Auto merge of #54581 - petrochenkov:cfgattr, r=alexcrichton

Accept trailing comma in `cfg_attr`

Fixes https://github.com/rust-lang/rust/issues/54463 (stable-to-beta regression)

src/test/ui/cfg-attr-trailing-comma.rs

@@ -0,0 +1,13 @@
+// compile-flags: --cfg TRUE
+
+#[cfg_attr(TRUE, inline,)] // OK
+fn f() {}
+
+#[cfg_attr(FALSE, inline,)] // OK
+fn g() {}
+
+#[cfg_attr(TRUE, inline,,)] //~ ERROR expected `)`, found `,`
+fn h() {}
+
+#[cfg_attr(FALSE, inline,,)] //~ ERROR expected `)`, found `,`
+fn i() {}

src/test/ui/cfg-attr-trailing-comma.stderr

@@ -0,0 +1,14 @@
+error: expected `)`, found `,`
+  --> $DIR/cfg-attr-trailing-comma.rs:9:25
+   |
+LL | #[cfg_attr(TRUE, inline,,)] //~ ERROR expected `)`, found `,`
+   |                         ^ expected `)`
+
+error: expected `)`, found `,`
+  --> $DIR/cfg-attr-trailing-comma.rs:12:26
+   |
+LL | #[cfg_attr(FALSE, inline,,)] //~ ERROR expected `)`, found `,`
+   |                          ^ expected `)`
+
+error: aborting due to 2 previous errors
+