Auto merge of #60669 - c410-f3r:attrs-fn, r=petrochenkov

Allow attributes in formal function parameters

Implements https://github.com/rust-lang/rust/issues/60406.

This is my first contribution to the compiler and since this is a large and complex project, I am not fully aware of the consequences of the changes I have made.

**TODO**

- [x] Forbid some built-in attributes.
- [x] Expand cfg/cfg_attr

1 files changed, 6 insertions, 0 deletions

diff --git a/src/librustc_data_structures/thin_vec.rs b/src/librustc_data_structures/thin_vec.rs
index 52f23f4893e..6692903cd4f 100644
--- a/src/librustc_data_structures/thin_vec.rs
+++ b/src/librustc_data_structures/thin_vec.rs
@@ -66,3 +66,9 @@ impl<T: HashStable<CTX>, CTX> HashStable<CTX> for ThinVec<T> {
         (**self).hash_stable(hcx, hasher)
     }
 }
+
+impl<T> Default for ThinVec<T> {
+    fn default() -> Self {
+        Self(None)
+    }
+}