Rollup merge of #51283 - kennytm:fix-51279-preempt-the-warning-song-and-dance, r=nikomatsakis

Deny #[cfg] and #[cfg_attr] on generic parameters.

Fix #51279.

Attributes on generic parameters are not expanded, meaning `#[cfg]`, `#[cfg_attr]` and attribute proc macros are entirely ignored on them.

This PR makes using the first two attributes an error, because if they are correctly expanded will affect the AST and change code behavior.

I'm beta-nominating this, because generic parameter attributes are stabilizing in 1.27, and if we did not reserve their usage, we may never be able to repurpose the meaning of these attributes in the Rust 2015 edition.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 83e7dd84cbf..29030783ca6 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -1412,6 +1412,11 @@ impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
         }
     }
 
+    fn fold_generic_param(&mut self, param: ast::GenericParam) -> ast::GenericParam {
+        self.cfg.disallow_cfg_on_generic_param(&param);
+        noop_fold_generic_param(param, self)
+    }
+
     fn fold_attribute(&mut self, at: ast::Attribute) -> Option<ast::Attribute> {
         // turn `#[doc(include="filename")]` attributes into `#[doc(include(file="filename",
         // contents="file contents")]` attributes