Auto merge of #57998 - niklasf:align-enum, r=nagisa

Allow #[repr(align(x))] on enums (#57996)

Tracking issue: #57996

Implements an extension of [RFC 1358](https://github.com/rust-lang/rfcs/blob/master/text/1358-repr-align.md) behind a feature flag (`repr_align_enum`). Originally introduced here for structs: #39999.

It seems like only HIR-level changes are required, since enums are already aware of their alignment (due to alignment of their limbs).

cc @bitshifter

1 files changed, 14 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 6c780369a0a..e7b9a884b5e 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -464,6 +464,9 @@ declare_features! (
 
     // #[optimize(X)]
     (active, optimize_attribute, "1.34.0", Some(54882), None),
+
+    // #[repr(align(X))] on enums
+    (active, repr_align_enum, "1.34.0", Some(57996), None),
 );
 
 declare_features! (
@@ -1700,6 +1703,17 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
+            ast::ItemKind::Enum(..) => {
+                for attr in attr::filter_by_name(&i.attrs[..], "repr") {
+                    for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
+                        if item.check_name("align") {
+                            gate_feature_post!(&self, repr_align_enum, attr.span,
+                                               "`#[repr(align(x))]` on enums is experimental");
+                        }
+                    }
+                }
+            }
+
             ast::ItemKind::Impl(_, polarity, defaultness, _, _, _, _) => {
                 if polarity == ast::ImplPolarity::Negative {
                     gate_feature_post!(&self, optin_builtin_traits,