Remove HashStable impl for [hir::Attribute].

This impl skips over a handful of attributes as if they aren't there.
This skipping originated all the way back in #36025, and was extended a
couple of times, e.g. in #36370. Those PRs don't have any explanation of
why the skipping exists. Removing the impl (and falling back to the
default impl for `[T]`) doesn't seem to have any effects.

Author: nnethercote

compiler/rustc_middle/src/ich/impls_syntax.rs

@@ -3,9 +3,6 @@
 
 use rustc_ast as ast;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_hir as hir;
-use rustc_span::{Symbol, sym};
-use smallvec::SmallVec;
 
 use super::StableHashingContext;
 
@@ -16,44 +13,6 @@ impl<'a> HashStable<StableHashingContext<'a>> for ast::NodeId {
     }
 }
 
-impl<'a> HashStable<StableHashingContext<'a>> for [hir::Attribute] {
-    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
-        if self.is_empty() {
-            self.len().hash_stable(hcx, hasher);
-            return;
-        }
-
-        // Some attributes are always ignored during hashing.
-        let filtered: SmallVec<[&hir::Attribute; 8]> = self
-            .iter()
-            .filter(|attr| {
-                attr.is_doc_comment().is_none()
-                    // FIXME(jdonszelmann) have a better way to handle ignored attrs
-                    && !attr.name().is_some_and(|ident| is_ignored_attr(ident))
-            })
-            .collect();
-
-        filtered.len().hash_stable(hcx, hasher);
-        for attr in filtered {
-            attr.hash_stable(hcx, hasher);
-        }
-    }
-}
-
-#[inline]
-fn is_ignored_attr(name: Symbol) -> bool {
-    const IGNORED_ATTRIBUTES: &[Symbol] = &[
-        sym::cfg_trace, // FIXME(#138844) should this really be ignored?
-        sym::rustc_if_this_changed,
-        sym::rustc_then_this_would_need,
-        sym::rustc_clean,
-        sym::rustc_partition_reused,
-        sym::rustc_partition_codegened,
-        sym::rustc_expected_cgu_reuse,
-    ];
-    IGNORED_ATTRIBUTES.contains(&name)
-}
-
 impl<'tcx> HashStable<StableHashingContext<'tcx>> for rustc_feature::Features {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'tcx>, hasher: &mut StableHasher) {
         // Unfortunately we cannot exhaustively list fields here, since the