Do not use HashSet for #[rustc_must_implement_one_of]

Author: WaffleLapkin

compiler/rustc_typeck/src/check/check.rs

@@ -980,8 +980,8 @@ fn check_impl_items_against_trait<'tcx>(
         // Check for missing items from trait
         let mut missing_items = Vec::new();
 
-        let mut must_implement_one_of: Option<FxHashSet<Ident>> =
-            trait_def.must_implement_one_of.as_deref().map(|slice| slice.iter().copied().collect());
+        let mut must_implement_one_of: Option<&[Ident]> =
+            trait_def.must_implement_one_of.as_deref();
 
         for &trait_item_id in tcx.associated_item_def_ids(impl_trait_ref.def_id) {
             let is_implemented = ancestors
@@ -1020,7 +1020,7 @@ fn check_impl_items_against_trait<'tcx>(
                 .find(|attr| attr.has_name(sym::rustc_must_implement_one_of))
                 .map(|attr| attr.span);
 
-            missing_items_must_implement_one_of_err(tcx, impl_span, &missing_items, attr_span);
+            missing_items_must_implement_one_of_err(tcx, impl_span, missing_items, attr_span);
         }
     }
 }

compiler/rustc_typeck/src/check/mod.rs

@@ -644,7 +644,7 @@ fn missing_items_err(
 fn missing_items_must_implement_one_of_err(
     tcx: TyCtxt<'_>,
     impl_span: Span,
-    missing_items: &FxHashSet<Ident>,
+    missing_items: &[Ident],
     annotation_span: Option<Span>,
 ) {
     let missing_items_msg =

src/test/ui/traits/default-method/rustc_must_implement_one_of.rs

@@ -39,6 +39,6 @@ impl Equal for T2 {
 }
 
 impl Equal for T3 {}
-//~^ not all trait items implemented, missing one of: `neq`, `eq`
+//~^ not all trait items implemented, missing one of: `eq`, `neq`
 
 fn main() {}

src/test/ui/traits/default-method/rustc_must_implement_one_of.stderr

@@ -1,8 +1,8 @@
-error[E0046]: not all trait items implemented, missing one of: `neq`, `eq`
+error[E0046]: not all trait items implemented, missing one of: `eq`, `neq`
   --> $DIR/rustc_must_implement_one_of.rs:41:1
    |
 LL | impl Equal for T3 {}
-   | ^^^^^^^^^^^^^^^^^ missing one of `neq`, `eq` in implementation
+   | ^^^^^^^^^^^^^^^^^ missing one of `eq`, `neq` in implementation
    |
 note: required because of this annotation
   --> $DIR/rustc_must_implement_one_of.rs:3:1