Use Arena inside hir::TraitMethod.

src/librustc/hir/intravisit.rs

@@ -850,7 +850,7 @@ pub fn walk_trait_item<'v, V: Visitor<'v>>(visitor: &mut V, trait_item: &'v Trai
             visitor.visit_ty(ty);
             walk_list!(visitor, visit_nested_body, default);
         }
-        TraitItemKind::Method(ref sig, TraitMethod::Required(ref param_names)) => {
+        TraitItemKind::Method(ref sig, TraitMethod::Required(param_names)) => {
             visitor.visit_id(trait_item.hir_id);
             visitor.visit_fn_decl(&sig.decl);
             for &param_name in param_names {

src/librustc/hir/lowering/item.rs

@@ -787,6 +787,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
             AssocItemKind::Fn(ref sig, None) => {
                 let names = self.lower_fn_params_to_names(&sig.decl);
+                let names: &[Ident] = self.arena.alloc_from_iter(names.into_iter());
                 let (generics, sig) =
                     self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))

src/librustc/hir/mod.rs

@@ -1946,9 +1946,9 @@ pub struct TraitItem<'hir> {
 
 /// Represents a trait method's body (or just argument names).
 #[derive(RustcEncodable, RustcDecodable, Debug, HashStable)]
-pub enum TraitMethod {
+pub enum TraitMethod<'hir> {
     /// No default body in the trait, just a signature.
-    Required(HirVec<Ident>),
+    Required(&'hir [Ident]),
 
     /// Both signature and body are provided in the trait.
     Provided(BodyId),
@@ -1960,7 +1960,7 @@ pub enum TraitItemKind<'hir> {
     /// An associated constant with an optional value (otherwise `impl`s must contain a value).
     Const(&'hir Ty, Option<BodyId>),
     /// A method with an optional body.
-    Method(FnSig<'hir>, TraitMethod),
+    Method(FnSig<'hir>, TraitMethod<'hir>),
     /// An associated type with (possibly empty) bounds and optional concrete
     /// type.
     Type(GenericBounds, Option<&'hir Ty>),

src/librustc_lint/nonstandard_style.rs

@@ -336,7 +336,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonSnakeCase {
     }
 
     fn check_trait_item(&mut self, cx: &LateContext<'_, '_>, item: &hir::TraitItem<'_>) {
-        if let hir::TraitItemKind::Method(_, hir::TraitMethod::Required(pnames)) = &item.kind {
+        if let hir::TraitItemKind::Method(_, hir::TraitMethod::Required(pnames)) = item.kind {
             self.check_snake_case(cx, "trait method", &item.ident);
             for param_name in pnames {
                 self.check_snake_case(cx, "variable", param_name);