Add a IsIdentity extension trait for CanonicalUserType

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -26,7 +26,7 @@ use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
 use rustc_middle::ty::{
-    self, AdtKind, CanonicalUserType, GenericParamDefKind, Ty, TyCtxt, UserType,
+    self, AdtKind, CanonicalUserType, GenericParamDefKind, IsIdentity, Ty, TyCtxt, UserType,
 };
 use rustc_middle::ty::{GenericArgKind, GenericArgsRef, UserArgs, UserSelfTy};
 use rustc_session::lint;
@@ -208,7 +208,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         debug!("fcx {}", self.tag());
 
         // FIXME: is_identity being on `UserType` and not `Canonical<UserType>` is awkward
-        if !canonical_user_type_annotation.value.is_identity() {
+        if !canonical_user_type_annotation.is_identity() {
             self.typeck_results
                 .borrow_mut()
                 .user_provided_types_mut()

compiler/rustc_middle/src/ty/mod.rs

@@ -106,8 +106,8 @@ pub use self::sty::{
 };
 pub use self::trait_def::TraitDef;
 pub use self::typeck_results::{
-    CanonicalUserType, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, TypeckResults,
-    UserType, UserTypeAnnotationIndex,
+    CanonicalUserType, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, IsIdentity,
+    TypeckResults, UserType, UserTypeAnnotationIndex,
 };
 
 pub mod _match;

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -607,11 +607,15 @@ pub enum UserType<'tcx> {
     TypeOf(DefId, UserArgs<'tcx>),
 }
 
-impl<'tcx> UserType<'tcx> {
+pub trait IsIdentity {
+    fn is_identity(&self) -> bool;
+}
+
+impl<'tcx> IsIdentity for CanonicalUserType<'tcx> {
     /// Returns `true` if this represents a substitution of the form `[?0, ?1, ?2]`,
     /// i.e., each thing is mapped to a canonical variable with the same index.
-    pub fn is_identity(&self) -> bool {
-        match self {
+    fn is_identity(&self) -> bool {
+        match self.value {
             UserType::Ty(_) => false,
             UserType::TypeOf(_, user_args) => {
                 if user_args.user_self_ty.is_some() {