Move special methods from ClosureKind back into rustc

compiler/rustc_middle/src/middle/lang_items.rs

@@ -36,6 +36,17 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// Given a [`ty::ClosureKind`], get the [`DefId`] of its corresponding `Fn`-family
+    /// trait, if it is defined.
+    pub fn fn_trait_kind_to_def_id(self, kind: ty::ClosureKind) -> Option<DefId> {
+        let items = self.lang_items();
+        match kind {
+            ty::ClosureKind::Fn => items.fn_trait(),
+            ty::ClosureKind::FnMut => items.fn_mut_trait(),
+            ty::ClosureKind::FnOnce => items.fn_once_trait(),
+        }
+    }
+
     /// Returns `true` if `id` is a `DefId` of [`Fn`], [`FnMut`] or [`FnOnce`] traits.
     pub fn is_fn_trait(self, id: DefId) -> bool {
         self.fn_trait_kind_from_def_id(id).is_some()

compiler/rustc_middle/src/ty/context.rs

@@ -151,30 +151,6 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     ) -> Self::Const {
         Const::new_bound(self, debruijn, var, ty)
     }
-
-    fn fn_def_id(self) -> Self::DefId {
-        self.require_lang_item(hir::LangItem::Fn, None)
-    }
-
-    fn fn_mut_def_id(self) -> Self::DefId {
-        self.require_lang_item(hir::LangItem::FnMut, None)
-    }
-
-    fn fn_once_def_id(self) -> Self::DefId {
-        self.require_lang_item(hir::LangItem::FnOnce, None)
-    }
-
-    fn i8_type(self) -> Self::Ty {
-        self.types.i8
-    }
-
-    fn i16_type(self) -> Self::Ty {
-        self.types.i16
-    }
-
-    fn i32_type(self) -> Self::Ty {
-        self.types.i32
-    }
 }
 
 type InternedSet<'tcx, T> = ShardedHashMap<InternedInSet<'tcx, T>, ()>;

compiler/rustc_middle/src/ty/sty.rs

@@ -2811,6 +2811,15 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Inverse of [`Ty::to_opt_closure_kind`].
+    pub fn from_closure_kind(tcx: TyCtxt<'tcx>, kind: ty::ClosureKind) -> Ty<'tcx> {
+        match kind {
+            ty::ClosureKind::Fn => tcx.types.i8,
+            ty::ClosureKind::FnMut => tcx.types.i16,
+            ty::ClosureKind::FnOnce => tcx.types.i32,
+        }
+    }
+
     /// Fast path helper for testing if a type is `Sized`.
     ///
     /// Returning true means the type is known to be sized. Returning