Remove `rustc_middle::ty::util::ExplicitSelf`.

It's an old (2017 or earlier) type that describes a `self` receiver.
It's only used in `rustc_hir_analysis` for two error messages, and much
of the complexity isn't used. I suspect it used to be used for more
things.

This commit removes it, and moves a greatly simplified version of the
`determine` method into `rustc_hir_analysis`, renamed as
`get_self_string`. The big comment on the method is removed because it
no longer seems relevant.

2 files changed, 22 insertions, 62 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index 84d07c711fa..a4ed15ba212 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -12,7 +12,6 @@ use rustc_hir::{self as hir, AmbigArg, GenericParamKind, ImplItemKind, intravisi
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::util;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::util::ExplicitSelf;
 use rustc_middle::ty::{
     self, BottomUpFolder, GenericArgs, GenericParamDefKind, Ty, TyCtxt, TypeFoldable, TypeFolder,
     TypeSuperFoldable, TypeVisitableExt, TypingMode, Upcast,
@@ -995,6 +994,26 @@ impl<'tcx> ty::FallibleTypeFolder<TyCtxt<'tcx>> for RemapHiddenTyRegions<'tcx> {
     }
 }
 
+/// Gets the string for an explicit self declaration, e.g. "self", "&self",
+/// etc.
+fn get_self_string<'tcx, P>(self_arg_ty: Ty<'tcx>, is_self_ty: P) -> String
+where
+    P: Fn(Ty<'tcx>) -> bool,
+{
+    if is_self_ty(self_arg_ty) {
+        "self".to_owned()
+    } else if let ty::Ref(_, ty, mutbl) = self_arg_ty.kind()
+        && is_self_ty(*ty)
+    {
+        match mutbl {
+            hir::Mutability::Not => "&self".to_owned(),
+            hir::Mutability::Mut => "&mut self".to_owned(),
+        }
+    } else {
+        format!("self: {self_arg_ty}")
+    }
+}
+
 fn report_trait_method_mismatch<'tcx>(
     infcx: &InferCtxt<'tcx>,
     mut cause: ObligationCause<'tcx>,
@@ -1020,12 +1039,7 @@ fn report_trait_method_mismatch<'tcx>(
             if trait_m.fn_has_self_parameter =>
         {
             let ty = trait_sig.inputs()[0];
-            let sugg = match ExplicitSelf::determine(ty, |ty| ty == impl_trait_ref.self_ty()) {
-                ExplicitSelf::ByValue => "self".to_owned(),
-                ExplicitSelf::ByReference(_, hir::Mutability::Not) => "&self".to_owned(),
-                ExplicitSelf::ByReference(_, hir::Mutability::Mut) => "&mut self".to_owned(),
-                _ => format!("self: {ty}"),
-            };
+            let sugg = get_self_string(ty, |ty| ty == impl_trait_ref.self_ty());
 
             // When the `impl` receiver is an arbitrary self type, like `self: Box<Self>`, the
             // span points only at the type `Box<Self`>, but we want to cover the whole
@@ -1238,12 +1252,7 @@ fn compare_self_type<'tcx>(
             .build_with_typing_env(ty::TypingEnv::non_body_analysis(tcx, method.def_id));
         let self_arg_ty = tcx.liberate_late_bound_regions(method.def_id, self_arg_ty);
         let can_eq_self = |ty| infcx.can_eq(param_env, untransformed_self_ty, ty);
-        match ExplicitSelf::determine(self_arg_ty, can_eq_self) {
-            ExplicitSelf::ByValue => "self".to_owned(),
-            ExplicitSelf::ByReference(_, hir::Mutability::Not) => "&self".to_owned(),
-            ExplicitSelf::ByReference(_, hir::Mutability::Mut) => "&mut self".to_owned(),
-            _ => format!("self: {self_arg_ty}"),
-        }
+        get_self_string(self_arg_ty, can_eq_self)
     };
 
     match (trait_m.fn_has_self_parameter, impl_m.fn_has_self_parameter) {
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index c0d4130336e..23c5c3488fc 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -1546,55 +1546,6 @@ impl<'tcx> Ty<'tcx> {
     }
 }
 
-pub enum ExplicitSelf<'tcx> {
-    ByValue,
-    ByReference(ty::Region<'tcx>, hir::Mutability),
-    ByRawPointer(hir::Mutability),
-    ByBox,
-    Other,
-}
-
-impl<'tcx> ExplicitSelf<'tcx> {
-    /// Categorizes an explicit self declaration like `self: SomeType`
-    /// into either `self`, `&self`, `&mut self`, `Box<Self>`, or
-    /// `Other`.
-    /// This is mainly used to require the arbitrary_self_types feature
-    /// in the case of `Other`, to improve error messages in the common cases,
-    /// and to make `Other` dyn-incompatible.
-    ///
-    /// Examples:
-    ///
-    /// ```ignore (illustrative)
-    /// impl<'a> Foo for &'a T {
-    ///     // Legal declarations:
-    ///     fn method1(self: &&'a T); // ExplicitSelf::ByReference
-    ///     fn method2(self: &'a T); // ExplicitSelf::ByValue
-    ///     fn method3(self: Box<&'a T>); // ExplicitSelf::ByBox
-    ///     fn method4(self: Rc<&'a T>); // ExplicitSelf::Other
-    ///
-    ///     // Invalid cases will be caught by `check_method_receiver`:
-    ///     fn method_err1(self: &'a mut T); // ExplicitSelf::Other
-    ///     fn method_err2(self: &'static T) // ExplicitSelf::ByValue
-    ///     fn method_err3(self: &&T) // ExplicitSelf::ByReference
-    /// }
-    /// ```
-    ///
-    pub fn determine<P>(self_arg_ty: Ty<'tcx>, is_self_ty: P) -> ExplicitSelf<'tcx>
-    where
-        P: Fn(Ty<'tcx>) -> bool,
-    {
-        use self::ExplicitSelf::*;
-
-        match *self_arg_ty.kind() {
-            _ if is_self_ty(self_arg_ty) => ByValue,
-            ty::Ref(region, ty, mutbl) if is_self_ty(ty) => ByReference(region, mutbl),
-            ty::RawPtr(ty, mutbl) if is_self_ty(ty) => ByRawPointer(mutbl),
-            _ if self_arg_ty.boxed_ty().is_some_and(is_self_ty) => ByBox,
-            _ => Other,
-        }
-    }
-}
-
 /// Returns a list of types such that the given type needs drop if and only if
 /// *any* of the returned types need drop. Returns `Err(AlwaysRequiresDrop)` if
 /// this type always needs drop.