Rollup merge of #128471 - camelid:rustdoc-self, r=notriddle

rustdoc: Fix handling of `Self` type in search index and refactor its representation

### Summary

- Add enum variant `clean::Type::SelfTy` and use it instead of `clean::Type::Generic(kw::SelfUpper)`.
- Stop treating `Self` as a generic in the search index.
- Remove struct formerly known as `clean::SelfTy` (constructed as representation of function receiver type). We're better off without it.

### Before

![image](https://github.com/user-attachments/assets/d257bdd8-3a62-4c71-84a5-9c950f2e4f00)

### After

![image](https://github.com/user-attachments/assets/8f6d3f22-92c1-41e3-9ab8-a881b66816c0)

r? ```@notriddle```
cc #127589 (comment)

src/librustdoc/clean/inline.rs

@@ -12,7 +12,7 @@ use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::hygiene::MacroKind;
-use rustc_span::symbol::{kw, sym, Symbol};
+use rustc_span::symbol::{sym, Symbol};
 use thin_vec::{thin_vec, ThinVec};
 use {rustc_ast as ast, rustc_hir as hir};
 
@@ -792,11 +792,7 @@ fn build_macro(
 fn filter_non_trait_generics(trait_did: DefId, mut g: clean::Generics) -> clean::Generics {
     for pred in &mut g.where_predicates {
         match *pred {
-            clean::WherePredicate::BoundPredicate {
-                ty: clean::Generic(ref s),
-                ref mut bounds,
-                ..
-            } if *s == kw::SelfUpper => {
+            clean::WherePredicate::BoundPredicate { ty: clean::SelfTy, ref mut bounds, .. } => {
                 bounds.retain(|bound| match bound {
                     clean::GenericBound::TraitBound(clean::PolyTrait { trait_, .. }, _) => {
                         trait_.def_id() != trait_did
@@ -812,13 +808,13 @@ fn filter_non_trait_generics(trait_did: DefId, mut g: clean::Generics) -> clean:
         clean::WherePredicate::BoundPredicate {
             ty:
                 clean::QPath(box clean::QPathData {
-                    self_type: clean::Generic(ref s),
+                    self_type: clean::Generic(_),
                     trait_: Some(trait_),
                     ..
                 }),
             bounds,
             ..
-        } => !(bounds.is_empty() || *s == kw::SelfUpper && trait_.def_id() == trait_did),
+        } => !bounds.is_empty() && trait_.def_id() != trait_did,
         _ => true,
     });
     g
@@ -832,9 +828,7 @@ fn separate_supertrait_bounds(
 ) -> (clean::Generics, Vec<clean::GenericBound>) {
     let mut ty_bounds = Vec::new();
     g.where_predicates.retain(|pred| match *pred {
-        clean::WherePredicate::BoundPredicate { ty: clean::Generic(ref s), ref bounds, .. }
-            if *s == kw::SelfUpper =>
-        {
+        clean::WherePredicate::BoundPredicate { ty: clean::SelfTy, ref bounds, .. } => {
             ty_bounds.extend(bounds.iter().cloned());
             false
         }

src/librustdoc/clean/mod.rs

@@ -1351,11 +1351,11 @@ pub(crate) fn clean_middle_assoc_item<'tcx>(
                 let self_arg_ty =
                     tcx.fn_sig(assoc_item.def_id).instantiate_identity().input(0).skip_binder();
                 if self_arg_ty == self_ty {
-                    item.decl.inputs.values[0].type_ = Generic(kw::SelfUpper);
+                    item.decl.inputs.values[0].type_ = SelfTy;
                 } else if let ty::Ref(_, ty, _) = *self_arg_ty.kind() {
                     if ty == self_ty {
                         match item.decl.inputs.values[0].type_ {
-                            BorrowedRef { ref mut type_, .. } => **type_ = Generic(kw::SelfUpper),
+                            BorrowedRef { ref mut type_, .. } => **type_ = SelfTy,
                             _ => unreachable!(),
                         }
                     }
@@ -1439,9 +1439,8 @@ pub(crate) fn clean_middle_assoc_item<'tcx>(
                         if trait_.def_id() != assoc_item.container_id(tcx) {
                             return true;
                         }
-                        match *self_type {
-                            Generic(ref s) if *s == kw::SelfUpper => {}
-                            _ => return true,
+                        if *self_type != SelfTy {
+                            return true;
                         }
                         match &assoc.args {
                             GenericArgs::AngleBracketed { args, constraints } => {
@@ -2228,6 +2227,8 @@ pub(crate) fn clean_middle_ty<'tcx>(
         ty::Param(ref p) => {
             if let Some(bounds) = cx.impl_trait_bounds.remove(&p.index.into()) {
                 ImplTrait(bounds)
+            } else if p.name == kw::SelfUpper {
+                SelfTy
             } else {
                 Generic(p.name)
             }

src/librustdoc/clean/simplify.rs

@@ -145,7 +145,6 @@ pub(crate) fn sized_bounds(cx: &mut DocContext<'_>, generics: &mut clean::Generi
     // should be handled when cleaning associated types.
     generics.where_predicates.retain(|pred| {
         if let WP::BoundPredicate { ty: clean::Generic(param), bounds, .. } = pred
-            && *param != rustc_span::symbol::kw::SelfUpper
             && bounds.iter().any(|b| b.is_sized_bound(cx))
         {
             sized_params.insert(*param);

src/librustdoc/clean/types.rs

@@ -34,10 +34,9 @@ use thin_vec::ThinVec;
 use {rustc_ast as ast, rustc_hir as hir};
 
 pub(crate) use self::ItemKind::*;
-pub(crate) use self::SelfTy::*;
 pub(crate) use self::Type::{
     Array, BareFunction, BorrowedRef, DynTrait, Generic, ImplTrait, Infer, Primitive, QPath,
-    RawPointer, Slice, Tuple,
+    RawPointer, SelfTy, Slice, Tuple,
 };
 use crate::clean::cfg::Cfg;
 use crate::clean::clean_middle_path;
@@ -1384,8 +1383,8 @@ pub(crate) struct FnDecl {
 }
 
 impl FnDecl {
-    pub(crate) fn self_type(&self) -> Option<SelfTy> {
-        self.inputs.values.get(0).and_then(|v| v.to_self())
+    pub(crate) fn receiver_type(&self) -> Option<&Type> {
+        self.inputs.values.get(0).and_then(|v| v.to_receiver())
     }
 }
 
@@ -1403,27 +1402,9 @@ pub(crate) struct Argument {
     pub(crate) is_const: bool,
 }
 
-#[derive(Clone, PartialEq, Debug)]
-pub(crate) enum SelfTy {
-    SelfValue,
-    SelfBorrowed(Option<Lifetime>, Mutability),
-    SelfExplicit(Type),
-}
-
 impl Argument {
-    pub(crate) fn to_self(&self) -> Option<SelfTy> {
-        if self.name != kw::SelfLower {
-            return None;
-        }
-        if self.type_.is_self_type() {
-            return Some(SelfValue);
-        }
-        match self.type_ {
-            BorrowedRef { ref lifetime, mutability, ref type_ } if type_.is_self_type() => {
-                Some(SelfBorrowed(lifetime.clone(), mutability))
-            }
-            _ => Some(SelfExplicit(self.type_.clone())),
-        }
+    pub(crate) fn to_receiver(&self) -> Option<&Type> {
+        if self.name == kw::SelfLower { Some(&self.type_) } else { None }
     }
 }
 
@@ -1477,6 +1458,8 @@ pub(crate) enum Type {
     DynTrait(Vec<PolyTrait>, Option<Lifetime>),
     /// A type parameter.
     Generic(Symbol),
+    /// The `Self` type.
+    SelfTy,
     /// A primitive (aka, builtin) type.
     Primitive(PrimitiveType),
     /// A function pointer: `extern "ABI" fn(...) -> ...`
@@ -1571,6 +1554,8 @@ impl Type {
             // If both sides are generic, this returns true.
             (_, Type::Generic(_)) => true,
             (Type::Generic(_), _) => false,
+            // `Self` only matches itself.
+            (Type::SelfTy, Type::SelfTy) => true,
             // Paths account for both the path itself and its generics.
             (Type::Path { path: a }, Type::Path { path: b }) => {
                 a.def_id() == b.def_id()
@@ -1642,7 +1627,7 @@ impl Type {
 
     pub(crate) fn is_self_type(&self) -> bool {
         match *self {
-            Generic(name) => name == kw::SelfUpper,
+            SelfTy => true,
             _ => false,
         }
     }
@@ -1700,7 +1685,7 @@ impl Type {
             Type::Pat(..) => PrimitiveType::Pat,
             RawPointer(..) => PrimitiveType::RawPointer,
             QPath(box QPathData { ref self_type, .. }) => return self_type.def_id(cache),
-            Generic(_) | Infer | ImplTrait(_) => return None,
+            Generic(_) | SelfTy | Infer | ImplTrait(_) => return None,
         };
         Primitive(t).def_id(cache)
     }

src/librustdoc/clean/utils.rs

@@ -468,7 +468,7 @@ pub(crate) fn resolve_type(cx: &mut DocContext<'_>, path: Path) -> Type {
     match path.res {
         Res::PrimTy(p) => Primitive(PrimitiveType::from(p)),
         Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } if path.segments.len() == 1 => {
-            Generic(kw::SelfUpper)
+            Type::SelfTy
         }
         Res::Def(DefKind::TyParam, _) if path.segments.len() == 1 => Generic(path.segments[0].name),
         _ => {

src/librustdoc/html/format.rs

@@ -1006,6 +1006,7 @@ fn fmt_type<'cx>(
 
     match *t {
         clean::Generic(name) => f.write_str(name.as_str()),
+        clean::SelfTy => f.write_str("Self"),
         clean::Type::Path { ref path } => {
             // Paths like `T::Output` and `Self::Output` should be rendered with all segments.
             let did = path.def_id();
@@ -1452,29 +1453,22 @@ impl clean::FnDecl {
 
         let last_input_index = self.inputs.values.len().checked_sub(1);
         for (i, input) in self.inputs.values.iter().enumerate() {
-            if let Some(selfty) = input.to_self() {
+            if let Some(selfty) = input.to_receiver() {
                 match selfty {
-                    clean::SelfValue => {
+                    clean::SelfTy => {
                         write!(f, "self")?;
                     }
-                    clean::SelfBorrowed(Some(ref lt), mutability) => {
-                        write!(
-                            f,
-                            "{amp}{lifetime} {mutability}self",
-                            lifetime = lt.print(),
-                            mutability = mutability.print_with_space(),
-                        )?;
-                    }
-                    clean::SelfBorrowed(None, mutability) => {
-                        write!(
-                            f,
-                            "{amp}{mutability}self",
-                            mutability = mutability.print_with_space(),
-                        )?;
+                    clean::BorrowedRef { lifetime, mutability, type_: box clean::SelfTy } => {
+                        write!(f, "{amp}")?;
+                        match lifetime {
+                            Some(lt) => write!(f, "{lt} ", lt = lt.print())?,
+                            None => {}
+                        }
+                        write!(f, "{mutability}self", mutability = mutability.print_with_space())?;
                     }
-                    clean::SelfExplicit(ref typ) => {
+                    _ => {
                         write!(f, "self: ")?;
-                        typ.print(cx).fmt(f)?;
+                        selfty.print(cx).fmt(f)?;
                     }
                 }
             } else {

src/librustdoc/html/render/mod.rs

@@ -58,7 +58,7 @@ use serde::{Serialize, Serializer};
 
 pub(crate) use self::context::*;
 pub(crate) use self::span_map::{collect_spans_and_sources, LinkFromSrc};
-use crate::clean::{self, ItemId, RenderedLink, SelfTy};
+use crate::clean::{self, ItemId, RenderedLink};
 use crate::error::Error;
 use crate::formats::cache::Cache;
 use crate::formats::item_type::ItemType;
@@ -1372,21 +1372,20 @@ fn render_deref_methods(
 
 fn should_render_item(item: &clean::Item, deref_mut_: bool, tcx: TyCtxt<'_>) -> bool {
     let self_type_opt = match *item.kind {
-        clean::MethodItem(ref method, _) => method.decl.self_type(),
-        clean::TyMethodItem(ref method) => method.decl.self_type(),
+        clean::MethodItem(ref method, _) => method.decl.receiver_type(),
+        clean::TyMethodItem(ref method) => method.decl.receiver_type(),
         _ => None,
     };
 
     if let Some(self_ty) = self_type_opt {
-        let (by_mut_ref, by_box, by_value) = match self_ty {
-            SelfTy::SelfBorrowed(_, mutability)
-            | SelfTy::SelfExplicit(clean::BorrowedRef { mutability, .. }) => {
+        let (by_mut_ref, by_box, by_value) = match *self_ty {
+            clean::Type::BorrowedRef { mutability, .. } => {
                 (mutability == Mutability::Mut, false, false)
             }
-            SelfTy::SelfExplicit(clean::Type::Path { path }) => {
+            clean::Type::Path { ref path } => {
                 (false, Some(path.def_id()) == tcx.lang_items().owned_box(), false)
             }
-            SelfTy::SelfValue => (false, false, true),
+            clean::Type::SelfTy => (false, false, true),
             _ => (false, false, false),
         };