Introduce DefPathData::AnonAssocTy.

PR #137977 changed `DefPathData::TypeNs` to contain `Option<Symbol>` to
account for RPITIT assoc types being anonymous. This commit changes it
back to `Symbol` and gives anonymous assoc types their own variant. It
makes things a bit nicer overall.

compiler/rustc_hir/src/definitions.rs

@@ -271,9 +271,8 @@ pub enum DefPathData {
     Use,
     /// A global asm item.
     GlobalAsm,
-    /// Something in the type namespace. Will be empty for RPITIT associated
-    /// types, which are given a synthetic name later, if necessary.
-    TypeNs(Option<Symbol>),
+    /// Something in the type namespace.
+    TypeNs(Symbol),
     /// Something in the value namespace.
     ValueNs(Symbol),
     /// Something in the macro namespace.
@@ -291,6 +290,8 @@ pub enum DefPathData {
     /// An existential `impl Trait` type node.
     /// Argument position `impl Trait` have a `TypeNs` with their pretty-printed name.
     OpaqueTy,
+    /// An anonymous associated type from an RPITIT.
+    AnonAssocTy,
     /// A synthetic body for a coroutine's by-move body.
     SyntheticCoroutineBody,
 }
@@ -413,9 +414,7 @@ impl DefPathData {
     pub fn get_opt_name(&self) -> Option<Symbol> {
         use self::DefPathData::*;
         match *self {
-            TypeNs(name) => name,
-
-            ValueNs(name) | MacroNs(name) | LifetimeNs(name) => Some(name),
+            TypeNs(name) | ValueNs(name) | MacroNs(name) | LifetimeNs(name) => Some(name),
 
             Impl
             | ForeignMod
@@ -426,6 +425,7 @@ impl DefPathData {
             | Ctor
             | AnonConst
             | OpaqueTy
+            | AnonAssocTy
             | SyntheticCoroutineBody => None,
         }
     }
@@ -433,14 +433,9 @@ impl DefPathData {
     pub fn name(&self) -> DefPathDataName {
         use self::DefPathData::*;
         match *self {
-            TypeNs(name) => {
-                if let Some(name) = name {
-                    DefPathDataName::Named(name)
-                } else {
-                    DefPathDataName::Anon { namespace: sym::synthetic }
-                }
+            TypeNs(name) | ValueNs(name) | MacroNs(name) | LifetimeNs(name) => {
+                DefPathDataName::Named(name)
             }
-            ValueNs(name) | MacroNs(name) | LifetimeNs(name) => DefPathDataName::Named(name),
             // Note that this does not show up in user print-outs.
             CrateRoot => DefPathDataName::Anon { namespace: kw::Crate },
             Impl => DefPathDataName::Anon { namespace: kw::Impl },
@@ -451,6 +446,7 @@ impl DefPathData {
             Ctor => DefPathDataName::Anon { namespace: sym::constructor },
             AnonConst => DefPathDataName::Anon { namespace: sym::constant },
             OpaqueTy => DefPathDataName::Anon { namespace: sym::opaque },
+            AnonAssocTy => DefPathDataName::Anon { namespace: sym::synthetic },
             SyntheticCoroutineBody => DefPathDataName::Anon { namespace: sym::synthetic },
         }
     }