Rollup merge of rust-lang#110660 - compiler-errors:placeholders-pretty, r=wesleywiser,BoxyUwU

Print ty placeholders pretty

Makes anon placeholders print like `!0` instead of `Placeholder { ... }`.

```
rustc_trait_selection::solve::compute_well_formed_goal goal=Goal{
    predicate: !0,
    param_env: ParamEnv{
      caller_bounds: [
        Binder(TraitPredicate(<!0 as std::marker::Copy>, polarity: Positive), []),
        Binder(TraitPredicate(<!0 as std::clone::Clone>, polarity: Positive), []),
        Binder(TraitPredicate(<!0 as std::marker::Sized>, polarity: Positive), []),
      ],
      reveal: UserFacing,
      constness: NotConst,
    }
  }
```

cc `@BoxyUwU` who might care about this formatting decision

Author: JohnTitor

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -738,7 +738,9 @@ pub trait PrettyPrinter<'tcx>:
                 }
             }
             ty::Placeholder(placeholder) => match placeholder.bound.kind {
-                ty::BoundTyKind::Anon => p!(write("Placeholder({:?})", placeholder)),
+                ty::BoundTyKind::Anon => {
+                    self.pretty_print_placeholder_var(placeholder.universe, placeholder.bound.var)?
+                }
                 ty::BoundTyKind::Param(_, name) => p!(write("{}", name)),
             },
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
@@ -1172,6 +1174,18 @@ pub trait PrettyPrinter<'tcx>:
         }
     }
 
+    fn pretty_print_placeholder_var(
+        &mut self,
+        ui: ty::UniverseIndex,
+        var: ty::BoundVar,
+    ) -> Result<(), Self::Error> {
+        if ui == ty::UniverseIndex::ROOT {
+            write!(self, "!{}", var.index())
+        } else {
+            write!(self, "!{}_{}", ui.index(), var.index())
+        }
+    }
+
     fn ty_infer_name(&self, _: ty::TyVid) -> Option<Symbol> {
         None
     }

compiler/rustc_type_ir/src/sty.rs

@@ -203,6 +203,10 @@ pub enum TyKind<I: Interner> {
     /// `for<'a, T> &'a (): Trait<T>` and then convert the introduced bound variables
     /// back to inference variables in a new inference context when inside of the query.
     ///
+    /// It is conventional to render anonymous bound types like `^N` or `^D_N`,
+    /// where `N` is the bound variable's anonymous index into the binder, and
+    /// `D` is the debruijn index, or totally omitted if the debruijn index is zero.
+    ///
     /// See the `rustc-dev-guide` for more details about
     /// [higher-ranked trait bounds][1] and [canonical queries][2].
     ///
@@ -212,6 +216,12 @@ pub enum TyKind<I: Interner> {
 
     /// A placeholder type, used during higher ranked subtyping to instantiate
     /// bound variables.
+    ///
+    /// It is conventional to render anonymous placeholer types like `!N` or `!U_N`,
+    /// where `N` is the placeholder variable's anonymous index (which corresponds
+    /// to the bound variable's index from the binder from which it was instantiated),
+    /// and `U` is the universe index in which it is instantiated, or totally omitted
+    /// if the universe index is zero.
     Placeholder(I::PlaceholderType),
 
     /// A type variable used during type checking.