Auto merge of #100032 - BoxyUwU:no_ty_in_placeholder_const, r=compiler-errors

make `PlaceholderConst` not store the type of the const

Currently the `Placeholder` variant on `ConstKind` is 28 bytes when with this PR its 8 bytes, i am not sure this is really useful at all rn since `Unevaluated` and `Value` variants are huge still but eventually it should be possible to get both down to 16 bytes 🤔. Mostly opening this to see if this change has any perf impact when done before it can make `ConstKind`/`ConstS` smaller

compiler/rustc_infer/src/infer/canonical/canonicalizer.rs

@@ -511,7 +511,9 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
             }
             ty::ConstKind::Placeholder(placeholder) => {
                 return self.canonicalize_const_var(
-                    CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderConst(placeholder) },
+                    CanonicalVarInfo {
+                        kind: CanonicalVarKind::PlaceholderConst(placeholder, ct.ty()),
+                    },
                     ct,
                 );
             }
@@ -695,11 +697,14 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
                             ..placeholder
                         })
                     }
-                    CanonicalVarKind::PlaceholderConst(placeholder) => {
+                    CanonicalVarKind::PlaceholderConst(placeholder, t) => {
-                        CanonicalVarKind::PlaceholderConst(ty::Placeholder {
+                        CanonicalVarKind::PlaceholderConst(
-                            universe: reverse_universe_map[&placeholder.universe],
+                            ty::Placeholder {
-                            ..placeholder
+                                universe: reverse_universe_map[&placeholder.universe],
-                        })
+                                ..placeholder
+                            },
+                            t,
+                        )
                     }
                 },
             })

compiler/rustc_infer/src/infer/canonical/mod.rs

@@ -144,13 +144,13 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                 )
                 .into(),
 
-            CanonicalVarKind::PlaceholderConst(ty::PlaceholderConst { universe, name }) => {
+            CanonicalVarKind::PlaceholderConst(ty::PlaceholderConst { universe, name }, ty) => {
                 let universe_mapped = universe_map(universe);
                 let placeholder_mapped = ty::PlaceholderConst { universe: universe_mapped, name };
                 self.tcx
                     .mk_const(ty::ConstS {
                         kind: ty::ConstKind::Placeholder(placeholder_mapped),
-                        ty: name.ty,
+                        ty,
                     })
                     .into()
             }

compiler/rustc_infer/src/infer/higher_ranked/mod.rs

@@ -97,7 +97,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 self.tcx.mk_const(ty::ConstS {
                     kind: ty::ConstKind::Placeholder(ty::PlaceholderConst {
                         universe: next_universe,
-                        name: ty::BoundConst { var: bound_var, ty },
+                        name: bound_var,
                     }),
                     ty,
                 })

compiler/rustc_infer/src/infer/mod.rs

@@ -2067,7 +2067,7 @@ fn replace_param_and_infer_substs_with_placeholder<'tcx>(
                     ty,
                     kind: ty::ConstKind::Placeholder(ty::PlaceholderConst {
                         universe: ty::UniverseIndex::ROOT,
-                        name: ty::BoundConst { ty, var: ty::BoundVar::from_usize(idx) },
+                        name: ty::BoundVar::from_usize(idx),
                     }),
                 })
                 .into()

compiler/rustc_middle/src/infer/canonical.rs

@@ -105,7 +105,7 @@ impl<'tcx> CanonicalVarInfo<'tcx> {
             CanonicalVarKind::Region(_) => true,
             CanonicalVarKind::PlaceholderRegion(..) => false,
             CanonicalVarKind::Const(..) => true,
-            CanonicalVarKind::PlaceholderConst(_) => false,
+            CanonicalVarKind::PlaceholderConst(_, _) => false,
         }
     }
 }
@@ -133,7 +133,7 @@ pub enum CanonicalVarKind<'tcx> {
     Const(ty::UniverseIndex, Ty<'tcx>),
 
     /// A "placeholder" that represents "any const".
-    PlaceholderConst(ty::PlaceholderConst<'tcx>),
+    PlaceholderConst(ty::PlaceholderConst<'tcx>, Ty<'tcx>),
 }
 
 impl<'tcx> CanonicalVarKind<'tcx> {
@@ -148,7 +148,7 @@ impl<'tcx> CanonicalVarKind<'tcx> {
             CanonicalVarKind::Region(ui) => ui,
             CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
             CanonicalVarKind::Const(ui, _) => ui,
-            CanonicalVarKind::PlaceholderConst(placeholder) => placeholder.universe,
+            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.universe,
         }
     }
 }

compiler/rustc_middle/src/ty/mod.rs

@@ -1200,7 +1200,7 @@ pub struct BoundConst<'tcx> {
     pub ty: Ty<'tcx>,
 }
 
-pub type PlaceholderConst<'tcx> = Placeholder<BoundConst<'tcx>>;
+pub type PlaceholderConst<'tcx> = Placeholder<BoundVar>;
 
 /// A `DefId` which, in case it is a const argument, is potentially bundled with
 /// the `DefId` of the generic parameter it instantiates.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -744,10 +744,7 @@ impl<'tcx> TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
             }
             ty::ConstKind::Bound(debruijn, bound_const) if debruijn >= self.current_index => {
                 let universe = self.universe_for(debruijn);
-                let p = ty::PlaceholderConst {
+                let p = ty::PlaceholderConst { universe, name: bound_const };
-                    universe,
-                    name: ty::BoundConst { var: bound_const, ty: ct.ty() },
-                };
                 self.mapped_consts.insert(p, bound_const);
                 self.infcx
                     .tcx

compiler/rustc_traits/src/chalk/mod.rs

@@ -76,7 +76,7 @@ pub(crate) fn evaluate_goal<'tcx>(
                         chalk_ir::UniverseIndex { counter: ui.index() },
                     ),
                     CanonicalVarKind::Const(_ui, _ty) => unimplemented!(),
-                    CanonicalVarKind::PlaceholderConst(_pc) => unimplemented!(),
+                    CanonicalVarKind::PlaceholderConst(_pc, _ty) => unimplemented!(),
                 }),
             ),
             value: obligation.value.lower_into(interner),