Rollup merge of #123170 - compiler-errors:const-statics, r=lcnr

Replace regions in const canonical vars' types with `'static` in next-solver canonicalizer

We shouldn't ever have non-static regions in consts on stable (or really any regions at all, lol).

The test I committed is less minimal than, e.g., https://github.com/rust-lang/rust/issues/123155?notification_referrer_id=NT_kwDOADgQyrMxMDAzNDU4MDI0OTozNjc0MzE0#issuecomment-2025472029 -- however, I believe that it actually portrays the underlying issue here a bit better than that one.

In the linked issue, we end up emitting a normalizes-to predicate for a const placeholder because we don't actually unify `false` and `""`. In the test I committed, we emit a normalizes-to predicate as a part of actually solving a negative coherence goal.

Fixes #123155
Fixes #118783

r? lcnr

compiler/rustc_middle/src/ty/region.rs

@@ -140,6 +140,10 @@ impl<'tcx> rustc_type_ir::new::Region<TyCtxt<'tcx>> for Region<'tcx> {
     fn new_anon_bound(tcx: TyCtxt<'tcx>, debruijn: ty::DebruijnIndex, var: ty::BoundVar) -> Self {
         Region::new_bound(tcx, debruijn, ty::BoundRegion { var, kind: ty::BoundRegionKind::BrAnon })
     }
+
+    fn new_static(tcx: TyCtxt<'tcx>) -> Self {
+        tcx.lifetimes.re_static
+    }
 }
 
 /// Region utilities

compiler/rustc_next_trait_solver/src/canonicalizer.rs

@@ -296,10 +296,7 @@ impl<Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeFolder<I>
         Region::new_anon_bound(self.interner(), self.binder_index, var)
     }
 
-    fn fold_ty(&mut self, t: I::Ty) -> I::Ty
-    where
-        I::Ty: TypeSuperFoldable<I>,
-    {
+    fn fold_ty(&mut self, t: I::Ty) -> I::Ty {
         let kind = match t.kind() {
             ty::Infer(i) => match i {
                 ty::TyVar(vid) => {
@@ -378,47 +375,48 @@ impl<Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeFolder<I>
         Ty::new_anon_bound(self.interner(), self.binder_index, var)
     }
 
-    fn fold_const(&mut self, c: I::Const) -> I::Const
-    where
-        I::Const: TypeSuperFoldable<I>,
-    {
+    fn fold_const(&mut self, c: I::Const) -> I::Const {
+        // We could canonicalize all consts with static types, but the only ones we
+        // *really* need to worry about are the ones that we end up putting into `CanonicalVarKind`
+        // since canonical vars can't reference other canonical vars.
+        let ty = c
+            .ty()
+            .fold_with(&mut RegionsToStatic { interner: self.interner(), binder: ty::INNERMOST });
         let kind = match c.kind() {
-            ty::ConstKind::Infer(i) => {
-                // FIXME: we should fold the ty too eventually
-                match i {
-                    ty::InferConst::Var(vid) => {
-                        assert_eq!(
-                            self.infcx.root_ct_var(vid),
-                            vid,
-                            "region vid should have been resolved fully before canonicalization"
-                        );
-                        assert_eq!(
-                            self.infcx.probe_ct_var(vid),
-                            None,
-                            "region vid should have been resolved fully before canonicalization"
-                        );
-                        CanonicalVarKind::Const(self.infcx.universe_of_ct(vid).unwrap(), c.ty())
-                    }
-                    ty::InferConst::EffectVar(_) => CanonicalVarKind::Effect,
-                    ty::InferConst::Fresh(_) => todo!(),
+            ty::ConstKind::Infer(i) => match i {
+                ty::InferConst::Var(vid) => {
+                    assert_eq!(
+                        self.infcx.root_ct_var(vid),
+                        vid,
+                        "region vid should have been resolved fully before canonicalization"
+                    );
+                    assert_eq!(
+                        self.infcx.probe_ct_var(vid),
+                        None,
+                        "region vid should have been resolved fully before canonicalization"
+                    );
+                    CanonicalVarKind::Const(self.infcx.universe_of_ct(vid).unwrap(), ty)
                 }
-            }
+                ty::InferConst::EffectVar(_) => CanonicalVarKind::Effect,
+                ty::InferConst::Fresh(_) => todo!(),
+            },
             ty::ConstKind::Placeholder(placeholder) => match self.canonicalize_mode {
                 CanonicalizeMode::Input => CanonicalVarKind::PlaceholderConst(
                     PlaceholderLike::new(placeholder.universe(), self.variables.len().into()),
-                    c.ty(),
+                    ty,
                 ),
                 CanonicalizeMode::Response { .. } => {
-                    CanonicalVarKind::PlaceholderConst(placeholder, c.ty())
+                    CanonicalVarKind::PlaceholderConst(placeholder, ty)
                 }
             },
             ty::ConstKind::Param(_) => match self.canonicalize_mode {
                 CanonicalizeMode::Input => CanonicalVarKind::PlaceholderConst(
                     PlaceholderLike::new(ty::UniverseIndex::ROOT, self.variables.len().into()),
-                    c.ty(),
+                    ty,
                 ),
                 CanonicalizeMode::Response { .. } => panic!("param ty in response: {c:?}"),
             },
+            // FIXME: See comment above -- we could fold the region separately or something.
             ty::ConstKind::Bound(_, _)
             | ty::ConstKind::Unevaluated(_)
             | ty::ConstKind::Value(_)
@@ -435,6 +433,35 @@ impl<Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeFolder<I>
             }),
         );
 
-        Const::new_anon_bound(self.interner(), self.binder_index, var, c.ty())
+        Const::new_anon_bound(self.interner(), self.binder_index, var, ty)
+    }
+}
+
+struct RegionsToStatic<I> {
+    interner: I,
+    binder: ty::DebruijnIndex,
+}
+
+impl<I: Interner> TypeFolder<I> for RegionsToStatic<I> {
+    fn interner(&self) -> I {
+        self.interner
+    }
+
+    fn fold_binder<T>(&mut self, t: I::Binder<T>) -> I::Binder<T>
+    where
+        T: TypeFoldable<I>,
+        I::Binder<T>: TypeSuperFoldable<I>,
+    {
+        self.binder.shift_in(1);
+        let t = t.fold_with(self);
+        self.binder.shift_out(1);
+        t
+    }
+
+    fn fold_region(&mut self, r: I::Region) -> I::Region {
+        match r.kind() {
+            ty::ReBound(db, _) if self.binder > db => r,
+            _ => Region::new_static(self.interner()),
+        }
     }
 }

compiler/rustc_type_ir/src/fold.rs

@@ -136,31 +136,21 @@ pub trait TypeFolder<I: Interner>: FallibleTypeFolder<I, Error = Never> {
         t.super_fold_with(self)
     }
 
-    fn fold_ty(&mut self, t: I::Ty) -> I::Ty
-    where
-        I::Ty: TypeSuperFoldable<I>,
-    {
+    fn fold_ty(&mut self, t: I::Ty) -> I::Ty {
         t.super_fold_with(self)
     }
 
     // The default region folder is a no-op because `Region` is non-recursive
-    // and has no `super_fold_with` method to call. That also explains the
-    // lack of `I::Region: TypeSuperFoldable<I>` bound on this method.
+    // and has no `super_fold_with` method to call.
     fn fold_region(&mut self, r: I::Region) -> I::Region {
         r
     }
 
-    fn fold_const(&mut self, c: I::Const) -> I::Const
-    where
-        I::Const: TypeSuperFoldable<I>,
-    {
+    fn fold_const(&mut self, c: I::Const) -> I::Const {
         c.super_fold_with(self)
     }
 
-    fn fold_predicate(&mut self, p: I::Predicate) -> I::Predicate
-    where
-        I::Predicate: TypeSuperFoldable<I>,
-    {
+    fn fold_predicate(&mut self, p: I::Predicate) -> I::Predicate {
         p.super_fold_with(self)
     }
 }
@@ -185,31 +175,21 @@ pub trait FallibleTypeFolder<I: Interner>: Sized {
         t.try_super_fold_with(self)
     }
 
-    fn try_fold_ty(&mut self, t: I::Ty) -> Result<I::Ty, Self::Error>
-    where
-        I::Ty: TypeSuperFoldable<I>,
-    {
+    fn try_fold_ty(&mut self, t: I::Ty) -> Result<I::Ty, Self::Error> {
         t.try_super_fold_with(self)
     }
 
     // The default region folder is a no-op because `Region` is non-recursive
-    // and has no `super_fold_with` method to call. That also explains the
-    // lack of `I::Region: TypeSuperFoldable<I>` bound on this method.
+    // and has no `super_fold_with` method to call.
     fn try_fold_region(&mut self, r: I::Region) -> Result<I::Region, Self::Error> {
         Ok(r)
     }
 
-    fn try_fold_const(&mut self, c: I::Const) -> Result<I::Const, Self::Error>
-    where
-        I::Const: TypeSuperFoldable<I>,
-    {
+    fn try_fold_const(&mut self, c: I::Const) -> Result<I::Const, Self::Error> {
         c.try_super_fold_with(self)
     }
 
-    fn try_fold_predicate(&mut self, p: I::Predicate) -> Result<I::Predicate, Self::Error>
-    where
-        I::Predicate: TypeSuperFoldable<I>,
-    {
+    fn try_fold_predicate(&mut self, p: I::Predicate) -> Result<I::Predicate, Self::Error> {
         p.try_super_fold_with(self)
     }
 }
@@ -234,10 +214,7 @@ where
         Ok(self.fold_binder(t))
     }
 
-    fn try_fold_ty(&mut self, t: I::Ty) -> Result<I::Ty, Never>
-    where
-        I::Ty: TypeSuperFoldable<I>,
-    {
+    fn try_fold_ty(&mut self, t: I::Ty) -> Result<I::Ty, Never> {
         Ok(self.fold_ty(t))
     }
 
@@ -245,17 +222,11 @@ where
         Ok(self.fold_region(r))
     }
 
-    fn try_fold_const(&mut self, c: I::Const) -> Result<I::Const, Never>
-    where
-        I::Const: TypeSuperFoldable<I>,
-    {
+    fn try_fold_const(&mut self, c: I::Const) -> Result<I::Const, Never> {
         Ok(self.fold_const(c))
     }
 
-    fn try_fold_predicate(&mut self, p: I::Predicate) -> Result<I::Predicate, Never>
-    where
-        I::Predicate: TypeSuperFoldable<I>,
-    {
+    fn try_fold_predicate(&mut self, p: I::Predicate) -> Result<I::Predicate, Never> {
         Ok(self.fold_predicate(p))
     }
 }

compiler/rustc_type_ir/src/interner.rs

@@ -2,13 +2,14 @@ use smallvec::SmallVec;
 use std::fmt::Debug;
 use std::hash::Hash;
 
+use crate::fold::TypeSuperFoldable;
 use crate::visit::{Flags, TypeSuperVisitable, TypeVisitable};
 use crate::{
     new, BoundVar, BoundVars, CanonicalVarInfo, ConstKind, DebugWithInfcx, RegionKind, TyKind,
     UniverseIndex,
 };
 
-pub trait Interner: Sized {
+pub trait Interner: Sized + Copy {
     type DefId: Copy + Debug + Hash + Eq;
     type AdtDef: Copy + Debug + Hash + Eq;
 
@@ -34,6 +35,7 @@ pub trait Interner: Sized {
         + Into<Self::GenericArg>
         + IntoKind<Kind = TyKind<Self>>
         + TypeSuperVisitable<Self>
+        + TypeSuperFoldable<Self>
         + Flags
         + new::Ty<Self>;
     type Tys: Copy + Debug + Hash + Eq + IntoIterator<Item = Self::Ty>;
@@ -57,6 +59,7 @@ pub trait Interner: Sized {
         + IntoKind<Kind = ConstKind<Self>>
         + ConstTy<Self>
         + TypeSuperVisitable<Self>
+        + TypeSuperFoldable<Self>
         + Flags
         + new::Const<Self>;
     type AliasConst: Copy + DebugWithInfcx<Self> + Hash + Eq;
@@ -82,7 +85,13 @@ pub trait Interner: Sized {
     type PlaceholderRegion: Copy + Debug + Hash + Eq + PlaceholderLike;
 
     // Predicates
-    type Predicate: Copy + Debug + Hash + Eq + TypeSuperVisitable<Self> + Flags;
+    type Predicate: Copy
+        + Debug
+        + Hash
+        + Eq
+        + TypeSuperVisitable<Self>
+        + TypeSuperFoldable<Self>
+        + Flags;
     type TraitPredicate: Copy + Debug + Hash + Eq;
     type RegionOutlivesPredicate: Copy + Debug + Hash + Eq;
     type TypeOutlivesPredicate: Copy + Debug + Hash + Eq;

compiler/rustc_type_ir/src/new.rs

@@ -6,6 +6,8 @@ pub trait Ty<I: Interner<Ty = Self>> {
 
 pub trait Region<I: Interner<Region = Self>> {
     fn new_anon_bound(interner: I, debruijn: DebruijnIndex, var: BoundVar) -> Self;
+
+    fn new_static(interner: I) -> Self;
 }
 
 pub trait Const<I: Interner<Const = Self>> {