Restrict const ty's regions to static when putting them in canonical var list

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

@@ -382,10 +382,14 @@ fn fold_const(&mut self, c: I::Const) -> I::Const
     where
         I::Const: TypeSuperFoldable<I>,
     {
+        // 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) => {
+            ty::ConstKind::Infer(i) => match i {
-                // FIXME: we should fold the ty too eventually
-                match i {
                 ty::InferConst::Var(vid) => {
                     assert_eq!(
                         self.infcx.root_ct_var(vid),
@@ -397,28 +401,28 @@ fn fold_const(&mut self, c: I::Const) -> I::Const
                         None,
                         "region vid should have been resolved fully before canonicalization"
                     );
-                        CanonicalVarKind::Const(self.infcx.universe_of_ct(vid).unwrap(), c.ty())
+                    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 +439,35 @@ fn fold_const(&mut self, c: I::Const) -> I::Const
             }),
         );
 
-        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/interner.rs

@@ -2,13 +2,14 @@
 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;

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>> {

tests/ui/coherence/negative-coherence/regions-in-canonical.rs

@@ -0,0 +1,23 @@
+//@ check-pass
+
+#![feature(adt_const_params)]
+//~^ WARN the feature `adt_const_params` is incomplete
+#![feature(with_negative_coherence, negative_impls)]
+
+pub trait A<const K: &'static str> {}
+pub trait C {}
+
+
+struct W<T>(T);
+
+// Negative coherence:
+// Proving `W<!T>: !A<"">` requires proving `CONST alias-eq ""`, which requires proving
+// `CONST normalizes-to (?1c: &str)`. The type's region is uniquified, so it ends up being
+// put in to the canonical vars list with an infer region => ICE.
+impl<T> C for T where T: A<""> {}
+impl<T> C for W<T> {}
+
+impl<T> !A<CONST> for W<T> {}
+const CONST: &str = "";
+
+fn main() {}

tests/ui/coherence/negative-coherence/regions-in-canonical.stderr

@@ -0,0 +1,11 @@
+warning: the feature `adt_const_params` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/regions-in-canonical.rs:3:12
+   |
+LL | #![feature(adt_const_params)]
+   |            ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #95174 <https://github.com/rust-lang/rust/issues/95174> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+