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Merged
rust-bors merged 1 commit into from
Apr 29, 2026
Merged

Canonicalize free regions from inputs as placeholders in root univ #155487

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46 changes: 38 additions & 8 deletions compiler/rustc_next_trait_solver/src/canonical/canonicalizer.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -410,14 +410,22 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
}

fn fold_region(&mut self, r: I::Region) -> I::Region {
// We canonicalize free regions from the input into placeholder regions so that
// region constraints created in nested contexts can be propagated back to the
// caller, instead of unifying them.
// See the following Zulip discussion for details:
// https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/A.20question.20on.20.23251/near/579240238
let kind = match r.kind() {
ty::ReBound(..) => return r,

// We don't canonicalize `ReStatic` in the `param_env` as we use it
// when checking whether a `ParamEnv` candidate is global.
ty::ReStatic => match self.canonicalize_mode {
CanonicalizeMode::Input(CanonicalizeInputKind::Predicate) => {
CanonicalVarKind::Region(ty::UniverseIndex::ROOT)
CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion::new_anon(
ty::UniverseIndex::ROOT,
self.variables.len().into(),
))
}
CanonicalizeMode::Input(CanonicalizeInputKind::ParamEnv)
| CanonicalizeMode::Response { .. } => return r,
Expand All @@ -431,24 +439,41 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
// `ReErased`. We may be able to short-circuit registering region
// obligations if we encounter a `ReErased` on one side, for example.
ty::ReErased | ty::ReError(_) => match self.canonicalize_mode {
CanonicalizeMode::Input(_) => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input(_) => {
CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion::new_anon(
ty::UniverseIndex::ROOT,
self.variables.len().into(),
))
}
CanonicalizeMode::Response { .. } => return r,
},

ty::ReEarlyParam(_) | ty::ReLateParam(_) => match self.canonicalize_mode {
CanonicalizeMode::Input(_) => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input(_) => {
CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion::new_anon(
ty::UniverseIndex::ROOT,
self.variables.len().into(),
))
}
CanonicalizeMode::Response { .. } => {
panic!("unexpected region in response: {r:?}")
}
},

ty::RePlaceholder(placeholder) => match self.canonicalize_mode {
// We canonicalize placeholder regions as existentials in query inputs.
CanonicalizeMode::Input(_) => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input(_) => {
CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion::new_anon(
ty::UniverseIndex::ROOT,
self.variables.len().into(),
))
}
CanonicalizeMode::Response { max_input_universe } => {
// If we have a placeholder region inside of a query, it must be from
// a new universe.
if max_input_universe.can_name(placeholder.universe()) {
// a new universe, unless from the root universe, which is used for
// canonicalization of any free region from the input.
if placeholder.universe() != ty::UniverseIndex::ROOT
&& max_input_universe.can_name(placeholder.universe())
{
panic!("new placeholder in universe {max_input_universe:?}: {r:?}");
}
CanonicalVarKind::PlaceholderRegion(placeholder)
Expand All @@ -462,7 +487,12 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
"region vid should have been resolved fully before canonicalization"
);
match self.canonicalize_mode {
CanonicalizeMode::Input(_) => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input(_) => {
CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion::new_anon(
ty::UniverseIndex::ROOT,
self.variables.len().into(),
))
}
CanonicalizeMode::Response { .. } => {
CanonicalVarKind::Region(self.delegate.universe_of_lt(vid).unwrap())
}
Expand Down
12 changes: 12 additions & 0 deletions compiler/rustc_next_trait_solver/src/canonical/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -211,6 +211,18 @@ where
} else {
// For placeholders which were already part of the input, we simply map this
// universal bound variable back the placeholder of the input.
//
// For `CanonicalVarKind::PlaceholderRegion`, this differs slightly: we
// canonicalize all free regions from the input into placeholders. This is
// unlike types or consts, where only input placeholders remain placeholders
// in the canonical form.
//
// We can still map these back to the original input regions, as we
// just instantiate the canonical variable with its corresponding
// `original_value`.
//
// For more information on why we canonicalize all input regions as
// placeholders, see the comment in `Canonicalizer::fold_region`.
original_values[kind.expect_placeholder_index()]
}
})
Expand Down
4 changes: 2 additions & 2 deletions ...ization-preserve-equality.borrowck.stderr → ...preserve-equality.borrowck_current.stderr
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
error: lifetime may not live long enough
--> $DIR/normalization-preserve-equality.rs:24:1
--> $DIR/normalization-preserve-equality.rs:27:1
|
LL | fn test_borrowck<'a, 'b>(_: (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>)) {
| ^^^^^^^^^^^^^^^^^--^^--^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -11,7 +11,7 @@ LL | fn test_borrowck<'a, 'b>(_: (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>))
= help: consider adding the following bound: `'a: 'b`

error: lifetime may not live long enough
--> $DIR/normalization-preserve-equality.rs:24:1
--> $DIR/normalization-preserve-equality.rs:27:1
|
LL | fn test_borrowck<'a, 'b>(_: (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>)) {
| ^^^^^^^^^^^^^^^^^--^^--^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand Down
13 changes: 8 additions & 5 deletions tests/ui/implied-bounds/normalization-preserve-equality.rs
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@@ -1,9 +1,12 @@
// Both revisions should pass. `borrowck` revision is a bug!
// All the revisions should pass. `borrowck_current` revision is a bug!
//
//@ revisions: wfcheck borrowck
//@ ignore-compare-mode-next-solver (explicit revisions)
//@ revisions: wfcheck borrowck_current borrowck_next
//@ [wfcheck] check-pass
//@ [borrowck] check-fail
//@ [borrowck] known-bug: #106569
//@ [borrowck_current] check-fail
//@ [borrowck_current] known-bug: #106569
//@ [borrowck_next] compile-flags: -Znext-solver
//@ [borrowck_next] check-pass

struct Equal<'a, 'b>(&'a &'b (), &'b &'a ()); // implies 'a == 'b

Expand All @@ -20,7 +23,7 @@ trait WfCheckTrait {}
#[cfg(wfcheck)]
impl<'a, 'b> WfCheckTrait for (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>) {}

#[cfg(borrowck)]
#[cfg(any(borrowck_current, borrowck_next))]
fn test_borrowck<'a, 'b>(_: (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>)) {
let _ = None::<Equal<'a, 'b>>;
}
Expand Down
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