Rollup merge of #136458 - compiler-errors:fix-3, r=lcnr
Do not deduplicate list of associated types provided by dyn principal
## Background
The way that we handle a dyn trait type's projection bounds is very *structural* today. A dyn trait is represented as a list of `PolyExistentialPredicate`s, which in most cases will be a principal trait (like `Iterator`) and a list of projections (like `Item = u32`). Importantly, the list of projections comes from user-written associated type bounds on the type *and* from elaborating the projections from the principal's supertraits.
For example, given a set of traits like:
```rust
trait Foo<T> {
type Assoc;
}
trait Bar<A, B>: Foo<A, Assoc = A> + Foo<B, Assoc = B> {}
```
For the type `dyn Bar<i32, u32>`, the list of projections will be something like `[Foo<i32>::Assoc = i32, Foo<u32>::Assoc = u32]`. We deduplicate these projections when they're identical, so for `dyn Bar<(), ()>` would be something like `[Foo<()>::Assoc = ()]`.
## Shortcomings 1: inference
We face problems when we begin to mix this structural notion of projection bounds with inference and associated type normalization. For example, let's try calling a generic function that takes `dyn Bar<A, B>` with a value of type `dyn Bar<(), ()>`:
```rust
trait Foo<T> {
type Assoc;
}
trait Bar<A, B>: Foo<A, Assoc = A> + Foo<B, Assoc = B> {}
fn call_bar<A, B>(_: &dyn Bar<A, B>) {}
fn test(x: &dyn Bar<(), ()>) {
call_bar(x);
// ^ ERROR mismatched types
}
```
```
error[E0308]: mismatched types
--> /home/mgx/test.rs:10:14
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10 | call_bar(x);
| -------- ^ expected trait `Bar<_, _>`, found trait `Bar<(), ()>`
```
What's going on here? Well, when calling `call_bar`, the generic signature `&dyn Bar<?A, ?B>` does not unify with `&dyn Bar<(), ()>` because the list of projections differ -- `[Foo<?A>::Assoc = ?A, Foo<?B>::Assoc = ?B]` vs `[Foo<()>::Assoc = ()]`.
A simple solution to this may be to unify the principal traits first, then attempt to deduplicate them after inference. In this case, if we constrain `?A = ?B = ()`, then we would be able to deduplicate those projections in the first list.
However, this idea is still pretty fragile, and it's not a complete solution.
## Shortcomings 2: normalization
Consider a slightly modified example:
```rust
//@ compile-flags: -Znext-solver
trait Mirror {
type Assoc;
}
impl<T> Mirror for T {
type Assoc = T;
}
fn call_bar(_: &dyn Bar<(), <() as Mirror>::Assoc>) {}
fn test(x: &dyn Bar<(), ()>) {
call_bar(x);
}
```
This fails in the new solver. In this example, we try to unify `dyn Bar<(), ()>` and `dyn Bar<(), <() as Mirror>::Assoc>`. We are faced with the same problem even though there are no inference variables, and making this work relies on eagerly and deeply normalizing all projections so that they can be structurally deduplicated.
This is incompatible with how we handle associated types in the new trait solver, and while we could perhaps support it with some major gymnastics in the new solver, it suggests more fundamental shortcomings with how we deal with projection bounds in the new solver.
## Shortcomings 3: redundant projections
Consider a final example:
```rust
trait Foo {
type Assoc;
}
trait Bar: Foo<Assoc = ()> {}
fn call_bar1(_: &dyn Bar) {}
fn call_bar2(_: &dyn Bar<Assoc = ()>) {}
fn main() {
let x: &dyn Bar<Assoc = _> = todo!();
call_bar1(x);
//~^ ERROR mismatched types
call_bar2(x);
//~^ ERROR mismatched types
}
```
In this case, we have a user-written associated type bound (`Assoc = _`) which overlaps the bound that comes from the supertrait projection of `Bar` (namely, `Foo<Assoc = ()>`). In a similar way to the two examples above, this causes us to have a projection list mismatch that the compiler is not able to deduplicate.
## Solution
### Do not deduplicate after elaborating projections when lowering `dyn` types
The root cause of this issue has to do with mismatches of the deduplicated projection list before and after substitution or inference. This PR aims to avoid these issues by *never* deduplicating the projection list after elaborating the list of projections from the *identity* substituted principal trait ref.
For example,
```rust
trait Foo<T> {
type Assoc;
}
trait Bar<A, B>: Foo<A, Assoc = A> + Foo<B, Assoc = B> {}
```
When computing the projections for `dyn Bar<(), ()>`, before this PR we'd elaborate `Bar<(), ()>` to find a (deduplicated) projection list of `[Foo<()>::Assoc = ()]`.
After this PR, we take the principal trait and use its *identity* substitutions `Bar<A, B>` during elaboration, giving us projections `[Foo<A>::Assoc = A, Foo<B>::Assoc = B]`. Only after this elaboration do we substitute `A = (), B = ()` to get `[Foo<()>::Assoc = (), Foo<()>::Assoc = ()]`. This allows the type to be unified with the projections for `dyn Bar<?A, ?B>`, which are `[Foo<?A>::Assoc = ?A, Foo<?B>::Assoc = ?B]`.
This helps us avoid shorcomings 1 noted above.
### Do not deduplicate projections when relating `dyn` types
Similarly, we also do not call deduplicate when relating dyn types. This means that the list of projections does not differ depending on if the type has been normalized or not, which should avoid shortcomings 2 noted above.
Following from the example above, when relating projection lists like `[Foo<()>::Assoc = (), Foo<()>::Assoc = ()]` and `[Foo<?A>::Assoc = ?A, Foo<?B>::Assoc = ?B]`, the latter won't be deduplicated to a list of length 1 which would immediately fail to relate to the latter which is a list of length 2.
### Implement proper precedence between supertrait and user-written projection bounds when lowering `dyn` types
```rust
trait Foo {
type Assoc;
}
trait Bar: Foo<Assoc = ()> {}
```
Given a type like `dyn Foo<Assoc = _>`, we used to previously include *both* the supertrait and user-written associated type bounds in the projection list, giving us `[Foo::Assoc = (), Foo::Assoc = _]`. This would never unify with `dyn Foo`. However, this PR implements a strategy which overwrites the supertrait associated type bound with the one provided by the user, giving us a projection list of `[Foo::Assoc = _]`.
Why is this OK? Well, if a user wrote an associated type bound that is unsatisfiable (e.g. `dyn Bar<Assoc = i32>`) then the dyn type would never implement `Bar` or `Foo` anyways. If the user wrote something that is either structurally equal or equal modulo normalization to the supertrait bound, then it should be unaffected. And if the user wrote something that needs inference guidance (e.g. `dyn Bar<Assoc = _>`), then it'll be constrained when proving `dyn Bar<Assoc = _>: Bar`.
Importantly, this differs from the strategy in https://github.com/rust-lang/rust/pull/133397, which preferred the *supertrait* bound and ignored the user-written bound. While that's also theoretically justifiable in its own way, it does lead to code which does not (and probably should not) compile either today or after this PR, like:
```rust
trait IteratorOfUnit: Iterator<Item = ()> {}
impl<T> IteratorOfUnit for T where T: Iterator<Item = ()> {}
fn main() {
let iter = [()].into_iter();
let iter: &dyn IteratorOfUnit<Item = i32> = &iter;
}
```
### Conclusion
This is a far less invasive change compared to #133397, and doesn't necessarily necessitate the addition of new lints or any breakage of existing code. While we could (and possibly should) eventually introduce lints to warn users of redundant or mismatched associated type bounds, we don't *need* to do so as part of fixing this unsoundness, which leads me to believe this is a much safer solution.
This commit is contained in:
Matthias Krüger
GitHub
commit
326072ac20
19 changed files with 297 additions and 118 deletions
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@ -1,4 +1,4 @@
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use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
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use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
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use rustc_errors::codes::*;
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use rustc_errors::struct_span_code_err;
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use rustc_hir as hir;
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@ -58,9 +58,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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}
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}
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let (trait_bounds, mut projection_bounds) =
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let (elaborated_trait_bounds, elaborated_projection_bounds) =
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traits::expand_trait_aliases(tcx, user_written_bounds.iter().copied());
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let (regular_traits, mut auto_traits): (Vec<_>, Vec<_>) = trait_bounds
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let (regular_traits, mut auto_traits): (Vec<_>, Vec<_>) = elaborated_trait_bounds
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.into_iter()
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.partition(|(trait_ref, _)| !tcx.trait_is_auto(trait_ref.def_id()));
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@ -103,29 +103,81 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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}
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}
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// Map the projection bounds onto a key that makes it easy to remove redundant
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// bounds that are constrained by supertraits of the principal def id.
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//
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// Also make sure we detect conflicting bounds from expanding a trait alias and
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// also specifying it manually, like:
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// ```
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// type Alias = Trait<Assoc = i32>;
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// let _: &dyn Alias<Assoc = u32> = /* ... */;
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// ```
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let mut projection_bounds = FxIndexMap::default();
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for (proj, proj_span) in elaborated_projection_bounds {
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let key = (
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proj.skip_binder().projection_term.def_id,
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tcx.anonymize_bound_vars(
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proj.map_bound(|proj| proj.projection_term.trait_ref(tcx)),
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),
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);
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if let Some((old_proj, old_proj_span)) =
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projection_bounds.insert(key, (proj, proj_span))
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&& tcx.anonymize_bound_vars(proj) != tcx.anonymize_bound_vars(old_proj)
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{
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let item = tcx.item_name(proj.item_def_id());
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self.dcx()
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.struct_span_err(
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span,
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format!(
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"conflicting associated type bounds for `{item}` when \
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expanding trait alias"
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),
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)
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.with_span_label(
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old_proj_span,
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format!("`{item}` is specified to be `{}` here", old_proj.term()),
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)
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.with_span_label(
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proj_span,
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format!("`{item}` is specified to be `{}` here", proj.term()),
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)
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.emit();
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}
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}
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let principal_trait = regular_traits.into_iter().next();
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let mut needed_associated_types = FxIndexSet::default();
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if let Some((principal_trait, spans)) = &principal_trait {
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let pred: ty::Predicate<'tcx> = (*principal_trait).upcast(tcx);
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for ClauseWithSupertraitSpan { pred, supertrait_span } in traits::elaborate(
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let mut needed_associated_types = vec![];
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if let Some((principal_trait, ref spans)) = principal_trait {
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let principal_trait = principal_trait.map_bound(|trait_pred| {
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assert_eq!(trait_pred.polarity, ty::PredicatePolarity::Positive);
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trait_pred.trait_ref
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});
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for ClauseWithSupertraitSpan { clause, supertrait_span } in traits::elaborate(
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tcx,
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[ClauseWithSupertraitSpan::new(pred, *spans.last().unwrap())],
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[ClauseWithSupertraitSpan::new(
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ty::TraitRef::identity(tcx, principal_trait.def_id()).upcast(tcx),
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*spans.last().unwrap(),
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)],
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)
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.filter_only_self()
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{
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debug!("observing object predicate `{pred:?}`");
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let clause = clause.instantiate_supertrait(tcx, principal_trait);
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debug!("observing object predicate `{clause:?}`");
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let bound_predicate = pred.kind();
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let bound_predicate = clause.kind();
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match bound_predicate.skip_binder() {
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ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) => {
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ty::ClauseKind::Trait(pred) => {
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// FIXME(negative_bounds): Handle this correctly...
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let trait_ref =
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tcx.anonymize_bound_vars(bound_predicate.rebind(pred.trait_ref));
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needed_associated_types.extend(
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tcx.associated_items(trait_ref.def_id())
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tcx.associated_items(pred.trait_ref.def_id)
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.in_definition_order()
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// We only care about associated types.
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.filter(|item| item.kind == ty::AssocKind::Type)
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// No RPITITs -- even with `async_fn_in_dyn_trait`, they are implicit.
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.filter(|item| !item.is_impl_trait_in_trait())
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// If the associated type has a `where Self: Sized` bound,
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// we do not need to constrain the associated type.
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@ -133,7 +185,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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.map(|item| (item.def_id, trait_ref)),
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);
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}
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ty::PredicateKind::Clause(ty::ClauseKind::Projection(pred)) => {
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ty::ClauseKind::Projection(pred) => {
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let pred = bound_predicate.rebind(pred);
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// A `Self` within the original bound will be instantiated with a
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// `trait_object_dummy_self`, so check for that.
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@ -161,8 +213,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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// `dyn MyTrait<MyOutput = X, Output = X>`, which is uglier but works. See
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// the discussion in #56288 for alternatives.
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if !references_self {
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// Include projections defined on supertraits.
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projection_bounds.push((pred, supertrait_span));
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let key = (
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pred.skip_binder().projection_term.def_id,
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tcx.anonymize_bound_vars(
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pred.map_bound(|proj| proj.projection_term.trait_ref(tcx)),
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),
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);
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if !projection_bounds.contains_key(&key) {
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projection_bounds.insert(key, (pred, supertrait_span));
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}
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}
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self.check_elaborated_projection_mentions_input_lifetimes(
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@ -182,12 +241,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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// types that we expect to be provided by the user, so the following loop
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// removes all the associated types that have a corresponding `Projection`
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// clause, either from expanding trait aliases or written by the user.
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for &(projection_bound, span) in &projection_bounds {
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for &(projection_bound, span) in projection_bounds.values() {
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let def_id = projection_bound.item_def_id();
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let trait_ref = tcx.anonymize_bound_vars(
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projection_bound.map_bound(|p| p.projection_term.trait_ref(tcx)),
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);
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needed_associated_types.swap_remove(&(def_id, trait_ref));
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if tcx.generics_require_sized_self(def_id) {
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tcx.emit_node_span_lint(
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UNUSED_ASSOCIATED_TYPE_BOUNDS,
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@ -198,9 +253,22 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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}
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}
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let mut missing_assoc_types = FxIndexSet::default();
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let projection_bounds: Vec<_> = needed_associated_types
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.into_iter()
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.filter_map(|key| {
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if let Some(assoc) = projection_bounds.get(&key) {
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Some(*assoc)
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} else {
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missing_assoc_types.insert(key);
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None
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}
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})
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.collect();
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if let Err(guar) = self.check_for_required_assoc_tys(
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principal_trait.as_ref().map_or(smallvec![], |(_, spans)| spans.clone()),
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needed_associated_types,
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missing_assoc_types,
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potential_assoc_types,
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hir_bounds,
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) {
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@ -266,7 +334,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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})
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});
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let existential_projections = projection_bounds.iter().map(|(bound, _)| {
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let existential_projections = projection_bounds.into_iter().map(|(bound, _)| {
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bound.map_bound(|mut b| {
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assert_eq!(b.projection_term.self_ty(), dummy_self);
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@ -291,12 +359,16 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
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})
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});
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let auto_trait_predicates = auto_traits.into_iter().map(|(trait_pred, _)| {
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assert_eq!(trait_pred.polarity(), ty::PredicatePolarity::Positive);
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assert_eq!(trait_pred.self_ty().skip_binder(), dummy_self);
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let mut auto_trait_predicates: Vec<_> = auto_traits
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.into_iter()
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.map(|(trait_pred, _)| {
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assert_eq!(trait_pred.polarity(), ty::PredicatePolarity::Positive);
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assert_eq!(trait_pred.self_ty().skip_binder(), dummy_self);
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ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(trait_pred.def_id()))
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});
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ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(trait_pred.def_id()))
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})
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.collect();
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auto_trait_predicates.dedup();
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// N.b. principal, projections, auto traits
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// FIXME: This is actually wrong with multiple principals in regards to symbol mangling
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.chain(auto_trait_predicates)
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.collect::<SmallVec<[_; 8]>>();
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v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
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v.dedup();
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let existential_predicates = tcx.mk_poly_existential_predicates(&v);
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// Use explicitly-specified region bound, unless the bound is missing.
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