Fix various bugs in `ty_kind_suggestion`
Consolidates two implementations of `ty_kind_suggestion`
Fixes some misuse of the empty param-env
Fixes a problem where we suggested `(42)` instead of `(42,)` for tuple suggestions
Suggest a value when `return;`, making it consistent with `break;`
Fixes#123906
Discard overflow obligations in `impl_may_apply`
Hacky fix for #123493. Throws away obligations that are overflowing in `impl_may_apply` when we recompute if an impl applies, since those will lead to fatal overflow if processed during fulfillment.
Something about #114811 (I think it's the predicate reordering) caused us to evaluate predicates differently in error reporting leading to fatal overflow, though I believe the underlying overflow is possible to hit since this code was rewritten to use fulfillment.
Fixes#123493
Don't rely on upvars being assigned just because coroutine-closure kind is assigned
Previously, code relied on the implicit assumption that if a coroutine-closure's kind variable was constrained, then its upvars were also constrained. This is because we assign all of them at once at the end up upvar analysis.
However, there's another way that a coroutine-closure's kind can be constrained: from a signature hint in closure signature deduction. After #123350, we use these hints, which means the implicit assumption above no longer holds.
This PR adds the necessary checks so that we don't ICE.
r? oli-obk
Pass list of defineable opaque types into canonical queries
This eliminates `DefiningAnchor::Bubble` for good and brings the old solver closer to the new one wrt cycles and nested obligations. At that point the difference between `DefiningAnchor::Bind([])` and `DefiningAnchor::Error` was academic. We only used the difference for some sanity checks, which actually had to be worked around in places, so I just removed `DefiningAnchor` entirely and just stored the list of opaques that may be defined.
fixes#108498
fixes https://github.com/rust-lang/rust/issues/116877
* [x] run crater
- https://github.com/rust-lang/rust/pull/122077#issuecomment-2013293931
Safe Transmute: Compute transmutability from `rustc_target::abi::Layout`
In its first step of computing transmutability, `rustc_transmutability` constructs a byte-level representation of type layout (`Tree`). Previously, this representation was computed for ADTs by inspecting the ADT definition and performing our own layout computations. This process was error-prone, verbose, and limited our ability to analyze many types (particularly default-repr types).
In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This helps ensure that layout optimizations are reflected our analyses, and increases the kinds of types we can now analyze, including:
- default repr ADTs
- transparent unions
- `UnsafeCell`-containing types
Overall, this PR expands the expressvity of `rustc_transmutability` to be much closer to the transmutability analysis performed by miri. Future PRs will work to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`, coroutines, etc.).
r? `@compiler-errors`
In its first step of computing transmutability, `rustc_transmutability`
constructs a byte-level representation of type layout (`Tree`). Previously, this
representation was computed for ADTs by inspecting the ADT definition and
performing our own layout computations. This process was error-prone, verbose,
and limited our ability to analyze many types (particularly default-repr types).
In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This
helps ensure that layout optimizations are reflected our analyses, and increases
the kinds of types we can now analyze, including:
- default repr ADTs
- transparent unions
- `UnsafeCell`-containing types
Overall, this PR expands the expressvity of `rustc_transmutability` to be much
closer to the transmutability analysis performed by miri. Future PRs will work
to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`,
coroutines, etc.).
Check def id before calling `match_projection_projections`
When I "inlined" `assemble_candidates_from_predicates` into `for_each_item_bound` in #120584, I forgot to copy over the check that actually made sure the def id of the candidate was equal to the def id of the obligation. This means that we normalize goal a bit too often even if it's not productive to do so.
This PR adds that def id check back.
Fixes#123448
do not ICE in `fn forced_ambiguity` if we get an error
see the comment. currently causing an ICE in typenum which we've been unable to minimize.
r? `@compiler-errors`
Cleanup: Rename `HAS_PROJECTIONS` to `HAS_ALIASES` etc.
The name of the bitflag `HAS_PROJECTIONS` and of its corresponding method `has_projections` is quite historical dating back to a time when projections were the only kind of alias type.
I think it's time to update it to clear up any potential confusion for newcomers and to reduce unnecessary friction during contributor onboarding.
r? types
change `NormalizesTo` to fully structurally normalize
notes in https://hackmd.io/wZ016dE4QKGIhrOnHLlThQ
need to also update the dev-guide once this PR lands. in short, the setup is now as follows:
`normalizes-to` internally implements one step normalization, applying that normalization to the `goal.predicate.term` causes the projected term to get recursively normalized. With this `normalizes-to` normalizes until the projected term is rigid, meaning that we normalize as many steps necessary, but at least 1.
To handle rigid aliases, we add another candidate only if the 1 to inf step normalization failed. With this `normalizes-to` is now full structural normalization. We can now change `AliasRelate` to simply emit `normalizes-to` goals for the rhs and lhs.
This avoids the concerns from https://github.com/rust-lang/trait-system-refactor-initiative/issues/103 and generally feels cleaner
some smaller DefiningOpaqueTypes::No -> Yes switches
r? `@compiler-errors`
These are some easy cases, so let's get them out of the way first.
I added tests exercising the specialization code paths that I believe weren't tested so far.
follow-up to https://github.com/rust-lang/rust/pull/117348
The reason is that in specialization graph computation we use `DefiningAnchor::Error`, so there's no difference anyway. And in the other use cases, we
* already errored in the specialization_graph computation, or
* already errored in coherence, or
* are comparing opaque types with inference variables already, or
* there are no opaque types involved
check `FnDef` return type for WF
better version of #106807, fixes#84533 (mostly). It's not perfect given that we still ignore WF requirements involving bound regions but I wasn't able to quickly write an example, so even if theoretically exploitable, it should be far harder to trigger.
This is strictly more restrictive than checking the return type for WF as part of the builtin `FnDef: FnOnce` impl (#106807) and moving to this approach in the future will not break any code.
~~It also agrees with my theoretical view of how this should behave~~
r? types
instantiate higher ranked goals outside of candidate selection
This PR modifies `evaluate` to more eagerly instantiate higher-ranked goals, preventing the `leak_check` during candidate selection from detecting placeholder errors involving that binder.
For a general background regarding higher-ranked region solving and the leak check, see https://hackmd.io/qd9Wp03cQVy06yOLnro2Kg.
> The first is something called the **leak check**. You can think of it as a "quick and dirty" approximation for the region check, which will come later. The leak check detects some kinds of errors early, essentially deciding between "this set of outlives constraints are guaranteed to result in an error eventually" or "this set of outlives constraints may be solvable".
## The ideal future
We would like to end up with the following idealized design to handle universal binders:
```rust
fn enter_forall<'tcx, T, R>(
forall: Binder<'tcx, T>,
f: impl FnOnce(T) -> R,
) -> R {
let new_universe = infcx.increment_universe_index();
let value = instantiate_binder_with_placeholders_in(new_universe, forall);
let result = f(value);
eagerly_handle_higher_ranked_region_constraints_in(new_universe);
infcx.decrement_universe_index();
assert!(!result.has_placeholders_in_or_above(new_universe));
result
}
```
That is, when universally instantiating a binder, anything using the placeholders has to happen inside of a limited scope (the closure `f`). After this closure has completed, all constraints involving placeholders are known.
We then handle any *external constraints* which name these placeholders. We destructure `TypeOutlives` constraints involving placeholders and eagerly handle any region constraints involving these placeholders. We do not return anything mentioning the placeholders created inside of this function to the caller.
Being able to eagerly handle *all* region constraints involving placeholders will be difficult due to complex `TypeOutlives` constraints, involving inference variables or alias types, and higher ranked implied bounds. The exact issues and possible solutions are out of scope of this FCP.
#### How does the leak check fit into this
The `leak_check` is an underapproximation of `eagerly_handle_higher_ranked_region_constraints_in`. It detects some kinds of errors involving placeholders from `new_universe`, but not all of them.
It only looks at region outlives constraints, ignoring `TypeOutlives`, and checks whether one of the following two conditions are met for **placeholders in or above `new_universe`**, in which case it results in an error:
- `'!p1: '!p2` a placeholder `'!p2` outlives a different placeholder `'!p1`
- `'!p1: '?2` an inference variable `'?2` outlives a placeholder `'!p1` *which it cannot name*
It does not handle all higher ranked region constraints, so we still return constraints involving placeholders from `new_universe` which are then (re)checked by `lexical_region_resolve` or MIR borrowck.
As we check higher ranked constraints in the full regionck anyways, the `leak_check` is not soundness critical. It's current only purpose is to move some higher ranked region errors earlier, enabling it to guide type inference and trait solving. Adding additional uses of the `leak_check` in the future would only strengthen inference and is therefore not breaking.
## Where do we use currently use the leak check
The `leak_check` is currently used in two places:
Coherence does not use a proper regionck, only relying on the `leak_check` called [at the end of the implicit negative overlap check](8b94152af6/compiler/rustc_trait_selection/src/traits/coherence.rs (L235-L238)). During coherence all parameters are instantiated with inference variables, so the only possible region errors are higher-ranked. We currently also sometimes make guesses when destructuring `TypeOutlives` constraints which can theoretically result in incorrect errors. This could result in overlapping impls.
We also use the `leak_check` [at the end of `fn evaluation_probe`](8b94152af6/compiler/rustc_trait_selection/src/traits/select/mod.rs (L607-L610)). This function is used during candidate assembly for `Trait` goals. Most notably we use [inside of `evaluate_candidate` during winnowing](0e4243538b/compiler/rustc_trait_selection/src/traits/select/mod.rs (L491-L502)). Conceptionally, it is as if we compute each candidate in a separate `enter_forall`.
## The current use in `fn evaluation_probe` is undesirable
Because we only instantiate a higher-ranked goal once inside of `fn evaluation_probe`, errors involving placeholders from that binder can impact selection. This results in inconsistent behavior ([playground](
*[playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=dac60ebdd517201788899ffa77364831)*)):
```rust
trait Leak<'a> {}
impl Leak<'_> for Box<u32> {}
impl Leak<'static> for Box<u16> {}
fn impls_leak<T: for<'a> Leak<'a>>() {}
trait IndirectLeak<'a> {}
impl<'a, T: Leak<'a>> IndirectLeak<'a> for T {}
fn impls_indirect_leak<T: for<'a> IndirectLeak<'a>>() {}
fn main() {
// ok
//
// The `Box<u16>` impls fails the leak check,
// meaning that we apply the `Box<u32>` impl.
impls_leak::<Box<_>>();
// error: type annotations needed
//
// While the `Box<u16>` impl would fail the leak check
// we have already instantiated the binder while applying
// the generic `IndirectLeak` impl, so during candidate
// selection of `Leak` we do not detect the placeholder error.
// Evaluation of `Box<_>: Leak<'!a>` is therefore ambiguous,
// resulting in `for<'a> Box<_>: Leak<'a>` also being ambiguous.
impls_indirect_leak::<Box<_>>();
}
```
We generally prefer `where`-bounds over implementations during candidate selection, both for [trait goals](11f32b73e0/compiler/rustc_trait_selection/src/traits/select/mod.rs (L1863-L1887)) and during [normalization](11f32b73e0/compiler/rustc_trait_selection/src/traits/project.rs (L184-L198)). However, we currently **do not** use the `leak_check` during candidate assembly in normalizing. This can result in inconsistent behavior:
```rust
trait Trait<'a> {
type Assoc;
}
impl<'a, T> Trait<'a> for T {
type Assoc = usize;
}
fn trait_bound<T: for<'a> Trait<'a>>() {}
fn projection_bound<T: for<'a> Trait<'a, Assoc = usize>>() {}
// A function with a trivial where-bound which is more
// restrictive than the impl.
fn function<T: Trait<'static, Assoc = usize>>() {
// ok
//
// Proving `for<'a> T: Trait<'a>` using the where-bound results
// in a leak check failure, so we use the more general impl,
// causing this to succeed.
trait_bound::<T>();
// error
//
// Proving the `Projection` goal `for<'a> T: Trait<'a, Assoc = usize>`
// does not use the leak check when trying the where-bound, causing us
// to prefer it over the impl, resulting in a placeholder error.
projection_bound::<T>();
// error
//
// Trying to normalize the type `for<'a> fn(<T as Trait<'a>>::Assoc)`
// only gets to `<T as Trait<'a>>::Assoc` once `'a` has been already
// instantiated, causing us to prefer the where-bound over the impl
// resulting in a placeholder error. Even if were were to also use the
// leak check during candidate selection for normalization, this
// case would still not compile.
let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
}
```
This is also likely to be more performant. It enables more caching in the new trait solver by simply [recursively calling the canonical query][new solver] after instantiating the higher-ranked goal.
It is also unclear how to add the leak check to normalization in the new solver. To handle https://github.com/rust-lang/trait-system-refactor-initiative/issues/1 `Projection` goals are implemented via `AliasRelate`. This again means that we instantiate the binder before ever normalizing any alias. Even if we were to avoid this, we lose the ability to [cache normalization by itself, ignoring the expected `term`](5bd5d214ef/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs (L34-L49)). We cannot replace the `term` with an inference variable before instantiating the binder, as otherwise `for<'a> T: Trait<Assoc<'a> = &'a ()>` breaks. If we only replace the term after instantiating the binder, we cannot easily evaluate the goal in a separate context, as [we'd then lose the information necessary for the leak check](11f32b73e0/compiler/rustc_next_trait_solver/src/canonicalizer.rs (L230-L232)). Adding this information to the canonical input also seems non-trivial.
## Proposed solution
I propose to instantiate the binder outside of candidate assembly, causing placeholders from higher-ranked goals to get ignored while selecting their candidate. This mostly[^1] matches the [current behavior of the new solver][new solver]. The impact of this change is therefore as follows:
```rust
trait Leak<'a> {}
impl Leak<'_> for Box<u32> {}
impl Leak<'static> for Box<u16> {}
fn impls_leak<T: for<'a> Leak<'a>>() {}
trait IndirectLeak<'a> {}
impl<'a, T: Leak<'a>> IndirectLeak<'a> for T {}
fn impls_indirect_leak<T: for<'a> IndirectLeak<'a>>() {}
fn guide_selection() {
// ok -> ambiguous
impls_leak::<Box<_>>();
// ambiguous
impls_indirect_leak::<Box<_>>();
}
trait Trait<'a> {
type Assoc;
}
impl<'a, T> Trait<'a> for T {
type Assoc = usize;
}
fn trait_bound<T: for<'a> Trait<'a>>() {}
fn projection_bound<T: for<'a> Trait<'a, Assoc = usize>>() {}
// A function which a trivial where-bound which is more
// restrictive than the impl.
fn function<T: Trait<'static, Assoc = usize>>() {
// ok -> error
trait_bound::<T>();
// error
projection_bound::<T>();
// error
let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
}
```
This does not change the behavior if candidates have higher ranked nested goals, as in this case the `leak_check` causes the nested goal to result in an error ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a74c25300b23db9022226de99d8a2fa6)):
```rust
trait LeakCheckFailure<'a> {}
impl LeakCheckFailure<'static> for () {}
trait Trait<T> {}
impl Trait<u32> for () where for<'a> (): LeakCheckFailure<'a> {}
impl Trait<u16> for () {}
fn impls_trait<T: Trait<U>, U>() {}
fn main() {
// ok
//
// It does not matter whether candidate assembly
// considers the placeholders from higher-ranked goal.
//
// Either `for<'a> (): LeakCheckFailure<'a>` has no
// applicable candidate or it has a single applicable candidate
// when then later results in an error. This allows us to
// infer `U` to `u16`.
impls_trait::<(), _>()
}
```
## Impact on existing crates
This is a **breaking change**. [A crater run](https://github.com/rust-lang/rust/pull/119820#issuecomment-1926862174) found 17 regressed crates with 7 root causes.
For a full analysis of all affected crates, see https://gist.github.com/lcnr/7c1c652f30567048ea240554a36ed95c.
---
I believe this breakage to be acceptable and would merge this change. I am confident that the new position of the leak check matches our idealized future and cannot envision any other consistent alternative. Where possible, I intend to open PRs fixing/avoiding the regressions before landing this PR.
I originally intended to remove the `coherence_leak_check` lint in the same PR. However, while I am confident in the *position* of the leak check, deciding on its exact behavior is left as future work, cc #112999. This PR therefore only moves the leak check while keeping the lint when relying on it in coherence.
[new solver]: https://github.com/rust-lang/rust/blob/master/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs#L479-L484
[^1]: the new solver has a separate cause of inconsistent behavior rn https://github.com/rust-lang/trait-system-refactor-initiative/issues/53#issuecomment-1914310171
r? `@nikomatsakis`
