by using an opaque type obligation to bubble up comparisons between opaque types and other types
Also uses proper obligation causes so that the body id works, because out of some reason nll uses body ids for logic instead of just diagnostics.
Continue work on associated const equality
This actually implements some more complex logic for assigning associated consts to values.
Inside of projection candidates, it now defers to a separate function for either consts or
types. To reduce amount of code, projections are now generic over T, where T is either a Type or
a Const. I can add some comments back later, but this was the fastest way to implement it.
It also now finds the correct type of consts in type_of.
---
The current main TODO is finding the const of the def id for the LeafDef.
Right now it works if the function isn't called, but once you use the trait impl with the bound it fails inside projection.
I was hoping to get some help in getting the `&'tcx ty::Const<'tcx>`, in addition to a bunch of other `todo!()`s which I think may not be hit.
r? `@oli-obk`
Updates #92827
Add note suggesting that predicate may be satisfied, but is not `const`
Not sure if we should be printing this in addition to, or perhaps _instead_ of the help message:
```
help: the trait `~const Add` is not implemented for `NonConstAdd`
```
Also added `ParamEnv::is_const` and `PolyTraitPredicate::is_const_if_const` and, in a separate commit, used those in other places instead of `== hir::Constness::Const`, etc.
r? ````@fee1-dead````
Only traverse attrs once while checking for coherence override attributes
In coherence, while checking for negative impls override attributes: only traverse the `DefId`s' attributes once.
This PR is an easy way to get back some of the small perf loss in #93175
Store a `Symbol` instead of an `Ident` in `AssocItem`
This is the same idea as #92533, but for `AssocItem` instead
of `VariantDef`/`FieldDef`.
With this change, we no longer have any uses of
`#[stable_hasher(project(...))]`
Properly track `DepNode`s in trait evaluation provisional cache
Fixes#92987
During evaluation of an auto trait predicate, we may encounter a cycle.
This causes us to store the evaluation result in a special 'provisional
cache;. If we later end up determining that the type can legitimately
implement the auto trait despite the cycle, we remove the entry from
the provisional cache, and insert it into the evaluation cache.
Additionally, trait evaluation creates a special anonymous `DepNode`.
All queries invoked during the predicate evaluation are added as
outoging dependency edges from the `DepNode`. This `DepNode` is then
store in the evaluation cache - if a different query ends up reading
from the cache entry, it will also perform a read of the stored
`DepNode`. As a result, the cached evaluation will still end up
(transitively) incurring all of the same dependencies that it would
if it actually performed the uncached evaluation (e.g. a call to
`type_of` to determine constituent types).
Previously, we did not correctly handle the interaction between the
provisional cache and the created `DepNode`. Storing an evaluation
result in the provisional cache would cause us to lose the `DepNode`
created during the evaluation. If we later moved the entry from the
provisional cache to the evaluation cache, we would use the `DepNode`
associated with the evaluation that caused us to 'complete' the cycle,
not the evaluatoon where we first discovered the cycle. As a result,
future reads from the evaluation cache would miss some incremental
compilation dependencies that would have otherwise been added if the
evaluation was *not* cached.
Under the right circumstances, this could lead to us trying to force
a query with a no-longer-existing `DefPathHash`, since we were missing
the (red) dependency edge that would have caused us to bail out before
attempting forcing.
This commit makes the provisional cache store the `DepNode` create
during the provisional evaluation. When we move an entry from the
provisional cache to the evaluation cache, we create a *new* `DepNode`
that has dependencies going to *both* of the evaluation `DepNodes` we
have available. This ensures that cached reads will incur all of
the necessary dependency edges.
Check `const Drop` impls considering `~const` Bounds
This PR adds logic to trait selection to account for `~const` bounds in custom `impl const Drop` for types, elaborates the `const Drop` check in `rustc_const_eval` to check those bounds, and steals some drop linting fixes from #92922, thanks `@DrMeepster.`
r? `@fee1-dead` `@oli-obk` <sup>(edit: guess I can't request review from two people, lol)</sup>
since each of you wrote and reviewed #88558, respectively.
Since the logic here is more complicated than what existed, it's possible that this is a perf regression. But it works correctly with tests, and that makes me happy.
Fixes#92881
- Also rename a trivial_const_drop to match style of other functions in
the util module.
- Also add a test for `const Drop` that doesn't depend on a `~const`
bound.
- Also comment a bit why we remove the const bound during dropck impl
check.
This is the same idea as #92533, but for `AssocItem` instead
of `VariantDef`/`FieldDef`.
With this change, we no longer have any uses of
`#[stable_hasher(project(...))]`
Fixes#92987
During evaluation of an auto trait predicate, we may encounter a cycle.
This causes us to store the evaluation result in a special 'provisional
cache;. If we later end up determining that the type can legitimately
implement the auto trait despite the cycle, we remove the entry from
the provisional cache, and insert it into the evaluation cache.
Additionally, trait evaluation creates a special anonymous `DepNode`.
All queries invoked during the predicate evaluation are added as
outoging dependency edges from the `DepNode`. This `DepNode` is then
store in the evaluation cache - if a different query ends up reading
from the cache entry, it will also perform a read of the stored
`DepNode`. As a result, the cached evaluation will still end up
(transitively) incurring all of the same dependencies that it would
if it actually performed the uncached evaluation (e.g. a call to
`type_of` to determine constituent types).
Previously, we did not correctly handle the interaction between the
provisional cache and the created `DepNode`. Storing an evaluation
result in the provisional cache would cause us to lose the `DepNode`
created during the evaluation. If we later moved the entry from the
provisional cache to the evaluation cache, we would use the `DepNode`
associated with the evaluation that caused us to 'complete' the cycle,
not the evaluatoon where we first discovered the cycle. As a result,
future reads from the evaluation cache would miss some incremental
compilation dependencies that would have otherwise been added if the
evaluation was *not* cached.
Under the right circumstances, this could lead to us trying to force
a query with a no-longer-existing `DefPathHash`, since we were missing
the (red) dependency edge that would have caused us to bail out before
attempting forcing.
This commit makes the provisional cache store the `DepNode` create
during the provisional evaluation. When we move an entry from the
provisional cache to the evaluation cache, we create a *new* `DepNode`
that has dependencies going to *both* of the evaluation `DepNodes` we
have available. This ensures that cached reads will incur all of
the necessary dependency edges.
Directly use ConstValue for single literals in blocks
Addresses the minimal repro in https://github.com/rust-lang/rust/issues/92186, but doesn't fix the underlying problem (which would be solved by solving the anon subst problem afaict).
I do, however, think that it makes sense in general to treat single literals in anon blocks as const values directly, especially in light of the problem that the issue refers to (anon const evaluation being postponed until infer variables in substs can be resolved, which was introduced by https://github.com/rust-lang/rust/pull/90023), i.e. while we do get warnings for those unnecessary braces, we should try to avoid errors caused by those braces if possible.
Replace use of `ty()` on term and use it in more places. This will allow more flexibility in the
future, but slightly worried it allows items which are consts which only accept types.
ProjectionPredicate should be able to handle both associated types and consts so this adds the
first step of that. It mainly just pipes types all the way down, not entirely sure how to handle
consts, but hopefully that'll come with time.
Replace `NestedVisitorMap` with generic `NestedFilter`
This is an attempt to make the `intravisit::Visitor` API simpler and "more const" with regard to nested visiting.
With this change, `intravisit::Visitor` does not visit nested things by default, unless you specify `type NestedFilter = nested_filter::OnlyBodies` (or `All`). `nested_visit_map` returns `Self::Map` instead of `NestedVisitorMap<Self::Map>`. It panics by default (unreachable if `type NestedFilter` is omitted).
One somewhat trixty thing here is that `nested_filter::{OnlyBodies, All}` live in `rustc_middle` so that they may have `type Map = map::Map` and so that `impl Visitor`s never need to specify `type Map` - it has a default of `Self::NestedFilter::Map`.
Remove deprecated LLVM-style inline assembly
The `llvm_asm!` was deprecated back in #87590 1.56.0, with intention to remove
it once `asm!` was stabilized, which already happened in #91728 1.59.0. Now it
is time to remove `llvm_asm!` to avoid continued maintenance cost.
Closes#70173.
Closes#92794.
Closes#87612.
Closes#82065.
cc `@rust-lang/wg-inline-asm`
r? `@Amanieu`
Include Projections when elaborating TypeOutlives
Fixes#92280
In `Elaborator`, we elaborate that `Foo<<Bar as Baz>::Assoc>: 'a` -> `<Bar as Baz>::Assoc: 'a`. This is the same rule that would be applied to any other `Param`. If there are escaping vars, we continue to do nothing.
r? `@nikomatsakis`
Prefer projection candidates instead of param_env candidates for Sized predicates
Fixes#89352
Also includes some drive by logging and verbose printing changes that I found useful when debugging this, but I can remove this if needed.
This is a little hacky - but imo no more than the rest of `candidate_should_be_dropped_in_favor_of`. Importantly, in a Chalk-like world, both candidates should be completely compatible.
r? ```@nikomatsakis```
Welcome opaque types into the fold
r? ```@nikomatsakis``` because idk who else to bug on the type_op changes
The commits have explanations in them. The TLDR is that
* 5c46002273 stops the "recurse and replace" scheme that replaces opaque types with their canonical inference var by just doing that ahead of time
* bdeeb07bf6 does not affect anything on master afaict, but since opaque types generate obligations when instantiated, and lazy TAIT instantiates opaque types *everywhere*, we need to properly handle obligations here instead of just hoping no problematic obligations ever come up.
Replace usages of vec![].into_iter with [].into_iter
`[].into_iter` is idiomatic over `vec![].into_iter` because its simpler and faster (unless the vec is optimized away in which case it would be the same)
So we should change all the implementation, documentation and tests to use it.
I skipped:
* `src/tools` - Those are copied in from upstream
* `src/test/ui` - Hard to tell if `vec![].into_iter` was used intentionally or not here and not much benefit to changing it.
* any case where `vec![].into_iter` was used because we specifically needed a `Vec::IntoIter<T>`
* any case where it looked like we were intentionally using `vec![].into_iter` to test it.
Normalize struct tail type when checking Pointee trait
Let's go ahead and implement the FIXMEs by properly normalizing the struct-tail type when satisfying a Pointee obligation. This should fix the ICE when we try to calculate a layout depending on `<Ty as Pointee>::Metadata` later.
Fixes#92128Fixes#92577
Additionally, mark the obligation as ambiguous if there are any infer types in that struct-tail type. This has the effect of causing `<_ as Pointee>::Metadata` to be properly replaced with an infer variable ([here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_trait_selection/src/traits/project.rs#L813)) and registered as an obligation... this turns out to be very important in unifying function parameters with formals that are assoc types.
Fixes#91446
rustc_metadata: Encode list of all crate's traits into metadata
While working on https://github.com/rust-lang/rust/pull/88679 I noticed that rustdoc is casually doing something quite expensive, something that is used only for error reporting in rustc - collecting all traits from all crates in the dependency tree.
This PR trades some minor extra time spent by metadata encoder in rustc for major gains for rustdoc (and for rustc runs with errors, which execute the `all_traits` query for better diagnostics).
Ignore other `PredicateKind`s in rustdoc auto trait finder
Fixes#92073
There's not really anything we can do with them, and they're
causing ICEs. I'm not using a wildcard match, as we should check
that any new `PredicateKind`s are handled properly by rustdoc.
Remove 'speculative evaluation' of predicates
Performing 'speculative evaluation' introduces caching bugs that
cannot be fixed without invasive changes to projection.
Hopefully, we can win back most of the performance lost by
re-adding 'cache completion'
Fixes#90662
This makes `Obligation` two words bigger, but avoids allocating a lot of
the time.
I previously tried this in #73983 and it didn't help much, but local
timings look more promising now.
Remove `SymbolStr`
This was originally proposed in https://github.com/rust-lang/rust/pull/74554#discussion_r466203544. As well as removing the icky `SymbolStr` type, it allows the removal of a lot of `&` and `*` occurrences.
Best reviewed one commit at a time.
r? `@oli-obk`
Instead of clearing out the cache entirely, we store
the intermediate evaluation result into the cache entry.
This accomplishes several things:
* We avoid the performance hit associated with re-evaluating
the sub-obligations
* We avoid causing issues with incremental compilation, since
the final evaluation result is always the same
* We avoid affecting other uses of the same `InferCtxt` which
might care about 'side effects' from processing the sub-obligations
(e,g. region constraints). Only code that is specifically aware
of the new 'complete' code is affected
Fixes#92073
There's not really anything we can do with them, and they're
causing ICEs. I'm not using a wildcard match, as we should check
that any new `PredicateKind`s are handled properly by rustdoc.
Performing 'speculative evaluation' introduces caching bugs that
cannot be fixed without invasive changes to projection.
Hopefully, we can win back most of the performance lost by
re-adding 'cache completion'
Fixes#90662
Sometimes an obligation depends on a later one, so we can't just process them in order like it was done previously.
This is not a problem in our test suite, but there may be ICEs out there and it will definitely be a problem with lazy TAIT.
extend `simplify_type`
might cause a slight perf inprovement and imo more accurately represents what types there are.
considering that I was going to use this in #85048 it seems like we might need this in the future anyways 🤷
Stabilize `iter::zip`
Hello all!
As the tracking issue (#83574) for `iter::zip` completed the final commenting period without any concerns being raised, I hereby submit this stabilization PR on the issue.
As the pull request that introduced the feature (#82917) states, the `iter::zip` function is a shorter way to zip two iterators. As it's generally a quality-of-life/ergonomic improvement, it has been integrated into the codebase without any trouble, and has been
used in many places across the rust compiler and standard library since March without any issues.
For more details, I would refer to `@cuviper's` original PR, or the [function's documentation](https://doc.rust-lang.org/std/iter/fn.zip.html).
* Point at RHS of associated type in obligation span
* Point at `impl` assoc type on projection error
* Reduce verbosity of recursive obligations
* Point at source of binding lifetime obligation
* Tweak "required bound" note
* Tweak "expected... found opaque (return) type" labels
* Point at set type in impl assoc type WF errors
Skip reborrows in AbstractConstBuilder
Fixes https://github.com/rust-lang/rust/issues/90455
Temporary fix to prevent confusing diagnostics that refer to implicit borrows and derefs until we allow borrows and derefs on constant expressions.
r? `@oli-obk`
Keep spans for generics in `#[derive(_)]` desugaring
Keep the spans for generics coming from a `derive`d Item, so that errors
and suggestions have better detail.
Fix#84003.
* Annotate `derive`d spans from the user's code with the appropciate context
* Add `Span::can_be_used_for_suggestion` to query if the underlying span
at the users' code
Rollup of 12 pull requests
Successful merges:
- #89954 (Fix legacy_const_generic doc arguments display)
- #91321 (Handle placeholder regions in NLL type outlive constraints)
- #91329 (Fix incorrect usage of `EvaluatedToOk` when evaluating `TypeOutlives`)
- #91364 (Improve error message for incorrect field accesses through raw pointers)
- #91387 (Clarify and tidy up explanation of E0038)
- #91410 (Move `#![feature(const_precise_live_drops)]` checks earlier in the pipeline)
- #91435 (Improve diagnostic for missing half of binary operator in `if` condition)
- #91444 (disable tests in Miri that take too long)
- #91457 (Add additional test from rust issue number 91068)
- #91460 (Document how `last_os_error` should be used)
- #91464 (Document file path case sensitivity)
- #91466 (Improve the comments in `Symbol::interner`.)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
Fix incorrect usage of `EvaluatedToOk` when evaluating `TypeOutlives`
A global predicate is not guarnatenteed to outlive all regions.
If the predicate involves late-bound regions, then it may fail
to outlive other regions (e.g. `for<'b> &'b bool: 'static` does not
hold)
We now only produce `EvaluatedToOk` when a global predicate has no
late-bound regions - in that case, the ony region that can be present
in the type is 'static
Cleanup: Eliminate ConstnessAnd
This is almost a behaviour-free change and purely a refactoring. "almost" because we appear to be using the wrong ParamEnv somewhere already, and this is now exposed by failing a test using the unstable `~const` feature.
We most definitely need to review all `without_const` and at some point should probably get rid of many of them by using `TraitPredicate` instead of `TraitRef`.
This is a continuation of https://github.com/rust-lang/rust/pull/90274.
r? `@oli-obk`
cc `@spastorino` `@ecstatic-morse`
A global predicate is not guarnatenteed to outlive all regions.
If the predicate involves late-bound regions, then it may fail
to outlive other regions (e.g. `for<'b> &'b bool: 'static` does not
hold)
We now only produce `EvaluatedToOk` when a global predicate has no
late-bound regions - in that case, the ony region that can be present
in the type is 'static
Make `TypeFolder::fold_*` return `Result`
Implements rust-lang/compiler-team#432.
Initially this is just a rebase of `@LeSeulArtichaut's` work in #85469 (abandoned; see https://github.com/rust-lang/rust/pull/85485#issuecomment-908781112). At that time, it caused a regression in performance that required some further exploration... with this rebased PR bors can hopefully report some perf analysis from which we can investigate further (if the regression is indeed still present).
r? `@jackh726` cc `@nikomatsakis`
Prior to PR #91205, checking for errors in the overall obligation
would check checking the `ParamEnv`, due to an incorrect
`super_visit_with` impl. With this bug fixed, we will now
bail out of impl candidate assembly if the `ParamEnv` contains
any error types.
In practice, this appears to be overly conservative - when an error
occurs early in compilation, we end up giving up early for some
predicates that we could have successfully evaluated without overflow.
By only checking for errors in the predicate itself, we avoid causing
additional spurious 'type annotations needed' errors after a 'real'
error has already occurred.
With this PR, the diagnostic changes caused by PR #91205 are reverted.
Be more thorough in using `ItemObligation` and `BindingObligation` when
evaluating obligations so that we can point at trait bounds that
introduced unfulfilled obligations. We no longer incorrectly point at
unrelated trait bounds (`substs-ppaux.verbose.stderr`).
In particular, we now point at trait bounds on method calls.
We no longer point at "obvious" obligation sources (we no longer have a
note pointing at `Trait` saying "required by a bound in `Trait`", like
in `associated-types-no-suitable-supertrait*`).
Address part of #89418.
This function parameter attribute was introduced in https://github.com/rust-lang/rust/pull/44866 as an intermediate step in implementing `impl Trait`, it's not necessary or used anywhere by itself.
Fix span for non-satisfied trivial trait bounds
The spans for "trait bound not satisfied" errors in trivial trait bounds referenced the entire item (fn, impl, struct) before.
Now they only reference the obligation itself (`String: Copy`)
Address #90869
Fixes incorrect handling of TraitRefs when emitting suggestions.
Closes#90804 , although there were more issues here that were hidden by the thing that caused this ICE.
Underlying problem was that substitutions were being thrown out, which not only leads to an ICE but also incorrect diagnostics. On top of that, in some cases the self types from the root obligations were being mixed in with those from derived obligations.
This makes a couple diagnostics arguable worse ("`B<C>` does not implement `Copy`" instead of "`C` does not implement `Copy`") but the worse diagnostics are at least still correct and that downside is in my opinion clearly outweighed by the benefits of fixing the ICE and unambiguously wrong diagnostics.
The spans for "trait bound not satisfied" errors in trivial trait bounds referenced the entire item (fn, impl, struct) before.
Now they only reference the obligation itself (`String: Copy`)
Address #90869
When suggesting references, substitutions were being forgotten and some types were misused. This led to at
least one ICE and other incorrectly emitted diagnostics. This has been fixed; in some cases this leads to
diagnostics changing, and tests have been adjusted.
Rollup of 7 pull requests
Successful merges:
- #89561 (Type inference for inline consts)
- #90035 (implement rfc-2528 type_changing-struct-update)
- #90613 (Allow to run a specific rustdoc-js* test)
- #90683 (Make `compiler-docs` only control the default instead of being a hard off-switch)
- #90685 (x.py: remove fixme by deleting code)
- #90701 (Record more artifact sizes during self-profiling.)
- #90723 (Better document `Box` and `alloc::alloc::box_free` connection)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
Type inference for inline consts
Fixes#78132Fixes#78174Fixes#81857Fixes#89964
Perform type checking/inference of inline consts in the same context as the outer def, similar to what is currently done to closure.
Doing so would require `closure_base_def_id` of the inline const to return the outer def, and since `closure_base_def_id` can be called on non-local crate (and thus have no HIR available), a new `DefKind` is created for inline consts.
The type of the generated anon const can capture lifetime of outer def, so we couldn't just use the typeck result as the type of the inline const's def. Closure has a similar issue, and it uses extra type params `CK, CS, U` to capture closure kind, input/output signature and upvars. I use a similar approach for inline consts, letting it have an extra type param `R`, and then `typeof(InlineConst<[paremt generics], R>)` would just be `R`. In borrowck region requirements are also propagated to the outer MIR body just like it's currently done for closure.
With this PR, inline consts in expression position are quitely usable now; however the usage in pattern position is still incomplete -- since those does not remain in the MIR borrowck couldn't verify the lifetime there. I have left an ignored test as a FIXME.
Some disucssions can be found on [this Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/inline.20consts.20typeck).
cc `````@spastorino````` `````@lcnr`````
r? `````@nikomatsakis`````
`````@rustbot````` label A-inference F-inline_const T-compiler
Use `is_global` in `candidate_should_be_dropped_in_favor_of`
This manifistated in #90195 with compiler being unable to keep
one candidate for a trait impl, if where is a global impl and more
than one trait bound in the where clause.
Before #87280 `candidate_should_be_dropped_in_favor_of` was using
`TypeFoldable::is_global()` that was enough to discard the two
`ParamCandidate`s. But #87280 changed it to use
`TypeFoldable::is_known_global()` instead, which is pessimistic, so
now the compiler drops the global impl instead (because
`is_known_global` is not sure) and then can't decide between the
two `ParamCandidate`s.
Switching it to use `is_global` again solves the issue.
Fixes#90195.
Revert "Add rustc lint, warning when iterating over hashmaps"
Fixes perf regressions introduced in https://github.com/rust-lang/rust/pull/90235 by temporarily reverting the relevant PR.
This manifistated in #90195 with compiler being unable to keep
one candidate for a trait impl, if where is a global impl and more
than one trait bound in the where clause.
Before #87280 `candidate_should_be_dropped_in_favor_of` was using
`TypeFoldable::is_global()` that was enough to discard the two
`ParamCandidate`s. But #87280 changed it to use
`TypeFoldable::is_known_global()` instead, which is pessimistic, so
now the compiler drops the global impl instead (because
`is_known_global` is not sure) and then can't decide between the
two `ParamCandidate`s.
Switching it to use `is_global` again solves the issue.
Fixes#90195.
Use the "nice E0277 errors"[1] for `!Send` `impl Future` from foreign crate
Partly address #78543 by making the error quieter.
We don't have access to the `typeck` tables from foreign crates, so we
used to completely skip the new code when checking foreign crates. Now,
we carry on and don't provide as nice output (we don't clarify *what* is
making the `Future: !Send`), but at least we no longer emit a sea of
derived obligations in the output.
[1]: https://blog.rust-lang.org/inside-rust/2019/10/11/AsyncAwait-Not-Send-Error-Improvements.html
r? `@tmandry`
Partly address #78543 by making the error quieter.
We don't have access to the `typeck` tables from foreign crates, so we
used to completely skip the new code when checking foreign crates. Now,
we carry on and don't provide as nice output (we don't clarify *what* is
making the `Future: !Send`), but at least we no longer emit a sea of
derived obligations in the output.
[1]: https://blog.rust-lang.org/inside-rust/2019/10/11/AsyncAwait-Not-Send-Error-Improvements.html
Implement coherence checks for negative trait impls
The main purpose of this PR is to be able to [move Error trait to core](https://github.com/rust-lang/project-error-handling/issues/3).
This feature is necessary to handle the following from impl on box.
```rust
impl From<&str> for Box<dyn Error> { ... }
```
Without having negative traits affect coherence moving the error trait into `core` and moving that `From` impl to `alloc` will cause the from impl to no longer compiler because of a potential future incompatibility. The compiler indicates that `&str` _could_ introduce an `Error` impl in the future, and thus prevents the `From` impl in `alloc` that would cause overlap with `From<E: Error> for Box<dyn Error>`. Adding `impl !Error for &str {}` with the negative trait coherence feature will disable this error by encoding a stability guarantee that `&str` will never implement `Error`, making the `From` impl compile.
We would have this in `alloc`:
```rust
impl From<&str> for Box<dyn Error> {} // A
impl<E> From<E> for Box<dyn Error> where E: Error {} // B
```
and this in `core`:
```rust
trait Error {}
impl !Error for &str {}
```
r? `@nikomatsakis`
This PR was built on top of `@yaahc` PR #85764.
Language team proposal: to https://github.com/rust-lang/lang-team/issues/96
This doesn't work properly yet, we would probably need to implement an
`assembly_neg_candidates` and consider things like `T: !AB` as `T: !A`
|| `T: !B`
Adopt let_else across the compiler
This performs a substitution of code following the pattern:
```
let <id> = if let <pat> = ... { identity } else { ... : ! };
```
To simplify it to:
```
let <pat> = ... { identity } else { ... : ! };
```
By adopting the `let_else` feature (cc #87335).
The PR also updates the syn crate because the currently used version of the crate doesn't support `let_else` syntax yet.
Note: Generally I'm the person who *removes* usages of unstable features from the compiler, not adds more usages of them, but in this instance I think it hopefully helps the feature get stabilized sooner and in a better state. I have written a [comment](https://github.com/rust-lang/rust/issues/87335#issuecomment-944846205) on the tracking issue about my experience and what I feel could be improved before stabilization of `let_else`.
Remove redundant member-constraint check
impl trait will, for each lifetime in the hidden type, register a "member constraint" that says the lifetime must be equal or outlive one of the lifetimes of the impl trait. These member constraints will be solved by borrowck
But, as you can see in the big red block of removed code, there was an ad-hoc check for member constraints happening at the site where they get registered. This check had some minor effects on diagnostics, but will fall down on its feet with my big type alias impl trait refactor. So we removed it and I pulled the removal out into a (hopefully) reviewable PR that works on master directly.
This performs a substitution of code following the pattern:
let <id> = if let <pat> = ... { identity } else { ... : ! };
To simplify it to:
let <pat> = ... { identity } else { ... : ! };
By adopting the let_else feature.
Add `const_eval_select` intrinsic
Adds an intrinsic that calls a given function when evaluated at compiler time, but generates a call to another function when called at runtime.
See https://github.com/rust-lang/const-eval/issues/7 for previous discussion.
r? `@oli-obk.`
