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Split Bound index into Canonical and Bound
See [#t-types/trait-system-refactor > perf `async-closures/post-mono-higher-ranked-hang.rs`](https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/perf.20.60async-closures.2Fpost-mono-higher-ranked-hang.2Ers.60/with/541535613) for context
Things compile and tests pass, but not sure if this actually solves the perf issue (edit: it does). Opening up this to do a perf (and maybe crater) run.
r? lcnr
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Remove the witness type from coroutine *args* (without actually removing the type)
This does as much of rust-lang/rust#144157 as we can without having to break rust-lang/rust#143545 and/or introduce some better way of handling higher ranked assumptions.
Namely, it:
* Stalls coroutines based off of the *coroutine* type rather than the witness type.
* Reworks the dtorck constraint hack to not rely on the witness type.
* Removes the witness type from the args of the coroutine, eagerly creating the type for nested obligations when needed (auto/clone impls).
I'll experiment with actually removing the witness type in a follow-up.
r? lcnr
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Optimize region constraints
r? `@lcnr`
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This commit changes it to store a `Region` instead of a `RegionVid` for the `Var` cases:
- We avoid having to call `Region::new_var` to re-create `Region`s from
`RegionVid`s in a few places, avoiding the interning process, giving a
small perf win. (At the cost of the type allowing some invalid
combinations of values.)
- All the cases now store two `Region`s, so the commit also separates
the `ConstraintKind` (a new type) from the `sub` and `sup` arguments
in `Constraint`.
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The fields are public, so this doesn't need a method, normal
deconstruction and/or field access is good enough.
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`-Zhigher-ranked-assumptions`: Consider WF of coroutine witness when proving outlives assumptions
### TL;DR
This PR introduces an unstable flag `-Zhigher-ranked-assumptions` which tests out a new algorithm for dealing with some of the higher-ranked outlives problems that come from auto trait bounds on coroutines. See:
* rust-lang/rust#110338
While it doesn't fix all of the issues, it certainly fixed many of them, so I'd like to get this landed so people can test the flag on their own code.
### Background
Consider, for example:
```rust
use std::future::Future;
trait Client {
type Connecting<'a>: Future + Send
where
Self: 'a;
fn connect(&self) -> Self::Connecting<'_>;
}
fn call_connect<C>(c: C) -> impl Future + Send
where
C: Client + Send + Sync,
{
async move { c.connect().await }
}
```
Due to the fact that we erase the lifetimes in a coroutine, we can think of the interior type of the async block as something like: `exists<'r, 's> { C, &'r C, C::Connecting<'s> }`. The first field is the `c` we capture, the second is the auto-ref that we perform on the call to `.connect()`, and the third is the resulting future we're awaiting at the first and only await point. Note that every region is uniquified differently in the interior types.
For the async block to be `Send`, we must prove that both of the interior types are `Send`. First, we have an `exists<'r, 's>` binder, which needs to be instantiated universally since we treat the regions in this binder as *unknown*[^exist]. This gives us two types: `{ &'!r C, C::Connecting<'!s> }`. Proving `&'!r C: Send` is easy due to a [`Send`](https://doc.rust-lang.org/nightly/std/marker/trait.Send.html#impl-Send-for-%26T) impl for references.
Proving `C::Connecting<'!s>: Send` can only be done via the item bound, which then requires `C: '!s` to hold (due to the `where Self: 'a` on the associated type definition). Unfortunately, we don't know that `C: '!s` since we stripped away any relationship between the interior type and the param `C`. This leads to a bogus borrow checker error today!
### Approach
Coroutine interiors are well-formed by virtue of them being borrow-checked, as long as their callers are invoking their parent functions in a well-formed way, then substitutions should also be well-formed. Therefore, in our example above, we should be able to deduce the assumption that `C: '!s` holds from the well-formedness of the interior type `C::Connecting<'!s>`.
This PR introduces the notion of *coroutine assumptions*, which are the outlives assumptions that we can assume hold due to the well-formedness of a coroutine's interior types. These are computed alongside the coroutine types in the `CoroutineWitnessTypes` struct. When we instantiate the binder when proving an auto trait for a coroutine, we instantiate the `CoroutineWitnessTypes` and stash these newly instantiated assumptions in the region storage in the `InferCtxt`. Later on in lexical region resolution or MIR borrowck, we use these registered assumptions to discharge any placeholder outlives obligations that we would otherwise not be able to prove.
### How well does it work?
I've added a ton of tests of different reported situations that users have shared on issues like rust-lang/rust#110338, and an (anecdotally) large number of those examples end up working straight out of the box! Some limitations are described below.
### How badly does it not work?
The behavior today is quite rudimentary, since we currently discharge the placeholder assumptions pretty early in region resolution. This manifests itself as some limitations on the code that we accept.
For example, `tests/ui/async-await/higher-ranked-auto-trait-11.rs` continues to fail. In that test, we must prove that a placeholder is equal to a universal for a param-env candidate to hold when proving an auto trait, e.g. `'!1 = 'a` is required to prove `T: Trait<'!1>` in a param-env that has `T: Trait<'a>`. Unfortunately, at that point in the MIR body, we only know that the placeholder is equal to some body-local existential NLL var `'?2`, which only gets equated to the universal `'a` when being stored into the return local later on in MIR borrowck.
This could be fixed by integrating these assumptions into the type outlives machinery in a more first-class way, and delaying things to the end of MIR typeck when we know the full relationship between existential and universal NLL vars. Doing this integration today is quite difficult today.
`tests/ui/async-await/higher-ranked-auto-trait-11.rs` fails because we don't compute the full transitive outlives relations between placeholders. In that test, we have in our region assumptions that some `'!1 = '!2` and `'!2 = '!3`, but we must prove `'!1 = '!3`.
This can be fixed by computing the set of coroutine outlives assumptions in a more transitive way, or as I mentioned above, integrating these assumptions into the type outlives machinery in a more first-class way, since it's already responsible for the transitive outlives assumptions of universals.
### Moving forward
I'm still quite happy with this implementation, and I'd like to land it for testing. I may work on overhauling both the way we compute these coroutine assumptions and also how we deal with the assumptions during (lexical/nll) region checking. But for now, I'd like to give users a chance to try out this new `-Zhigher-ranked-assumptions` flag to uncover more shortcomings.
[^exist]: Instantiating this binder with infer regions would be incomplete, since we'd be asking for *some* instantiation of the interior types, not proving something for *all* instantiations of the interior types.
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This commit does the following:
- Replaces use of rustc_type_ir by rustc_middle
- Removes the rustc_type_ir dependency
- The DelayedSet type is exposed by rustc_middle so everything can be
accessed through rustc_middle in a coherent manner.
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Remove `weak` alias terminology
I find the "weak" alias terminology to be quite confusing. It implies the existence of "strong" aliases (which do not exist) and I'm not really sure what about weak aliases is "weak". I much prefer "free alias" as the term. I think it's much more obvious what it means as "free function" is a well defined term that already exists in rust.
It's also a little confusing given "weak alias" is already a term in linker/codegen spaces which are part of the compiler too. Though I'm not particularly worried about that as it's usually very obvious if you're talking about the type system or not lol. I'm also currently trying to write documentation about aliases and it's somewhat awkward/confusing to be talking about *weak* aliases, when I'm not really sure what the basis for that as the term actually *is*.
I would also be happy to just find out there's a nice meaning behind calling them "weak" aliases :-)
r? `@oli-obk`
maybe we want a types MCP to decide on a specific naming here? or maybe we think its just too late to go back on this naming decision ^^'
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I found these by grepping for `&[a-z_\.]*\.clone()`, i.e. expressions
like `&a.b.clone()`, which are sometimes unnecessary clones, and also
looking at clones nearby to cases like that.
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There's an existing fast path for the `type_op_prove_predicate`
predicate, checking for trivially `Sized` types, which can be re-used
when evaluating obligations within queries. This should improve
performance, particularly in anticipation of new sizedness traits being
added which can take advantage of this.
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Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>
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bounds hack
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- Remove `Result` that couldn't be Err on valid compilation.
- Always compute errors on failure.
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HIR type checking no longer runs dropck, so we may get new errors when
we run it in borrowck. If this happens then rerun the query in a local
infcx and report errors for it.
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Revert #135914: Remove usages of `QueryNormalizer` in the compiler
Reverts #135914.
r? jackh726
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r=jackh726"
This reverts commit 556d901c36511560e0ae8ce3058507121a2fb2f0, reversing
changes made to be15391703babf217aaef3c854213a7fcd70e00b.
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