Reword resolve errors caused by likely missing crate in dep tree
Reword label and add `help`:
```
error[E0432]: unresolved import `some_novel_crate`
--> f704.rs:1:5
|
1 | use some_novel_crate::Type;
| ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `some_novel_crate`
|
= help: if you wanted to use a crate named `some_novel_crate`, use `cargo add some_novel_crate` to add it to your `Cargo.toml`
```
Fix#133137.
Properly report error when object type param default references self
I accidentally broke this error for cases where a type parameter references `Self` via a projection (i.e. `trait Foo<Arg = Self::Bar> {}`). This PR fixes that, and also makes the error a bit easier to understand.
Fixes#135918
```
error[E0432]: unresolved import `some_novel_crate`
--> file.rs:1:5
|
1 | use some_novel_crate::Type;
| ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `some_novel_crate`
```
On resolve errors where there might be a missing crate, mention `cargo add foo`:
```
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
--> $DIR/conflicting-impl-with-err.rs:4:11
|
LL | impl From<nope::Thing> for Error {
| ^^^^ use of unresolved module or unlinked crate `nope`
|
= help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
```
- Convert `run-pass` to `check-pass`, the test is about closure
inference based on expected type, does not need to run-pass.
- Dropped unnecessary ignores.
- Change test to check only.
- Don't ignore `wasm` or `sgx`.
- Gate test to be Unix only because Unix `CommandExt` influences the
suggestion.
- Run rustfix on the suggestion.
Use `structurally_normalize` instead of manual `normalizes-to` goals in alias relate errors
r? `@lcnr`
I added `structurally_normalize_term` so that code that is generic over ty or const can use the structurally normalize helpers. See `tests/ui/traits/next-solver/diagnostics/alias_relate_error_uses_structurally_normalize.rs` for a description of the reason for the (now fixed) ICEs
This CL makes a number of small changes to dyn compatibility errors:
- "object safety" has been renamed to "dyn-compatibility" throughout
- "Convert to enum" suggestions are no longer generated when there
exists a type-generic impl of the trait or an impl for `dyn OtherTrait`
- Several error messages are reorganized for user readability
Additionally, the dyn compatibility error creation code has been
split out into functions.
cc #132713
cc #133267
remove support for the (unstable) #[start] attribute
As explained by `@Noratrieb:`
`#[start]` should be deleted. It's nothing but an accidentally leaked implementation detail that's a not very useful mix between "portable" entrypoint logic and bad abstraction.
I think the way the stable user-facing entrypoint should work (and works today on stable) is pretty simple:
- `std`-using cross-platform programs should use `fn main()`. the compiler, together with `std`, will then ensure that code ends up at `main` (by having a platform-specific entrypoint that gets directed through `lang_start` in `std` to `main` - but that's just an implementation detail)
- `no_std` platform-specific programs should use `#![no_main]` and define their own platform-specific entrypoint symbol with `#[no_mangle]`, like `main`, `_start`, `WinMain` or `my_embedded_platform_wants_to_start_here`. most of them only support a single platform anyways, and need cfg for the different platform's ways of passing arguments or other things *anyways*
`#[start]` is in a super weird position of being neither of those two. It tries to pretend that it's cross-platform, but its signature is a total lie. Those arguments are just stubbed out to zero on ~~Windows~~ wasm, for example. It also only handles the platform-specific entrypoints for a few platforms that are supported by `std`, like Windows or Unix-likes. `my_embedded_platform_wants_to_start_here` can't use it, and neither could a libc-less Linux program.
So we have an attribute that only works in some cases anyways, that has a signature that's a total lie (and a signature that, as I might want to add, has changed recently, and that I definitely would not be comfortable giving *any* stability guarantees on), and where there's a pretty easy way to get things working without it in the first place.
Note that this feature has **not** been RFCed in the first place.
*This comment was posted [in May](https://github.com/rust-lang/rust/issues/29633#issuecomment-2088596042) and so far nobody spoke up in that issue with a usecase that would require keeping the attribute.*
Closes https://github.com/rust-lang/rust/issues/29633
try-job: x86_64-gnu-nopt
try-job: x86_64-msvc-1
try-job: x86_64-msvc-2
try-job: test-various
Rework dyn trait lowering to stop being so intertwined with trait alias expansion
This PR reworks the trait object lowering code to stop handling trait aliases so funky, and removes the `TraitAliasExpander` in favor of a much simpler design. This refactoring is important for making the code that I'm writing in https://github.com/rust-lang/rust/pull/133397 understandable and easy to maintain, so the diagnostics regressions are IMO inevitable.
In the old trait object lowering code, we used to be a bit sloppy with the lists of traits in their unexpanded and expanded forms. This PR largely rewrites this logic to expand the trait aliases *once* and handle them more responsibly throughout afterwards.
Please review this with whitespace disabled.
r? lcnr
Some small nits to the borrowck suggestions for mutating a map through index
1. Suggesting users to either use `.insert` or `.get_mut` (which do totally different things) can be a bit of a footgun, so let's make that a bit more nuanced.
2. I find the suggestion of `.get_mut(|val| { *val = whatever; })` to be a bit awkward. I changed this to be an if-let instead.
3. Fix a bug which was suppressing the structured suggestion for some mutations via the index operator on `HashMap`/`BTreeMap`.
r? estebank or reassign
borrowck diagnostics: make `add_move_error_suggestions` use the HIR rather than `SourceMap`
This PR aims to fix#132806 by rewriting `add_move_error_suggestions`[^1]. Previously, it manually scanned the source text to find a leading `&`, which isn't always going to produce a correct result (see: that issue). Admittedly, the HIR visitor in this PR introduces a lot of boilerplate, but hopefully the logic at its core isn't too complicated (I go over it in the comments). I also tried a simpler version that didn't use a HIR visitor and suggested adding `ref` always, but the `&ref x` suggestions really didn't look good. As a bonus for the added complexity though, it's now able to produce nice `&`-removing suggestions in more cases.
I tried to do this such that it avoids edition-dependent checks and its suggestions can be applied together with those from the match ergonomics 2024 migration lint. I haven't added tests for that since the details of match ergonomics 2024 are still being sorted out, but I can try if desired once that's finalized.
[^1]: In brief, it fires on patterns where users try to bind by-value in such a way that moves out of a reference to a non-Copy type (including slice references with non-copy elements). The suggestions are to change the binding's mode to be by-reference, either by removing[^2] an enclosing `&`/`&mut` or adding `ref` to the binding.
[^2]: Incidentally, I find the terminology of "consider removing the borrow" a bit confusing for a suggestion to remove a `&` pattern in order to make bindings borrow rather than move. I'm not sure what a good, concise way to explain that would be though, and that should go in a separate PR anyway.
Fix ICE when multiple supertrait substitutions need assoc but only one is provided
Dyn traits must have all of their associated types constrained either by:
1. writing them in the dyn trait itself as an associated type bound, like `dyn Iterator<Item = u32>`,
2. A supertrait bound, like `trait ConstrainedIterator: Iterator<Item = u32> {}`, then you may write `dyn ConstrainedIterator` which doesn't need to mention `Item`.
However, the object type lowering code did not consider the fact that there may be multiple supertraits with different substitutions, so it just used the associated type's *def id* as a key for keeping track of which associated types are missing:
1fc691e6dd/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs (L131)
This means that we can have missing associated types when there are mutliple supertraits with different substitutions and only one of them is constrained, like:
```rust
trait Sup<T> {
type Assoc: Default;
}
impl<T: Default> Sup<T> for () {
type Assoc = T;
}
impl<T: Default, U: Default> Dyn<T, U> for () {}
trait Dyn<A, B>: Sup<A, Assoc = A> + Sup<B> {}
```
The above example allows you to name `<dyn Dyn<i32, u32> as Sup<u32>>::Assoc` even though it is not possible to project since it's neither constrained by a manually written projection bound or a supertrait bound. This successfully type-checks, but leads to a codegen ICE since we are not able to project the associated type.
This PR fixes the validation for checking that a dyn trait mentions all of its associated type bounds. This is theoretically a breaking change, since you could technically use that `dyn Dyn<A, B>` type mentionedin the example above without actually *projecting* to the bad associated type, but I don't expect it to ever be relevant to a user since it's almost certainly a bug. This is corroborated with the crater results[^crater], which show no failures[^unknown].
Crater: https://github.com/rust-lang/rust/pull/133392#issuecomment-2508769703Fixes#133388
[^crater]: I cratered this originally with #133397, which is a PR that is stacked on top, then re-ran crater with just the failures from that PR.
[^unknown]: If you look at the crater results, it shows all of the passes as "unknown". I believe this is a crater bug, since looking at the results manually shows them as passes.
Suggest using deref in patterns
Fixes#132784
This changes the following code:
```rs
use std::sync::Arc;
fn main() {
let mut x = Arc::new(Some(1));
match x {
Some(_) => {}
None => {}
}
}
```
to output
```rs
error[E0308]: mismatched types
--> src/main.rs:5:9
|
LL | match x {
| - this expression has type `Arc<Option<{integer}>>`
...
LL | Some(_) => {}
| ^^^^^^^ expected `Arc<Option<{integer}>>`, found `Option<_>`
|
= note: expected struct `Arc<Option<{integer}>>`
found enum `Option<_>`
help: consider dereferencing to access the inner value using the Deref trait
|
LL | match *x {
| ~~
```
instead of
```rs
error[E0308]: mismatched types
--> src/main.rs:5:9
|
4 | match x {
| - this expression has type `Arc<Option<{integer}>>`
5 | Some(_) => {}
| ^^^^^^^ expected `Arc<Option<{integer}>>`, found `Option<_>`
|
= note: expected struct `Arc<Option<{integer}>>`
found enum `Option<_>`
```
This makes it more obvious that a Deref is available, and gives a suggestion on how to use it in order to fix the issue at hand.
Rollup of 8 pull requests
Successful merges:
- #134252 (Fix `Path::is_absolute` on Hermit)
- #134254 (Fix building `std` for Hermit after `c_char` change)
- #134255 (Update includes in `/library/core/src/error.rs`.)
- #134261 (Document the symbol Visibility enum)
- #134262 (Arbitrary self types v2: adjust diagnostic.)
- #134265 (Rename `ty_def_id` so people will stop using it by accident)
- #134271 (Arbitrary self types v2: better feature gate test)
- #134274 (Add check-pass test for `&raw`)
r? `@ghost`
`@rustbot` modify labels: rollup
Tweak multispan rendering to reduce output length
Consider comments and bare delimiters the same as an "empty line" for purposes of hiding rendered code output of long multispans. This results in more aggressive shortening of rendered output without losing too much context, specially in `*.stderr` tests that have "hidden" comments. We do that check not only on the first 4 lines of the multispan, but now also on the previous to last line as well.
The recently landed PR to adjust arbitrary self types was a bit
overenthusiastic, advising folks to use the new Receiver trait even
before it's been stabilized. Revert to the older wording of the lint in
such cases.
Stabilize async closures (RFC 3668)
# Async Closures Stabilization Report
This report proposes the stabilization of `#![feature(async_closure)]` ([RFC 3668](https://rust-lang.github.io/rfcs/3668-async-closures.html)). This is a long-awaited feature that increases the expressiveness of the Rust language and fills a pressing gap in the async ecosystem.
## Stabilization summary
* You can write async closures like `async || {}` which return futures that can borrow from their captures and can be higher-ranked in their argument lifetimes.
* You can express trait bounds for these async closures using the `AsyncFn` family of traits, analogous to the `Fn` family.
```rust
async fn takes_an_async_fn(f: impl AsyncFn(&str)) {
futures::join(f("hello"), f("world")).await;
}
takes_an_async_fn(async |s| { other_fn(s).await }).await;
```
## Motivation
Without this feature, users hit two major obstacles when writing async code that uses closures and `Fn` trait bounds:
- The inability to express higher-ranked async function signatures.
- That closures cannot return futures that borrow from the closure captures.
That is, for the first, we cannot write:
```rust
// We cannot express higher-ranked async function signatures.
async fn f<Fut>(_: impl for<'a> Fn(&'a u8) -> Fut)
where
Fut: Future<Output = ()>,
{ todo!() }
async fn main() {
async fn g(_: &u8) { todo!() }
f(g).await;
//~^ ERROR mismatched types
//~| ERROR one type is more general than the other
}
```
And for the second, we cannot write:
```rust
// Closures cannot return futures that borrow closure captures.
async fn f<Fut: Future<Output = ()>>(_: impl FnMut() -> Fut)
{ todo!() }
async fn main() {
let mut xs = vec![];
f(|| async {
async fn g() -> u8 { todo!() }
xs.push(g().await);
});
//~^ ERROR captured variable cannot escape `FnMut` closure body
}
```
Async closures provide a first-class solution to these problems.
For further background, please refer to the [motivation section](https://rust-lang.github.io/rfcs/3668-async-closures.html#motivation) of the RFC.
## Major design decisions since RFC
The RFC had left open the question of whether we would spell the bounds syntax for async closures...
```rust
// ...as this...
fn f() -> impl AsyncFn() -> u8 { todo!() }
// ...or as this:
fn f() -> impl async Fn() -> u8 { todo!() }
```
We've decided to spell this as `AsyncFn{,Mut,Once}`.
The `Fn` family of traits is special in many ways. We had originally argued that, due to this specialness, that perhaps the `async Fn` syntax could be adopted without having to decide whether a general `async Trait` mechanism would ever be adopted. However, concerns have been raised that we may not want to use `async Fn` syntax unless we would pursue more general trait modifiers. Since there remain substantial open questions on those -- and we don't want to rush any design work there -- it makes sense to ship this needed feature using the `AsyncFn`-style bounds syntax.
Since we would, in no case, be shipping a generalized trait modifier system anytime soon, we'll be continuing to see `AsyncFoo` traits appear across the ecosystem regardless. If we were to ever later ship some general mechanism, we could at that time manage the migration from `AsyncFn` to `async Fn`, just as we'd be enabling and managing the migration of many other traits.
Note that, as specified in RFC 3668, the details of the `AsyncFn*` traits are not exposed and they can only be named via the "parentheses sugar". That is, we can write `T: AsyncFn() -> u8` but not `T: AsyncFn<Output = u8>`.
Unlike the `Fn` traits, we cannot project to the `Output` associated type of the `AsyncFn` traits. That is, while we can write...
```rust
fn f<F: Fn() -> u8>(_: F::Output) {}
```
...we cannot write:
```rust
fn f<F: AsyncFn() -> u8>(_: F::Output) {}
//~^ ERROR
```
The choice of `AsyncFn{,Mut,Once}` bounds syntax obviates, for our purposes here, another question decided after that RFC, which was how to order bound modifiers such as `for<'a> async Fn()`.
Other than answering the open question in the RFC on syntax, nothing has changed about the design of this feature between RFC 3668 and this stabilization.
## What is stabilized
For those interested in the technical details, please see [the dev guide section](https://rustc-dev-guide.rust-lang.org/coroutine-closures.html) I authored.
#### Async closures
Other than in how they solve the problems described above, async closures act similarly to closures that return async blocks, and can have parts of their signatures specified:
```rust
// They can have arguments annotated with types:
let _ = async |_: u8| { todo!() };
// They can have their return types annotated:
let _ = async || -> u8 { todo!() };
// They can be higher-ranked:
let _ = async |_: &str| { todo!() };
// They can capture values by move:
let x = String::from("hello, world");
let _ = async move || do_something(&x).await };
```
When called, they return an anonymous future type corresponding to the (not-yet-executed) body of the closure. These can be awaited like any other future.
What distinguishes async closures is that, unlike closures that return async blocks, the futures returned from the async closure can capture state from the async closure. For example:
```rust
let vec: Vec<String> = vec![];
let closure = async || {
vec.push(ready(String::from("")).await);
};
```
The async closure captures `vec` with some `&'closure mut Vec<String>` which lives until the closure is dropped. Every call to `closure()` returns a future which reborrows that mutable reference `&'call mut Vec<String>` which lives until the future is dropped (e.g. it is `await`ed).
As another example:
```rust
let string: String = "Hello, world".into();
let closure = async move || {
ready(&string).await;
};
```
The closure is marked with `move`, which means it takes ownership of the string by *value*. The future that is returned by calling `closure()` returns a future which borrows a reference `&'call String` which lives until the future is dropped (e.g. it is `await`ed).
#### Async fn trait family
To support the lending capability of async closures, and to provide a first-class way to express higher-ranked async closures, we introduce the `AsyncFn*` family of traits. See the [corresponding section](https://rust-lang.github.io/rfcs/3668-async-closures.html#asyncfn) of the RFC.
We stabilize naming `AsyncFn*` via the "parenthesized sugar" syntax that normal `Fn*` traits can be named. The `AsyncFn*` trait can be used anywhere a `Fn*` trait bound is allowed, such as:
```rust
/// In return-position impl trait:
fn closure() -> impl AsyncFn() { async || {} }
/// In trait bounds:
trait Foo<F>: Sized
where
F: AsyncFn()
{
fn new(f: F) -> Self;
}
/// in GATs:
trait Gat {
type AsyncHasher<T>: AsyncFn(T) -> i32;
}
```
Other than using them in trait bounds, the definitions of these traits are not directly observable, but certain aspects of their behavior can be indirectly observed such as the fact that:
* `AsyncFn::async_call` and `AsyncFnMut::async_call_mut` return a future which is *lending*, and therefore borrows the `&self` lifetime of the callee.
```rust
fn by_ref_call(c: impl AsyncFn()) {
let fut = c();
drop(c);
// ^ Cannot drop `c` since it is borrowed by `fut`.
}
```
* `AsyncFnOnce::async_call_once` returns a future that takes ownership of the callee.
```rust
fn by_ref_call(c: impl AsyncFnOnce()) {
let fut = c();
let _ = c();
// ^ Cannot call `c` since calling it takes ownership the callee.
}
```
* All currently-stable callable types (i.e., closures, function items, function pointers, and `dyn Fn*` trait objects) automatically implement `AsyncFn*() -> T` if they implement `Fn*() -> Fut` for some output type `Fut`, and `Fut` implements `Future<Output = T>`.
* This is to make sure that `AsyncFn*()` trait bounds have maximum compatibility with existing callable types which return futures, such as async function items and closures which return boxed futures.
* For now, this only works currently for *concrete* callable types -- for example, a argument-position impl trait like `impl Fn() -> impl Future<Output = ()>` does not implement `AsyncFn()`, due to the fact that a `AsyncFn`-if-`Fn` blanket impl does not exist in reality. This may be relaxed in the future. Users can work around this by wrapping their type in an async closure and calling it. I expect this to not matter much in practice, as users are encouraged to write `AsyncFn` bounds directly.
```rust
fn is_async_fn(_: impl AsyncFn(&str)) {}
async fn async_fn_item(s: &str) { todo!() }
is_async_fn(s);
// ^^^ This works.
fn generic(f: impl Fn() -> impl Future<Output = ()>) {
is_async_fn(f);
// ^^^ This does not work (yet).
}
```
#### The by-move future
When async closures are called with `AsyncFn`/`AsyncFnMut`, they return a coroutine that borrows from the closure. However, when they are called via `AsyncFnOnce`, we consume that closure, and cannot return a coroutine that borrows from data that is now dropped.
To work around around this limitation, we synthesize a separate future type for calling the async closure via `AsyncFnOnce`.
This future executes identically to the by-ref future returned from calling the async closure, except for the fact that it has a different set of captures, since we must *move* the captures from the parent async into the child future.
#### Interactions between async closures and the `Fn*` family of traits
Async closures always implement `FnOnce`, since they always can be called once. They may also implement `Fn` or `FnMut` if their body is compatible with the calling mode (i.e. if they do not mutate their captures, or they do not capture their captures, respectively) and if the future returned by the async closure is not *lending*.
```rust
let id = String::new();
let mapped: Vec</* impl Future */> =
[/* elements */]
.into_iter()
// `Iterator::map` takes an `impl FnMut`
.map(async |element| {
do_something(&id, element).await;
})
.collect();
```
See [the dev guide](https://rustc-dev-guide.rust-lang.org/coroutine-closures.html#follow-up-when-do-async-closures-implement-the-regular-fn-traits) for a detailed explanation for the situations where this may not be possible due to the lending nature of async closures.
#### Other notable features of async closures shared with synchronous closures
* Async closures are `Copy` and/or `Clone` if their captures are `Copy`/`Clone`.
* Async closures do closure signature inference: If an async closure is passed to a function with a `AsyncFn` or `Fn` trait bound, we can eagerly infer the argument types of the closure. More details are provided in [the dev guide](https://rustc-dev-guide.rust-lang.org/coroutine-closures.html#closure-signature-inference).
#### Lints
This PR also stabilizes the `CLOSURE_RETURNING_ASYNC_BLOCK` lint as an `allow` lint. This lints on "old-style" async closures:
```rust
#![warn(closure_returning_async_block)]
let c = |x: &str| async {};
```
We should encourage users to use `async || {}` where possible. This lint remains `allow` and may be refined in the future because it has a few false positives (namely, see: "Where do we expect rewriting `|| async {}` into `async || {}` to fail?")
An alternative that could be made at the time of stabilization is to put this lint behind another gate, so we can decide to stabilize it later.
## What isn't stabilized (aka, potential future work)
#### `async Fn*()` bound syntax
We decided to stabilize async closures without the `async Fn*()` bound modifier syntax. The general direction of this syntax and how it fits is still being considered by T-lang (e.g. in [RFC 3710](https://github.com/rust-lang/rfcs/pull/3710)).
#### Naming the futures returned by async closures
This stabilization PR does not provide a way of naming the futures returned by calling `AsyncFn*`.
Exposing a stable way to refer to these futures is important for building async-closure-aware combinators, and will be an important future step.
#### Return type notation-style bounds for async closures
The RFC described an RTN-like syntax for putting bounds on the future returned by an async closure:
```rust
async fn foo(x: F) -> Result<()>
where
F: AsyncFn(&str) -> Result<()>,
// The future from calling `F` is `Send` and `'static`.
F(..): Send + 'static,
{}
```
This stabilization PR does not stabilize that syntax yet, which remains unimplemented (though will be soon).
#### `dyn AsyncFn*()`
`AsyncFn*` are not dyn-compatible yet. This will likely be implemented in the future along with the dyn-compatibility of async fn in trait, since the same issue (dealing with the future returned by a call) applies there.
## Tests
Tests exist for this feature in [`tests/ui/async-await/async-closures`](5b54286640/tests/ui/async-await/async-closures).
<details>
<summary>A selected set of tests:</summary>
* Lending behavior of async closures
* `tests/ui/async-await/async-closures/mutate.rs`
* `tests/ui/async-await/async-closures/captures.rs`
* `tests/ui/async-await/async-closures/precise-captures.rs`
* `tests/ui/async-await/async-closures/no-borrow-from-env.rs`
* Async closures may be higher-ranked
* `tests/ui/async-await/async-closures/higher-ranked.rs`
* `tests/ui/async-await/async-closures/higher-ranked-return.rs`
* Async closures may implement `Fn*` traits
* `tests/ui/async-await/async-closures/is-fn.rs`
* `tests/ui/async-await/async-closures/implements-fnmut.rs`
* Async closures may be cloned
* `tests/ui/async-await/async-closures/clone-closure.rs`
* Ownership of the upvars when `AsyncFnOnce` is called
* `tests/ui/async-await/async-closures/drop.rs`
* `tests/ui/async-await/async-closures/move-is-async-fn.rs`
* `tests/ui/async-await/async-closures/force-move-due-to-inferred-kind.rs`
* `tests/ui/async-await/async-closures/force-move-due-to-actually-fnonce.rs`
* Closure signature inference
* `tests/ui/async-await/async-closures/signature-deduction.rs`
* `tests/ui/async-await/async-closures/sig-from-bare-fn.rs`
* `tests/ui/async-await/async-closures/signature-inference-from-two-part-bound.rs`
</details>
## Remaining bugs and open issues
* https://github.com/rust-lang/rust/issues/120694 tracks moving onto more general `LendingFn*` traits. No action needed, since it's not observable.
* https://github.com/rust-lang/rust/issues/124020 - Polymorphization ICE. Polymorphization needs to be heavily reworked. No action needed.
* https://github.com/rust-lang/rust/issues/127227 - Tracking reworking the way that rustdoc re-sugars bounds.
* The part relevant to to `AsyncFn` is fixed by https://github.com/rust-lang/rust/pull/132697.
## Where do we expect rewriting `|| async {}` into `async || {}` to fail?
* Fn pointer coercions
* Currently, it is not possible to coerce an async closure to an fn pointer like regular closures can be. This functionality may be implemented in the future.
```rust
let x: fn() -> _ = async || {};
```
* Argument capture
* Like async functions, async closures always capture their input arguments. This is in contrast to something like `|t: T| async {}`, which doesn't capture `t` unless it is used in the async block. This may affect the `Send`-ness of the future or affect its outlives.
```rust
fn needs_send_future(_: impl Fn(NotSendArg) -> Fut)
where
Fut: Future<Output = ()>,
{}
needs_send_future(async |_| {});
```
## History
#### Important feature history
- https://github.com/rust-lang/rust/pull/51580
- https://github.com/rust-lang/rust/pull/62292
- https://github.com/rust-lang/rust/pull/120361
- https://github.com/rust-lang/rust/pull/120712
- https://github.com/rust-lang/rust/pull/121857
- https://github.com/rust-lang/rust/pull/123660
- https://github.com/rust-lang/rust/pull/125259
- https://github.com/rust-lang/rust/pull/128506
- https://github.com/rust-lang/rust/pull/127482
## Acknowledgements
Thanks to `@oli-obk` for reviewing the bulk of the work for this feature. Thanks to `@nikomatsakis` for his design blog posts which generated interest for this feature, `@traviscross` for feedback and additions to this stabilization report. All errors are my own.
r? `@ghost`
Consider comments and bare delimiters the same as an "empty line" for purposes of hiding rendered code output of long multispans. This results in more aggressive shortening of rendered output without losing too much context, specially in `*.stderr` tests that have "hidden" comments.
In this new version of Arbitrary Self Types, we no longer use the Deref trait
exclusively when working out which self types are valid. Instead, we follow a
chain of Receiver traits. This enables methods to be called on smart pointer
types which fundamentally cannot support Deref (for instance because they are
wrappers for pointers that don't follow Rust's aliasing rules).
This includes:
* Changes to tests appropriately
* New tests for:
* The basics of the feature
* Ensuring lifetime elision works properly
* Generic Receivers
* A copy of the method subst test enhanced with Receiver
This is really the heart of the 'arbitrary self types v2' feature, and
is the most critical commit in the current PR.
Subsequent commits are focused on:
* Detecting "shadowing" problems, where a smart pointer type can hide
methods in the pointee.
* Diagnostics and cleanup.
Naming: in this commit, the "Autoderef" type is modified so that it no
longer solely focuses on the "Deref" trait, but can now consider the
"Receiver" trait instead. Should it be renamed, to something like
"TraitFollower"? This was considered, but rejected, because
* even in the Receiver case, it still considers built-in derefs
* the name Autoderef is short and snappy.
Move most tests for `-l` and `#[link(..)]` into `tests/ui/link-native-libs`
Tests for the closely-related `-l` flag and `#[link(..)]` attribute are spread across a few different directories, and in some cases have ended up in a test directory intended for other linker-related functionality.
This PR moves most of them into a single `tests/ui/link-native-libs` directory.
---
Part of #133895.
try-job: i686-mingw
r? jieyouxu
Add lint against function pointer comparisons
This is kind of a follow-up to https://github.com/rust-lang/rust/pull/117758 where we added a lint against wide pointer comparisons for being ambiguous and unreliable; well function pointer comparisons are also unreliable. We should IMO follow a similar logic and warn people about it.
-----
## `unpredictable_function_pointer_comparisons`
*warn-by-default*
The `unpredictable_function_pointer_comparisons` lint checks comparison of function pointer as the operands.
### Example
```rust
fn foo() {}
let a = foo as fn();
let _ = a == foo;
```
### Explanation
Function pointers comparisons do not produce meaningful result since they are never guaranteed to be unique and could vary between different code generation units. Furthermore different function could have the same address after being merged together.
----
This PR also uplift the very similar `clippy::fn_address_comparisons` lint, which only linted on if one of the operand was an `ty::FnDef` while this PR lints proposes to lint on all `ty::FnPtr` and `ty::FnDef`.
```@rustbot``` labels +I-lang-nominated
~~Edit: Blocked on https://github.com/rust-lang/libs-team/issues/323 being accepted and it's follow-up pr~~
Get rid of HIR const checker
As far as I can tell, the HIR const checker was implemented in https://github.com/rust-lang/rust/pull/66170 because we were not able to issue useful const error messages in the MIR const checker.
This seems to have changed in the last 5 years, probably due to work like #90532. I've tweaked the diagnostics slightly and think the error messages have gotten *better* in fact.
Thus I think the HIR const checker has reached the end of its usefulness, and we can retire it.
cc `@RalfJung`
