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async fn presence affects an unrelated error message #66312

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tguser402 opened this issue Nov 11, 2019 · 10 comments · Fixed by #70376
Closed

async fn presence affects an unrelated error message #66312

tguser402 opened this issue Nov 11, 2019 · 10 comments · Fixed by #70376
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A-async-await Area: Async & Await A-traits Area: Trait system AsyncAwait-Polish Async-await issues that are part of the "polish" area AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. C-bug Category: This is a bug. P-high High priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@tguser402
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tguser402 commented Nov 11, 2019

The following code

trait Test<T> {
    fn is_some(self: T);
}

fn f() {
    let x = Some(2);
    if x.is_some() {
        println!("Some");
    }
}

produces an expected error:

error[E0307]: invalid `self` parameter type: T
 --> src/lib.rs:2:22
  |
2 |     fn is_some(self: T);
  |                      ^
  |
  = note: type of `self` must be `Self` or a type that dereferences to it
  = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)

If I make f async,

trait Test<T> {
    fn is_some(self: T);
}

async fn f() {
    let x = Some(2);
    if x.is_some() {
        println!("Some");
    }
}

the original error is replaced with this:

error[E0308]: mismatched types
 --> src/lib.rs:7:8
  |
7 |     if x.is_some() {
  |        ^^^^^^^^^^^ expected bool, found ()
  |
  = note: expected type `bool`
             found type `()`

Meta

rustc --version --verbose:

rustc 1.39.0 (4560ea788 2019-11-04)
binary: rustc
commit-hash: 4560ea788cb760f0a34127156c78e2552949f734
commit-date: 2019-11-04
host: x86_64-unknown-linux-gnu
release: 1.39.0
LLVM version: 9.0
@jonas-schievink jonas-schievink added A-async-await Area: Async & Await A-traits Area: Trait system C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. I-nominated labels Nov 11, 2019
@jonas-schievink
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Nominating because:

  • The trait impl does not exist, the compiler hallucinated it into existence
  • Inherent methods are supposed to take precedence over trait methods

This might indicate a quite serious bug in the compiler.

@Centril
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Centril commented Nov 12, 2019

I would suggest stable- and beta-nominating the fix for this PR; it seems like this could warrant a point release. cc @rust-lang/release @rust-lang/lang

@Centril Centril added the P-high High priority label Nov 12, 2019
@pietroalbini
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@rustbot modify labels: beta-nominated stable-nominated

@rustbot rustbot added beta-nominated Nominated for backporting to the compiler in the beta channel. stable-nominated Nominated for backporting to the compiler in the stable channel. labels Nov 12, 2019
@Centril
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Centril commented Nov 12, 2019

Phew; looks like precedence works correctly in non-error code at least:

trait Test {
    fn is_some(&self) -> u8;
}

impl<T> Test for Option<T> {
    fn is_some(&self) -> u8 {
        self.is_some() as u8
    }
}

async fn f() {
    let x = Some(2);
    let _: () = x.is_some(); // type is bool.
}

Crisis averted? :)

@nikomatsakis
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@rustbot assign @csmoe

@nikomatsakis nikomatsakis added AsyncAwait-Polish Async-await issues that are part of the "polish" area AsyncAwait-OnDeck AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. and removed I-nominated labels Nov 12, 2019
@pnkfelix
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unnominating for back ports; we don't backport nominate issues, just Pull Requests.

@pnkfelix pnkfelix removed beta-nominated Nominated for backporting to the compiler in the beta channel. stable-nominated Nominated for backporting to the compiler in the stable channel. labels Nov 14, 2019
@nikomatsakis
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@rfcbot claim

I'll take a look, since @csmoe is finding this to be mighty tricky to track down

@tguser402
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I managed to get similar symptoms without async but with #![feature(arbitrary_self_types)]. But this time the trait compiles.

#![feature(arbitrary_self_types)]
trait Test<T: core::ops::Deref<Target = Self>> {
    fn is_some(self: T);
}

fn f() {
    let x = Some(2);
    if x.is_some() {
        println!("Some");
    }
}

Playground link

error[E0277]: the trait bound `std::option::Option<{integer}>: std::ops::Deref` is not satisfied
 --> src/lib.rs:8:10
  |
8 |     if x.is_some() {
  |          ^^^^^^^ the trait `std::ops::Deref` is not implemented for `std::option::Option<{integer}>`

error[E0308]: mismatched types
 --> src/lib.rs:8:8
  |
8 |     if x.is_some() {
  |        ^^^^^^^^^^^ expected `bool`, found `()`

error: aborting due to 2 previous errors

@nikomatsakis
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I am 99% sure that the diagnosis I posted on #67651 also applies here.

@nikomatsakis
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The async case is fixed by #68884, I think we should break out the arbitrary self types case into a distinct bug.

@tmandry tmandry added this to High priority in wg-async-foundations triage Feb 11, 2020
@tmandry tmandry added this to To do in wg-async work Feb 11, 2020
@tmandry tmandry moved this from To do to In progress in wg-async work Mar 3, 2020
@tmandry tmandry moved this from In progress to Done in wg-async work Mar 24, 2020
@tmandry tmandry moved this from Done to In progress in wg-async work Mar 24, 2020
@tmandry tmandry assigned tmandry and unassigned nikomatsakis Mar 24, 2020
Centril added a commit to Centril/rust that referenced this issue Mar 25, 2020
Centril added a commit to Centril/rust that referenced this issue Mar 25, 2020
Centril added a commit to Centril/rust that referenced this issue Mar 25, 2020
bors added a commit to rust-lang-ci/rust that referenced this issue Mar 25, 2020
Rollup of 7 pull requests

Successful merges:

 - rust-lang#70331 (Increase verbosity when using update syntax with private fields)
 - rust-lang#70349 (move `hir_id_validation` to `rustc_passes` + simplify `hir::map` code)
 - rust-lang#70361 (Update backtrace crate to 0.3.46)
 - rust-lang#70364 (resolve: Remove `rustc_attrs` as a standalone feature gate)
 - rust-lang#70369 (Fix smaller issues with invalid placeholder type errors)
 - rust-lang#70373 (normalize some imports & prefer direct ones)
 - rust-lang#70376 (Add test for rust-lang#66312)

Failed merges:

r? @ghost
@bors bors closed this as completed in 1a21c28 Mar 25, 2020
wg-async work automation moved this from In progress to Done Mar 25, 2020
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Labels
A-async-await Area: Async & Await A-traits Area: Trait system AsyncAwait-Polish Async-await issues that are part of the "polish" area AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. C-bug Category: This is a bug. P-high High priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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9 participants