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False positive for "lifetime must be declared" error in 2018 preview #51008

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alexcrichton opened this issue May 23, 2018 · 4 comments
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False positive for "lifetime must be declared" error in 2018 preview #51008

alexcrichton opened this issue May 23, 2018 · 4 comments
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@nikomatsakis
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A-edition-2018 Area: The 2018 edition F-rust_2018_preview `#![feature(rust_2018_preview)]`

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@alexcrichton
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This code:

#![feature(rust_2018_preview)]

trait A {
    
}

impl<F> A for F where F: FnOnce(&i32) {}

fn main() {}

yields:

error[E0687]: lifetimes used in `fn` or `Fn` syntax must be explicitly declared using `<...>` binders
 --> src/main.rs:7:33
  |
7 | impl<F> A for F where F: FnOnce(&i32) {}
  |                                 ^ in-band lifetime definition

error: aborting due to previous error

For more information about this error, try `rustc --explain E0687`.
@alexcrichton alexcrichton added the F-rust_2018_preview `#![feature(rust_2018_preview)]` label May 23, 2018
@nikomatsakis
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cc @cramertj =)

@nikomatsakis
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(I'm investigating a bit)

@nikomatsakis
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Got a fix coming

@nikomatsakis nikomatsakis self-assigned this May 30, 2018
@nikomatsakis nikomatsakis mentioned this issue May 30, 2018
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#51220

bors added a commit that referenced this issue May 31, 2018
@bors
Auto merge of #51220 - nikomatsakis:issue-51008-false-positive-lifeti…
c1287c0 
…me-must-be-declared, r=cramertj

reset anonymous-lifetime-mode as we enter `()` scopes

Background:

The anonymous lifetime mode is used to prohibit elided lifetimes where
they didn't used to be permitted, and instead require that `'_` be
used. For example:

```rust
impl Trait for Ref<T> { .. }
//             ^^^^^^ ERROR: should be `Ref<'_, T>`
```

When we are parsing the parts of the impl header, we enter into an alternate mode called `CreateParameter`. In this mode, we give an error for things like `Ref<T>`, but for elided lifetimes in a reference type like `&T` we make the elided lifetime into an in-band lifetime:

https://github.com/rust-lang/rust/blob/4f99f37b7e213d69a489884f651adfc6d217cef5/src/librustc/hir/lowering.rs#L4017-L4035

This was not intended to change behavior because we only enter into that mode in contexts where elision was not historically permitted. However, the problem is that we fail to reset the mode when we enter into bounds like `Fn(&u32)`, where elision *was* allowed -- the same occurs for fn types like `fn(&u32`). This PR restores the original mode in those contexts.

Fixes #51008

r? @cramertj
@fmease fmease added the A-edition-2018 Area: The 2018 edition label Dec 21, 2024
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