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rustdoc can't handle existential types #58624

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QuietMisdreavus opened this issue Feb 21, 2019 · 4 comments
Closed

rustdoc can't handle existential types #58624

QuietMisdreavus opened this issue Feb 21, 2019 · 4 comments
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A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. F-type_alias_impl_trait `#[feature(type_alias_impl_trait)]` requires-nightly This issue requires a nightly compiler in some way. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue.

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@QuietMisdreavus
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#![feature(existential_type)]

pub trait SomeTrait {
    fn some_fn(&self) {}
    
    type SomeType;
}

struct SomeStruct;

impl SomeTrait for SomeStruct {
    type SomeType = String;
}

pub existential type Asdf: SomeTrait;
pub fn asdf() -> Asdf { SomeStruct }

This sample compiles just fine, but when you run it in rustdoc...

$ rustdoc +nightly c.rs
error[E0277]: the trait bound `(): SomeTrait` is not satisfied
  --> c.rs:15:1
   |
15 | pub existential type Asdf: SomeTrait;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SomeTrait` is not implemented for `()`
   |
   = note: the return type of a function must have a statically known size

error: Compilation failed, aborting rustdoc

$ rustdoc +nightly --version
rustdoc 1.34.0-nightly (f66e4697a 2019-02-20)

My personal guess is that the fact that rustdoc runs everybody_loops on the function that defines this type means that it no longer returns the actual type - instead it returns !, though the compiler picks up () for the error message.

@QuietMisdreavus QuietMisdreavus added T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. labels Feb 21, 2019
@QuietMisdreavus
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cc #34511

@oli-obk
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oli-obk commented Feb 21, 2019

The same thing happens for plain impl trait:

trait Foo {}

fn foo() -> impl Foo {
    loop {}
}

gives

error[E0277]: the trait bound `(): Foo` is not satisfied
 --> src/lib.rs:3:13
  |
3 | fn foo() -> impl Foo {
  |             ^^^^^^^^ the trait `Foo` is not implemented for `()`
  |
  = note: the return type of a function must have a statically known size

I'm not sure how to handle this, even if I turn on #![feature(never_type)], I get

error[E0277]: the trait bound `!: Foo` is not satisfied
 --> src/lib.rs:5:13
  |
5 | fn foo() -> impl Foo {
  |             ^^^^^^^^ the trait `Foo` is not implemented for `!`
  |
  = note: the return type of a function must have a statically known size

@Centril Centril added F-type_alias_impl_trait `#[feature(type_alias_impl_trait)]` requires-nightly This issue requires a nightly compiler in some way. labels Jul 28, 2019
@alecmocatta
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see also #58011

@varkor
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varkor commented Jan 12, 2020

Closing as a duplicate of #65863 (which has more information).

@varkor varkor closed this as completed Jan 12, 2020
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Labels
A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. F-type_alias_impl_trait `#[feature(type_alias_impl_trait)]` requires-nightly This issue requires a nightly compiler in some way. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue.
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