Autotrait bounds on dyn-safe trait methods

[dtolnay]

This PR is a successor to #106604 implementing the approach encouraged by #106604 (comment).

I propose making it legal to use autotraits as trait bounds on the Self type of trait methods in a trait object. #51443 (comment) justifies why this use case is particularly important in the context of the async-trait crate.

#![feature(auto_traits)]
#![deny(where_clauses_object_safety)]

auto trait AutoTrait {}

trait MyTrait {
    fn f(&self) where Self: AutoTrait;
}

fn main() {
    let _: &dyn MyTrait;
}

Previously this would fail with:

error: the trait `MyTrait` cannot be made into an object
 --> src/main.rs:7:8
  |
7 |     fn f(&self) where Self: AutoTrait;
  |        ^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/main.rs:7:8
  |
6 | trait MyTrait {
  |       ------- this trait cannot be made into an object...
7 |     fn f(&self) where Self: AutoTrait;
  |        ^ ...because method `f` references the `Self` type in its `where` clause
  = help: consider moving `f` to another trait

In order for this to be sound without hitting #50781, I further propose that we disallow handwritten autotrait impls that apply to trait objects. Both of the following were previously allowed (on nightly) and no longer allowed in my proposal:

auto trait AutoTrait {}

trait MyTrait {}
impl AutoTrait for dyn MyTrait {}  // NOT ALLOWED

impl<T: ?Sized> AutoTrait for T {}  // NOT ALLOWED

(impl<T> AutoTrait for T {} remains allowed.)

After this change, traits with a default impl are implemented for a trait object if and only if the autotrait is one of the trait object's trait bounds (or a supertrait of a bound). In other words dyn Trait + AutoTrait always implements AutoTrait while dyn Trait never implements AutoTrait.

Fixes dtolnay/async-trait#228.

r? @lcnr

[dtolnay]

Mentioning @cjgillot + @nbdd0121 since you had opinions about #106604.

[lcnr]

see the PR description for details

@rfcbot fcp merge

[rfcbot]

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[lcnr]

@bors r+

[bors]

📌 Commit fa84c6f has been approved by lcnr

It is now in the queue for this repository.

[dtolnay]

• fa84c6f..94ad442—squash commits from the code review
• 94ad442..182ebb5—clean rebase
• 182ebb5..4501d3a—resolve conflict with Compute generator saved locals on MIR #101692

@bors r=lcnr

[bors]

📌 Commit 4501d3a has been approved by lcnr

It is now in the queue for this repository.