Support ?Trait bounds in supertraits and dyn Trait under a feature gate

[Bryanskiy]

This patch allows maybe polarity bounds under a feature gate. The only language change here is that corresponding hard errors are replaced by feature gates. Example:

#![feature(allow_maybe_polarity)]
...
trait Trait1 : ?Trait { ... } // ok
fn foo(_: Box<(dyn Trait2 + ?Trait)>) {} // ok
fn bar<T: ?Sized + ?Trait>(_: &T) {} // ok

Maybe bounds still don't do anything (except for Sized trait), however this patch will allow us to experiment with default auto traits.

This is a part of the MCP: Low level components for async drop

[rustbot]

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Nilstrieb]

for reference, is there any language team shepherd for all of this default bound experimentation? I only ever see random work and don't remember seeing design documents or anything like that

[petrochenkov]

@Nilstrieb
I'll make a compiler team MCP for core components for async drop enabling library experimentation soon.
@traviscross is currently the Async WG liaison for this stuff (I actually assumed he was on lang team as well, but apparently not).

[Bryanskiy]

@rustbot author

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[Bryanskiy]

@rustbot ready

[rustbot]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[petrochenkov]

Need to link to rust-lang/compiler-team#727 from this PR description.
@rustbot author

[petrochenkov]

Implementation LGTM, except the commit message needs to be more human oriented.

I'm going to pass this to r? @traviscross in cases some additional lang team approval is needed, but otherwise r=me.

[Bryanskiy]

In the last update MisplacedRelaxTraitBound was replaced by feature gate.

[traviscross]

@rustbot labels +T-lang +I-lang-nominated

We'll need to discuss this in a lang meeting for visibility and to ensure this experiment is properly approved, so let's nominate this.

https://lang-team.rust-lang.org/how_to/experiment.html

@Bryanskiy / @petrochenkov: If you could perhaps write up a summary for the lang team to review as part of this nomination about the nature of this part of the experiment, what the motivation for it is, and how this fits into the larger body of work, that work be helpful.

[petrochenkov]

Summary:

• Initial support for auto traits with default bounds #120706 introduces a way to add new auto traits that are appended to all bound lists by default, similarly to existing Sized. Such traits may include Leak, SyncDrop or similar, see Initial support for auto traits with default bounds #120706 (comment) for more detailed motivation.
• To opt out from bounds added by default the ?Trait syntax is used, but such "maybe" bounds are not supported in some contexts like supertrait lists and dyn Trait + ... lists, because Sized is not added by default in those context.
• This PR adds a feature for supporting trait Trait1: ?Trait2, dyn Trait1 + ?Trait2 and also multiple maybe bounds in the same list ?Trait1 + ?Trait2, because the new traits need to be added by default in those contexts too, and ?Sized + ?Leak may also make sense.
• We need this to be available in bootstrap compiler, to make experiments on standard library without adding too many #[cfg(not(bootstrap))]s
• Larger compiler team MCP including this feature - MCP: Low level components for async drop compiler-team#727, it gives some more context

[bors]

☔ The latest upstream changes (presumably #122132) made this pull request unmergeable. Please resolve the merge conflicts.

[Bryanskiy]

In the last update #120706 (comment) was fixed.

[bors]

☔ The latest upstream changes (presumably #122713) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@Bryanskiy what's the status of this? thanks