delegation: add support for infers in generics

[aerooneqq]

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This PR adds support for generating generic params for lifetimes and types/consts infers ('_. _). We support only single infers, if they are nested then we do nothing and eventually an error will be emitted after inheriting this unsound signature (i.e., reuse foo::<Vec<_>> is not supported).

The basic idea is:

fn foo<'a, 'b: 'b, T, const N: usize>() {}

reuse foo::<'_, String, _> as bar;

// Desugaring:
fn bar<'b, const N: usize>() {
  foo::<'b, String, N>()
}

So we generated params and lifetimes for provided infers. Note that in case of lifetimes we may have early and late bound lifetimes in child segment. We process only early-bound lifetimes as they are present in signature function generics (tcx.generics_of(sig_id)). Moreover since this PR we started to explicitly propagate early-bound lifetimes in generated delegation's call, so the warning about specifying lifetimes when late-bound lifetimes are present is suppressed for delegation segments.

Next, we limit the number of processed infers with the number of generic params in the signature function, so if we have fn foo<X, Y>() {} and we wrote reuse foo::<_, _, _, _>; we will not generate 4 generic params in delegation, we will generate 2 (X and Y), two remaining infers will not be processed and this will result in an error.

Considering free-to-trait reuses this PR extends the number of supported cases, as now we always generate Self generic param when needed:

trait Trait<'a, X> {
    fn method<'b, const M: usize>(&self) where 'b:'b { }
    fn r#static<'b, Y, const B: bool>() { }
}

impl <'a, X> Trait<'a, X> for () { }

reuse Trait::<'_, _>::method::<'_, _> as foo;
reuse <_ as Trait<'_, _>>::method::<'_, _> as foo1;
reuse <() as Trait<'_, _>>::method::<'_, _> as foo2;
reuse <_ as Trait<'_, _>>::r#static::<_, _> as foo3;
reuse <() as Trait<'_, _>>::r#static::<_, _> as foo4;
reuse Trait::<'_, _>::r#static::<_, _> as foo5;

// Desugaring:
#[attr = Inline(Hint)]
fn foo<'a, 'b, Self, X, const M: _>(self: _) -> _ where 'a:'a,
    'b:'b { <Self as Trait::<'a, X>>::method::<'b, M>(self) }
#[attr = Inline(Hint)]
fn foo1<'a, 'b, Self, X, const M: _>(self: _) -> _ where 'a:'a,
    'b:'b { <Self as Trait::<'a, X>>::method::<'b, M>(self) }
#[attr = Inline(Hint)]
fn foo2<'a, 'b, X, const M: _>(self: _) -> _ where 'a:'a,
    'b:'b { <() as Trait::<'a, X>>::method::<'b, M>(self) }
#[attr = Inline(Hint)]
fn foo3<'a, Self, X, Y, const B: _>() -> _ where
    'a:'a { <Self as Trait::<'a, X>>::r#static::<Y, B>() }
#[attr = Inline(Hint)]
fn foo4<'a, X, Y, const B: _>() -> _ where
    'a:'a { <() as Trait::<'a, X>>::r#static::<Y, B>() }
#[attr = Inline(Hint)]
fn foo5<'a, Self, X, Y, const B: _>() -> _ where
    'a:'a { <Self as Trait::<'a, X>>::r#static::<Y, B>() }

Note that we generated Self in foo5 reuse. With that done the error about self-type specification is removed.

Finally this PR greatly simplifies generic args for signature inheritance generation code.

Part of #118212.
r? @petrochenkov

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[petrochenkov]

(I started reviewing, will continue tomorrow.)

[aerooneqq]

@rustbot ready

[petrochenkov]

r=me after cleaning up git history.
@rustbot author
@bors delegate+

[rust-bors]

✌️ @aerooneqq, you can now approve this pull request!

If @petrochenkov told you to "r=me" after making some further change, then please make that change and post @bors r=petrochenkov.

View changes since this delegation.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[aerooneqq]

@bors r=petrochenkov

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  TOOLSTATE_PUBLISH: 1
  DEPLOY_ALT: 1
##[endgroup]
    Updating crates.io index
error: failed to get `time` as a dependency of package `cookie_store v0.21.1`
    ... which satisfies dependency `cookie_store = "^0.21.1"` of package `ureq v3.0.8`
    ... which satisfies dependency `ureq = "^3"` of package `citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)`

Caused by:
  failed to load source for dependency `time`

Caused by:

[rust-bors]

⌛ Testing commit 7d42183 with merge 64682ad...

Workflow: https://github.com/rust-lang/rust/actions/runs/28009206977

[jhpratt]

@bors yield

[rust-bors]

Auto build was cancelled. Cancelled workflows:

• https://github.com/rust-lang/rust/actions/runs/28009206977

The next pull request likely to be tested is #158288.