delegation: add support for infers in generics

[aerooneqq]

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

[aerooneqq]

Do we need this diagnostic? Other option is to skip incorrect infers and error will be emitted later during signature inheritance.

View changes since the review

[aerooneqq]

Do we need to care about ast::TyKind::Paren or we can just use matches!(ty.kind, ast::TyKind::Infer)?

View changes since the review

[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]

Nit: the annotations were reordered, and it makes the changes look larger than they actually are.
(Same below.)

View changes since the review

[petrochenkov]

Could you add comments telling what the variants mean?

View changes since the review

[petrochenkov]

Could you add comments telling what the fields mean?
E.g. what is the difference between child_seg_id and child_seg_id_for_sig.

View changes since the review

[petrochenkov]

(I started reviewing, will continue tomorrow.)