Delegation: support generics for delegation from free functions

[Bryanskiy]

(The PR was split from #123958, explainer - https://github.com/Bryanskiy/posts/blob/master/delegation%20in%20generic%20contexts.md)

This PR implements generics inheritance from free functions to free functions and trait methods.

free functions to free functions:

fn to_reuse<T: Clone>(_: T) {}

reuse to_reuse as bar;
// desugaring:
fn bar<T: Clone>(x: T) {
  to_reuse(x)
}

Generics, predicates and signature are simply copied. Generic arguments in paths are ignored during generics inheritance:

fn to_reuse<T: Clone>(_: T) {}

reuse to_reuse::<u8> as bar;
// desugaring:
fn bar<T: Clone>(x: T) {
  to_reuse::<u8>(x) // ERROR: mismatched types
}

Due to implementation limitations callee path is lowered without modifications. Therefore, it is a compilation error at the moment.

free functions to trait methods:

trait Trait<'a, A> {
    fn foo<'b, B>(&self, x: A, y: B) {...}
}

reuse Trait::foo;
// desugaring:
fn foo<'a, 'b, This: Trait<'a, A>, A, B>(this: &This, x: A, y: B) {
  Trait::foo(this, x, y)
}

The inheritance is similar to the previous case but with some corrections:

• Self parameter converted into T: Trait
• generic parameters need to be reordered so that lifetimes go first

Arguments are similarly ignored.

---

In the future, we plan to support generic inheritance for delegating from all contexts to all contexts (from free/trait/impl to free/trait /impl). These cases were considered first as the simplest from the implementation perspective.

[rustbot]

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[rustbot]

HIR ty lowering was modified

cc @fmease

[Bryanskiy]

сс @petrochenkov

[Bryanskiy]

@rustbot author

[petrochenkov]

r? types
cc @fmease

[Bryanskiy]

@rustbot ready

[lcnr]

r? fmease

[petrochenkov]

@bors r+

[bors]

📌 Commit f2f9aab has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit f2f9aab with merge 1ddedba...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 1ddedba to master...

[rust-timer]

Finished benchmarking commit (1ddedba): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.8%]	11
Regressions ❌ (secondary)	0.5%	[0.1%, 1.2%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	2
All ❌✅ (primary)	0.4%	[0.2%, 0.8%]	11

Max RSS (memory usage)

Results (primary 0.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.7%	[1.6%, 3.8%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.1%	[-4.1%, -4.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-4.1%, 3.8%]	3

Cycles

Results (primary -2.2%, secondary 2.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.2%	[-2.2%, -2.2%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 770.079s -> 769.935s (-0.02%)
Artifact size: 331.86 MiB -> 331.88 MiB (0.00%)

[petrochenkov]

The perf regression is addressed in #128441.

[Kobzol]

@rustbot label: +perf-regression-triaged