[lifetimes] add same-type default lifetime inference

[atrick]

• [docs] update lifetime defaults to prioritize the same-type rule

• [NFC] lifetime inference: check for prior defaults before error
  Don't diagnose a lifetime error if a previous default already handled the
  non-Escapable output in question.

• [lifetimes] add same-type default lifetime inference
  Infer @Lifetime(result: copy arg) for every (result: R, arg: A) pair
  such that R == A and 'arg' is not 'inout'.

[atrick]

@swift-ci test

[slavapestov]

Please add some tests where the outer nominal is generic, and the method is generic, otherwise you won’t spot the incorrect usage of interface types and archetypes I pointed out in the review

[atrick]

@slavapestov I did not expect this case to work with the current PR, but it does

protocol P_NE {
  associatedtype T: ~Escapable
}

protocol Q_NE {
  associatedtype U: ~Escapable
}

struct AssociatedNE<P: P_NE, Q: Q_NE> {}

extension AssociatedNE where P.T == Q.U {
  func foo(a: P.T) -> Q.U {...}
}

Mapping Q.U into foos environment normalizes it to P.T so the trivial type comparison succeeds

paramTy == resultTy

Now I'm wondering if I need to even look at protocol requirements here. Is this really a sufficient same-type check?

[atrick]

@swift-ci test

[atrick]

@swift-ci test source compatibility

[atrick]

@swift-ci benchmark