Allow quantification of lifetimes outside the self type in where clauses

[nrc]

Closes #20022

r? @nikomatsakis

[jroesch]

The patch looks good, I think it is wise to prefer the hack for the time being.

[nikomatsakis]

This looks good but don't we want some tests of some kind?

[nikomatsakis]

(PS sorry it took me so long to review, I didn't notice this for a while)

[nrc]

ah, crap, I forgot to git add the test...

[nrc]

rebased and added tests

[nikomatsakis]

Seems reasonable. I'd sort of like a test that shows that the correct requirements are being put onto the caller (e.g. a compile-fail test where the requirement is for<'a> &'a T : Foo but the caller only knows that &'x T : Foo holds for some particular 'x)

[nrc]

@nikomatsakis test added

[nikomatsakis]

@bors r+ c50a8f930245aa2a5a52e230c68a298c84f5a8a1

[bors]

⌛ Testing commit c50a8f9 with merge 5f2ab7f...

[bors]

💔 Test failed - auto-mac-32-opt

[nrc]

@bors r=nmatsakis f9c577e

[bors]

⌛ Testing commit f9c577e with merge de8bc44...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt