proc bounds and closure lifetimes are parsed incorrectly

[Kimundi]

<kimundi> If you have a function that takes a reference to a closure the lifetime specifier becomes confusing
<kimundi> fn foo(&|T| -> U), fn foo(&'a |T| -> U) and fn foo(&'a 'a |T| -> U) are all valid, and in the second one you can't tell which one of both the lifetimes is supposed to belong to
<nmatsakis> kimundi: this is parser bug, 'a |T| shouldn't parse
<nmatsakis> it should be |T|:'a

<nmatsakis> the parser may well be wrong
<nmatsakis> maybe got overlooked

<kimundi> well, if I change to that syntax I get compile errors
<kimundi> this works: impl<'a> StringPrefixMatch for 'a |$self_anon| -> bool
<kimundi> this doesn't: impl<'a> StringPrefixMatch for |$self_anon|:'a -> bool
<kimundi> I get a "'static is the only valid region allowed here" error

<nmatsakis> kimundi: I think it's just a parser bug
<nmatsakis> kimundi: yes, I can understand why that would occur
<kimundi> why?

<nmatsakis> kimundi: basically because some cleanup is needed
<nmatsakis> we are interpreting |...|:K with K as a list of bounds , which is correct,
<nmatsakis> but we currently haven't implemented support for arbitrary lifetimes in bounds,
<nmatsakis> which is wrong,
<nmatsakis> and instead we have special case treatment for closures,
<kimundi> Ah, so it only accepts the 'static bound there
<nmatsakis> which kind of fit ok with the old syntax,
<nmatsakis> but in the newer one we really should be just using general lifetimes in bounds

[Kimundi]

cc @nikomatsakis

[pnkfelix]

cc me

[pnkfelix]

Note in particular that attempting to make 'a |T| -> U a type expression leads to ambiguities when parsing a generic instantiation where you are trying to pass a closure type as the type parameter, like Foo<'a |T| -> U>, because the parser will eagerly treat the Foo<'a prefix as a lifetime parameter being passed to Foo, and thus it expects a comma next.

(An easy work-around for the type being is to use a type declaration like
type closure<'a, T, U> = 'a |T| -> U;
and then you can instantiate Foo on the type closure<'a, T, U>.)

[flaper87]

cc @flaper87

[nikomatsakis]

Another parser issue: something like proc:Send doesn't parse

[nikomatsakis]

Nominating.

[pnkfelix]

Assigning 1.0, P-backcompat-lang

[emberian]

cc me