`impl Trait` Lifetime Elision

[cramertj]

The following ICEs:
fn foo(x: &bool) -> impl Into<&bool> { x }

The message is: error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:618: escaping regions in predicate Obligation(predicate=Binder(TraitPredicate(<_ as std::convert::Into<&bool>>)),depth=0) --> src/main.rs:6:21.

Should we explicitly disallow lifetime elision in impl Trait?

[eddyb]

The ICE should not be fixed, but rather elision be disabled, unless someone can point me to an accepted RFC allowing this kind of elision. cc @rust-lang/lang

[nikomatsakis]

So @cramertj and I discussed this over IRC today. The plan is to support elision as follows:

• Lifetimes can be elided, but if you just write impl Trait it does not capture &self. To capture, you must write impl Trait + '_.
• We will effectively desugar by mapping '_ to an add'l lifetime parameter on the abstract type that is being created. So we would have:

fn foo(x: &u32) -> impl Trait<'_> { ... }

desugaring to something like:

abstract type Foo<'a>: Trait<'a>;
fn foo(x: &u32) -> Foo<'_> { ... }

To implement this, we have to first modify HIR lowerring to detect elided lifetimes when it is scraping the impl trait bounds for additional parameters. We can then add a special parameter (perhaps naming it '_?).

Then we have to modify name resolution as follows. When we are lifetime-resolving the trait bounds (Trait<'a> in the above example):

rust/src/librustc/middle/resolve_lifetime.rs

Line 496 in 833785b

self.with(scope, |_old_scope, this| {

we would insert an elision scope:

rust/src/librustc/middle/resolve_lifetime.rs

Lines 248 to 253 in 833785b

	/// A scope which either determines unspecified lifetimes or errors
	/// on them (e.g. due to ambiguity). For more details, see `Elide`.
	Elision {
	elide: Elide,
	s: ScopeRef<'a>
	},

with Elide::Exact set to a reference to this (early-bound) extra parameter:

rust/src/librustc/middle/resolve_lifetime.rs

Lines 271 to 272 in 833785b

	/// Always use this one lifetime.
	Exact(Region),

We will also insert a reference to the elided lifetime into the Foo<'_> reference, which can just resolve as normal.

[nikomatsakis]

An example from #46565:

struct Parent
{
    children: Option<Vec<String>>
}

impl Parent {
    pub fn children() -> impl Iterator<Item=&str> {
        self.children.unwrap_or(Vec::new()).iter()
    }
}