promotion tweaks

[RalfJung]

r? @oli-obk @ecstatic-morse

[ecstatic-morse]

This looks good to me apart from the mention about promotion of references to types with interior mutability. Perhaps we could include the following as an example?

const X: Cell<i32> = Cell::new(5); // ok
fn main() {
    let x: &'static _ = &X; // not ok
}

[RalfJung]

This looks good to me apart from the mention about promotion of references to types with interior mutability. Perhaps we could include the following as an example?

Done.

[ecstatic-morse]

It's worth noting that these restrictions apply only in the context lifetime extension. We can initialize an array, which also uses the implicit rules, with a constant containing interior mutability (i.e. [CELL; 4]). My revisions failed to do this.

[RalfJung]

But [&CELL; 4] gets rejected, I hope?

[ecstatic-morse]

That compiles on stable because &T is Copy for all T. Any use of it will cause a borrow check error since the access to CELL has a temporary lifetime. Promotion fails for &CELL even if it's in an array initializer if that's what you're asking.

[RalfJung]

Promotion fails for &CELL even if it's in an array initializer if that's what you're asking.

Good.

But then behavior seems to be the same for arrays and lifetime extension? It's just that lifetime extension always involves a borrow.

[RalfJung]

@ecstatic-morse any more concerns/comments?

[ecstatic-morse]

r=me