Code sometimes uses UniqueImmBorrow and not MutBorrow

[nikomatsakis]

Discussion here:

rust-lang/rust#86995 (comment)

[nikomatsakis]

Relevant code:

https://github.com/rust-lang/rust/blob/efeea79a6bb6867b220471e0396b82b6b68ebc4c/compiler/rustc_typeck/src/check/upvar.rs#L1495-L1503

The problem here is that this code is still oriented around a single variable, and not a place.

[nikomatsakis]

Given foo: &mut (u32, u32), there are a few scenarios to consider:

• If the closure mutates foo.0 but we wind up capturing foo, we only need a "unique" borrow of foo.
• If the closure mutates foo.0 and we capture foo.0, we need a "mut" borrow.

In short, the need for a "mut" borrow depends on what data we ultimately capture.

[arora-aman]

https://hackmd.io/dKtOGyJgSjuf-ztGD7I7kw