feat: Closure capture inlay hints

[Veykril]

I opted for a fictional move(foo, &bar, &mut qux) syntax here, disabled by default as these are not correct rust syntax and hence could cause confusion.

[Veykril]

@HKalbasi fwiw I still think this is wrong. qux is not a mutable binding so we shouldn't be allowed to take a mutable borrow here. Do you mind pointing me to the part in the rustc code base for this (assuming you have it at hand)?

[HKalbasi]

qux is not mutable, but we are taking a mutable reference to the *qux, not qux itself. For example this code:

    let mut t1 = 2;
    let mut t2 = 5;
    let mut x = &mut t1;
    let mut y = || {
        //&x;
        *x = 3;
    };
    x = &mut t2;
    y();

compiles since y is capturing *x by mutable borrow, so x itself is free to mutate. But if the //&x; line becomes uncommented, y will capture the whole x by unique immutable borrow, and we will get compile error.

[HKalbasi]

I think it will becomes less confusing if we render the whole place.

[Veykril]

I think my confusion comes from thinking of old capture logic? Also the rust reference I suppose is outdated then. You are certainly right that we should be rendering the place we capture, as with this mere field captures will render incorrectly already I believe.

[HKalbasi]

I think my confusion comes from thinking of old capture logic? Also the rust reference I suppose is outdated then

Yes the rust reference section for closure is pre edition-2021, and is currently in a misleading state.

[Veykril]

Turns out there is a PR for this already so I'll dig into that for now rust-lang/reference#1059

[Veykril]

The new writing still refers to this as a unique immutable borrow fwiw https://github.com/sexxi-goose/reference/blob/closure-doc/src/types/closure.md#unique-immutable-borrows-in-captures

[matklad]

🔥 🧨 🚒

[Veykril]

We probably want an image showcasing more complex captures for the changelog, will cook those up some later time this week
@bors r+

[bors]

📌 Commit 4c5fd19 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4c5fd19 with merge d3ce333...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing d3ce333 to master...

[lnicola]

We probably want an image showcasing more complex captures for the changelog

I'll use the existing one, it seems complex enough to me 😅.