Enum MutBorrowKind

pub enum MutBorrowKind {
    Default,
    TwoPhaseBorrow,
    ClosureCapture,
}

Variants

Default

TwoPhaseBorrow

This borrow arose from method-call auto-ref. (i.e., adjustment::Adjust::Borrow)

ClosureCapture

Data must be immutable but not aliasable. This kind of borrow cannot currently be expressed by the user and is used only in implicit closure bindings. It is needed when the closure is borrowing or mutating a mutable referent, e.g.:

let mut z = 3;
let x: &mut isize = &mut z;
let y = || *x += 5;

If we were to try to translate this closure into a more explicit form, we’d encounter an error with the code as written:

struct Env<'a> { x: &'a &'a mut isize }
let mut z = 3;
let x: &mut isize = &mut z;
let y = (&mut Env { x: &x }, fn_ptr);  // Closure is pair of env and fn
fn fn_ptr(env: &mut Env) { **env.x += 5; }

This is then illegal because you cannot mutate an &mut found in an aliasable location. To solve, you’d have to translate with an &mut borrow:

struct Env<'a> { x: &'a mut &'a mut isize }
let mut z = 3;
let x: &mut isize = &mut z;
let y = (&mut Env { x: &mut x }, fn_ptr); // changed from &x to &mut x
fn fn_ptr(env: &mut Env) { **env.x += 5; }

Now the assignment to **env.x is legal, but creating a mutable pointer to x is not because x is not mutable. We could fix this by declaring x as let mut x. This is ok in user code, if awkward, but extra weird for closures, since the borrow is hidden.

So we introduce a ClosureCapture borrow – user will not have to mark the variable containing the mutable reference as mut, as they didn’t ever intend to mutate the mutable reference itself. We still mutable capture it in order to mutate the pointed value through it (but not mutating the reference itself).

This solves the problem. For simplicity, we don’t give users the way to express this borrow, it’s just used when translating closures.