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 you the closure
is borrowing or mutating a mutable referent, e.g.:
let x: &mut int = ...;
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 { x: & &mut int }
let x: &mut int = ...;
let y = (&mut Env { &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 a &mut found
in an aliasable location. To solve, you'd have to translate with
an &mut borrow:
struct Env { x: & &mut int }
let x: &mut int = ...;
let y = (&mut Env { &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 "unique imm" borrow -- the referent is
immutable, but not aliasable. This solves the problem. For
simplicity, we don't give users the way to express this
borrow, it's just used when translating closures.
