pub enum BorrowKind {
ImmBorrow,
UniqueImmBorrow,
MutBorrow,
}
Variants
ImmBorrow | Data must be immutable and is aliasable.
|
UniqueImmBorrow | 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 isize = ...;
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 isize }
let x: &mut isize = ...;
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 isize }
let x: &mut isize = ...;
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.
|
MutBorrow | Data is mutable and not aliasable.
|
Methods
Returns a mutability m
such that an &m T
pointer could be used to obtain this borrow
kind. Because borrow kinds are richer than mutabilities, we sometimes have to pick a
mutability that is stronger than necessary so that it at least would permit the borrow in
question.
Trait Implementations
Derived Implementations
fn hash<__H>(&self, __arg_0: &mut __H) where __H: Hasher
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