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.

Trait Implementations

impl Clone for MutBorrowKind

fn clone(&self) -> MutBorrowKind

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for MutBorrowKind

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<__D: TyDecoder> Decodable<__D> for MutBorrowKind

fn decode(__decoder: &mut __D) -> Self

impl<__E: TyEncoder> Encodable<__E> for MutBorrowKind

fn encode(&self, __encoder: &mut __E)

impl Hash for MutBorrowKind

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<'__ctx> HashStable<StableHashingContext<'__ctx>> for MutBorrowKind

fn hash_stable( &self, __hcx: &mut StableHashingContext<'__ctx>, __hasher: &mut StableHasher, )

impl Ord for MutBorrowKind

fn cmp(&self, other: &MutBorrowKind) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for MutBorrowKind

fn eq(&self, other: &MutBorrowKind) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for MutBorrowKind

fn partial_cmp(&self, other: &MutBorrowKind) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for MutBorrowKind

impl Eq for MutBorrowKind

impl StructuralPartialEq for MutBorrowKind

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 1 byte

Size for each variant:

• Default: 0 bytes
• TwoPhaseBorrow: 0 bytes
• ClosureCapture: 0 bytes