Enum FakeReadCause

pub enum FakeReadCause {
    ForMatchGuard,
    ForMatchedPlace(Option<LocalDefId>),
    ForGuardBinding,
    ForLet(Option<LocalDefId>),
    ForIndex,
}

The FakeReadCause describes the type of pattern why a FakeRead statement exists.

Variants

ForMatchGuard

Inject a fake read of the borrowed input at the end of each guards code.

This should ensure that you cannot change the variant for an enum while you are in the midst of matching on it.

ForMatchedPlace(Option<LocalDefId>)

let x: !; match x {} doesn’t generate any read of x so we need to generate a read of x to check that it is initialized and safe.

If a closure pattern matches a Place starting with an Upvar, then we introduce a FakeRead for that Place outside the closure, in such a case this option would be Some(closure_def_id). Otherwise, the value of the optional LocalDefId will be None.

ForGuardBinding

A fake read of the RefWithinGuard version of a bind-by-value variable in a match guard to ensure that its value hasn’t change by the time we create the OutsideGuard version.

ForLet(Option<LocalDefId>)

Officially, the semantics of

let pattern = <expr>;

is that <expr> is evaluated into a temporary and then this temporary is into the pattern.

However, if we see the simple pattern let var = <expr>, we optimize this to evaluate <expr> directly into the variable var. This is mostly unobservable, but in some cases it can affect the borrow checker, as in #53695. Therefore, we insert a “fake read” here to ensure that we get appropriate errors.

If a closure pattern matches a Place starting with an Upvar, then we introduce a FakeRead for that Place outside the closure, in such a case this option would be Some(closure_def_id). Otherwise, the value of the optional DefId will be None.

ForIndex

If we have an index expression like

(*x)[1][{ x = y; 4}]

then the first bounds check is invalidated when we evaluate the second index expression. Thus we create a fake borrow of x across the second indexer, which will cause a borrow check error.

Trait Implementations

impl Clone for FakeReadCause

fn clone(&self) -> FakeReadCause

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for FakeReadCause

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

Formats the value using the given formatter.

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

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

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

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

impl Hash for FakeReadCause

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 FakeReadCause

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

impl PartialEq for FakeReadCause

fn eq(&self, other: &FakeReadCause) -> 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<'tcx> TypeFoldable<TyCtxt<'tcx>> for FakeReadCause

fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>( self, _: &mut F, ) -> Result<Self, F::Error>

The entry point for folding. To fold a value t with a folder f call: t.try_fold_with(f).

fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, _: &mut F) -> Self

A convenient alternative to try_fold_with for use with infallible folders. Do not override this method, to ensure coherence with try_fold_with.

impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for FakeReadCause

fn visit_with<F: TypeVisitor<TyCtxt<'tcx>>>(&self, _: &mut F) -> F::Result

The entry point for visiting. To visit a value t with a visitor v call: t.visit_with(v).

impl Copy for FakeReadCause

impl StructuralPartialEq for FakeReadCause

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: 8 bytes

Size for each variant:

• ForMatchGuard: 0 bytes
• ForMatchedPlace: 4 bytes
• ForGuardBinding: 0 bytes
• ForLet: 4 bytes
• ForIndex: 0 bytes