FakeReadCause in rustc_middle::mir - Rust
Description: The `FakeReadCause` describes the type of pattern why a FakeRead statement exists.

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## FakeReadCause

# Enum FakeReadCause

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

Expand description

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

## Variants§

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### ForMatchGuard

A fake read injected into a match guard to ensure that the discriminants that are being matched on aren’t modified while the match guard is being evaluated.

At the beginning of each match guard, a fake borrow is inserted for each discriminant accessed in the entire `match` statement.

Then, at the end of the match guard, a `FakeRead(ForMatchGuard)` is inserted to keep the fake borrows alive until that point.

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

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### ForMatchedPlace(Option<LocalDefId>)

Fake read of the scrutinee of a `match` or destructuring `let` (i.e. `let` with non-trivial pattern).

In `match x { ... }`, we generate a `FakeRead(ForMatchedPlace, x)` and insert it into the `otherwise_block` (which is supposed to be unreachable for irrefutable pattern-matches like `match` or `let`).

This is necessary because `let x: !; match x {}` doesn’t generate any actual read of x, so we need to generate a `FakeRead` to check that it is initialized.

If the `FakeRead(ForMatchedPlace)` is being performed with a closure that doesn’t capture the required upvars, the `FakeRead` within the closure is omitted entirely.

To make sure that this is still sound, if a closure matches against a Place starting with an Upvar, we hoist the `FakeRead` to the definition point of the closure.

If the `FakeRead` comes from being hoisted out of a closure like this, we record the `LocalDefId` of the closure. Otherwise, the `Option` will be `None`.

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### ForGuardBinding

A fake read injected into a match guard to ensure that the places bound by the pattern are immutable for the duration of the match guard.

Within a match guard, references are created for each place that the pattern creates a binding for — this is known as the `RefWithinGuard` version of the variables. To make sure that the references stay alive until the end of the match guard, and properly prevent the places in question from being modified, a `FakeRead(ForGuardBinding)` is inserted at the end of the match guard.

For details on how these references are created, see the extensive documentation on `bind_matched_candidate_for_guard` in `rustc_mir_build`.

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### 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 `FakeRead(ForLet)` immediately after each `let` with a trivial pattern.

FIXME: `ExprUseVisitor` has an entirely different opinion on what `FakeRead(ForLet)` is supposed to mean. If it was accurate to what MIR lowering does, would it even make sense to hoist these out of closures like `ForMatchedPlace`?

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### ForIndex

Currently, index expressions overloaded through the `Index` trait get lowered differently than index expressions with builtin semantics for arrays and slices — the latter will emit code to perform bound checks, and then return a MIR place that will only perform the indexing “for real” when it gets incorporated into an instruction.

This is observable in the fact that the following compiles:

    fn f(x: &mut [&mut [u32]], i: usize) {
        x[i][x[i].len() - 1] += 1;
    }

However, we need to be careful to not let the user invalidate the bound check with an expression like

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

Here, the first bounds check would be invalidated when we evaluate the second index expression. To make sure that this doesn’t happen, we create a fake borrow of `x` and hold it while we evaluate the second index.

This borrow is kept alive by a `FakeRead(ForIndex)` at the end of its scope.

## Trait Implementations§

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### impl Clone for FakeReadCause

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#### fn clone(&self) -> FakeReadCause

Returns a duplicate of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for FakeReadCause

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

Formats the value using the given formatter. Read more
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### impl<'tcx, __D: TyDecoder<'tcx>> Decodable<__D> for FakeReadCause

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

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### impl<'tcx, __E: TyEncoder<'tcx>> Encodable<__E> for FakeReadCause

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

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### impl Hash for FakeReadCause

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

Feeds this value into the given `Hasher`. Read more
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#### fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
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### impl<'__ctx> HashStable<StableHashingContext<'__ctx>> for FakeReadCause

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

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### impl PartialEq for FakeReadCause

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

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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### impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for FakeReadCause

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#### 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)`. Read more
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#### fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, _: &mut F) -> Self

The entry point for folding. To fold a value `t` with a folder `f` call: `t.fold_with(f)`. Read more
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### impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for FakeReadCause

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#### 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)`. Read more
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### impl Copy for FakeReadCause

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### impl StructuralPartialEq for FakeReadCause

## Auto Trait Implementations§

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### impl DynSend for FakeReadCause

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### impl DynSync for FakeReadCause

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### impl Freeze for FakeReadCause

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### impl RefUnwindSafe for FakeReadCause

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### impl Send for FakeReadCause

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### impl Sync for FakeReadCause

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### impl Unpin for FakeReadCause

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### impl UnsafeUnpin for FakeReadCause

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### impl UnwindSafe for FakeReadCause

## Blanket Implementations§

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### impl<T> Aligned for T

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#### const ALIGN: Alignment

Alignment of `Self`.
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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> AnyEq for T
where T: Any + PartialEq,

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#### fn equals(&self, other: &(dyn Any + 'static)) -> bool

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#### fn as_any(&self) -> &(dyn Any + 'static)

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### impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for T
where T: Copy,

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#### fn allocate_on(self, arena: &'tcx Arena<'tcx>) -> &'tcx mut T

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#### fn allocate_from_iter( arena: &'tcx Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'tcx mut [T]

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### impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for T
where T: Copy,

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#### fn allocate_on(self, arena: &'tcx Arena<'tcx>) -> &'tcx mut T

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#### fn allocate_from_iter( arena: &'tcx Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'tcx mut [T]

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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T, R> CollectAndApply<T, R> for T

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#### fn collect_and_apply<I, F>(iter: I, f: F) -> R
where I: Iterator<Item = T>, F: FnOnce(&[T]) -> R,

Equivalent to `f(&iter.collect::<Vec<_>>())`.

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#### type Output = R

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### impl<'tcx, T> DepNodeKey<'tcx> for T
where T: for<'a> HashStable<StableHashingContext<'a>> + Debug,

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#### default fn key_fingerprint_style() -> KeyFingerprintStyle

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#### default fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint

This method turns a query key into an opaque `Fingerprint` to be used in `DepNode`.
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#### default fn try_recover_key(_: TyCtxt<'tcx>, _: &DepNode) -> Option<T>

This method tries to recover the query key from the given `DepNode`, something which is needed when forcing `DepNode`s during red-green evaluation. The query system will only call this method if `fingerprint_style()` is not `FingerprintStyle::Opaque`. It is always valid to return `None` here, in which case incremental compilation will treat the query as having changed instead of forcing it.
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### impl<T> DynClone for T
where T: Clone,

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#### fn __clone_box(&self, _: Private) -> *mut ()

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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T> Instrument for T

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#### fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided `Span`, returning an `Instrumented` wrapper. Read more
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#### fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current `Span`, returning an `Instrumented` wrapper. Read more
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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> IntoEither for T

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#### fn into_either(self, into_left: bool) -> Either<Self, Self>

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left` is `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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#### fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
where F: FnOnce(&Self) -> bool,

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left(&self)` returns `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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### impl<K> IntoQueryKey<K> for K

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#### fn into_query_key(self) -> K

Argument conversion from `Self` to `K`. This should always be a very cheap conversion, e.g. `LocalDefId::to_def_id`.
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### impl<'tcx, T> IsSuggestable<'tcx> for T
where T: TypeVisitable<TyCtxt<'tcx>> + TypeFoldable<TyCtxt<'tcx>>,

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#### fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool

Whether this makes sense to suggest in a diagnostic. Read more
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#### fn make_suggestable( self, tcx: TyCtxt<'tcx>, infer_suggestable: bool, placeholder: Option<Ty<'tcx>>, ) -> Option<T>

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### impl<T> MaybeResult<T> for T

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#### type Error = !

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#### fn from(_: Result<T, <T as MaybeResult<T>>::Error>) -> T

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#### fn to_result(self) -> Result<T, <T as MaybeResult<T>>::Error>

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### impl<T> Pointable for T

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#### const ALIGN: usize

The alignment of pointer.
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#### type Init = T

The type for initializers.
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#### unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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#### unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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#### unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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#### unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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### impl<T> Same for T

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#### type Output = T

Should always be `Self`
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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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### impl<I, T> TypeVisitableExt<I> for T
where I: Interner, T: TypeVisitable<I>,

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#### fn has_type_flags(&self, flags: TypeFlags) -> bool

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#### fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool

Returns `true` if `self` has any late-bound regions that are either bound by `binder` or bound by some binder outside of `binder`. If `binder` is `ty::INNERMOST`, this indicates whether there are any late-bound regions that appear free.
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#### fn error_reported(&self) -> Result<(), <I as Interner>::ErrorGuaranteed>

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#### fn non_region_error_reported( &self, ) -> Result<(), <I as Interner>::ErrorGuaranteed>

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#### fn has_vars_bound_above(&self, binder: DebruijnIndex) -> bool

Returns `true` if this type has any regions that escape `binder` (and hence are not bound by it).
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#### fn has_escaping_bound_vars(&self) -> bool

Return `true` if this type has regions that are not a part of the type. For example, `for<'a> fn(&'a i32)` return `false`, while `fn(&'a i32)` would return `true`. The latter can occur when traversing through the former. Read more
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#### fn has_aliases(&self) -> bool

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#### fn has_opaque_types(&self) -> bool

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#### fn has_coroutines(&self) -> bool

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#### fn references_error(&self) -> bool

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#### fn has_non_region_param(&self) -> bool

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#### fn has_infer_regions(&self) -> bool

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#### fn has_infer_types(&self) -> bool

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#### fn has_non_region_infer(&self) -> bool

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#### fn has_infer(&self) -> bool

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#### fn has_placeholders(&self) -> bool

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#### fn has_non_region_placeholders(&self) -> bool

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#### fn has_param(&self) -> bool

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#### fn has_free_regions(&self) -> bool

“Free” regions in this context means that it has any region that is not (a) erased or (b) late-bound.
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#### fn has_erased_regions(&self) -> bool

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#### fn has_erasable_regions(&self) -> bool

True if there are any un-erased free regions.
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#### fn is_global(&self) -> bool

Indicates whether this value references only ‘global’ generic parameters that are the same regardless of what fn we are in. This is used for caching.
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#### fn has_bound_regions(&self) -> bool

True if there are any late-bound regions
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#### fn has_non_region_bound_vars(&self) -> bool

True if there are any late-bound non-region variables
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#### fn has_bound_vars(&self) -> bool

True if there are any bound variables
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#### fn still_further_specializable(&self) -> bool

Indicates whether this value still has parameters/placeholders/inference variables which could be replaced later, in a way that would change the results of `impl` specialization.
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#### fn has_non_region_error(&self) -> bool

True if a type or const error is reachable
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### impl<I, T, U> Upcast<I, U> for T
where U: UpcastFrom<I, T>,

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#### fn upcast(self, interner: I) -> U

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### impl<I, T> UpcastFrom<I, T> for T

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#### fn upcast_from(from: T, _tcx: I) -> T

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### impl<V, T> VZip<V> for T
where V: MultiLane<T>,

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#### fn vzip(self) -> V

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### impl<T> WithSubscriber for T

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#### fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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#### fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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### impl<T> ErasedDestructor for T
where T: 'static,

## 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