Enum rustc_hir_typeck::expectation::Expectation

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pub enum Expectation<'tcx> {
    NoExpectation,
    ExpectHasType(Ty<'tcx>),
    ExpectCastableToType(Ty<'tcx>),
    ExpectRvalueLikeUnsized(Ty<'tcx>),
}
Expand description

When type-checking an expression, we propagate downward whatever type hint we are able in the form of an Expectation.

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NoExpectation

We know nothing about what type this expression should have.

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ExpectHasType(Ty<'tcx>)

This expression should have the type given (or some subtype).

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ExpectCastableToType(Ty<'tcx>)

This expression will be cast to the Ty.

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ExpectRvalueLikeUnsized(Ty<'tcx>)

This rvalue expression will be wrapped in & or Box and coerced to &Ty or Box<Ty>, respectively. Ty is [A] or Trait.

Implementations§

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impl<'a, 'tcx> Expectation<'tcx>

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pub(crate) fn adjust_for_branches( &self, fcx: &FnCtxt<'a, 'tcx> ) -> Expectation<'tcx>

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pub(crate) fn rvalue_hint( fcx: &FnCtxt<'a, 'tcx>, ty: Ty<'tcx> ) -> Expectation<'tcx>

Provides an expectation for an rvalue expression given an optional hint, which is not required for type safety (the resulting type might be checked higher up, as is the case with &expr and box expr), but is useful in determining the concrete type.

The primary use case is where the expected type is a fat pointer, like &[isize]. For example, consider the following statement:

let x: &isize = &[1, 2, 3];

In this case, the expected type for the &[1, 2, 3] expression is &[isize]. If however we were to say that [1, 2, 3] has the expectation ExpectHasType([isize]), that would be too strong – [1, 2, 3] does not have the type [isize] but rather [isize; 3]. It is only the &[1, 2, 3] expression as a whole that can be coerced to the type &[isize]. Therefore, we propagate this more limited hint, which still is useful, because it informs integer literals and the like. See the test case test/ui/coerce-expect-unsized.rs and #20169 for examples of where this comes up,.

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fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx>

Resolves expected by a single level if it is a variable. If there is no expected type or resolution is not possible (e.g., no constraints yet present), just returns self.

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pub(crate) fn to_option(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>>

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pub(crate) fn only_has_type(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>>

It sometimes happens that we want to turn an expectation into a hard constraint (i.e., something that must be satisfied for the program to type-check). only_has_type will return such a constraint, if it exists.

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pub(crate) fn coercion_target_type( self, fcx: &FnCtxt<'a, 'tcx>, span: Span ) -> Ty<'tcx>

Like only_has_type, but instead of returning None if no hard constraint exists, creates a fresh type variable.

Trait Implementations§

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impl<'tcx> Clone for Expectation<'tcx>

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fn clone(&self) -> Expectation<'tcx>

Returns a copy 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<'tcx> Debug for Expectation<'tcx>

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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> Copy for Expectation<'tcx>

Auto Trait Implementations§

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impl<'tcx> DynSend for Expectation<'tcx>

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impl<'tcx> DynSync for Expectation<'tcx>

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impl<'tcx> Freeze for Expectation<'tcx>

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impl<'tcx> !RefUnwindSafe for Expectation<'tcx>

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impl<'tcx> Send for Expectation<'tcx>

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impl<'tcx> Sync for Expectation<'tcx>

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impl<'tcx> Unpin for Expectation<'tcx>

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impl<'tcx> !UnwindSafe for Expectation<'tcx>

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

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

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

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

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

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fn allocate_from_iter<'a>( arena: &'a Arena<'tcx>, iter: impl IntoIterator<Item = T> ) -> &'a 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, 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<T> Filterable for T

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fn filterable( self, filter_name: &'static str ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>

Creates a filterable data provider with the given name for debugging. Read more
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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<P> IntoQueryParam<P> for P

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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<'tcx, T> ToPredicate<'tcx, T> for T

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fn to_predicate(self, _tcx: TyCtxt<'tcx>) -> T

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

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fn vzip(self) -> V

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impl<Tcx, T> Value<Tcx> for T
where Tcx: DepContext,

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default fn from_cycle_error( tcx: Tcx, cycle_error: &CycleError, _guar: ErrorGuaranteed ) -> T

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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<'a, T> Captures<'a> for T
where T: ?Sized,

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impl<T> ErasedDestructor for T
where T: 'static,

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impl<T> MaybeSendSync for T
where T: Send + Sync,

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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: 16 bytes

Size for each variant:

  • NoExpectation: 0 bytes
  • ExpectHasType: 8 bytes
  • ExpectCastableToType: 8 bytes
  • ExpectRvalueLikeUnsized: 8 bytes