Enum rustc_typeck::check::expectation::Expectation
source · [−]pub enum Expectation<'tcx> {
NoExpectation,
ExpectHasType(Ty<'tcx>),
ExpectCastableToType(Ty<'tcx>),
ExpectRvalueLikeUnsized(Ty<'tcx>),
IsLast(Span),
}
Expand description
When type-checking an expression, we propagate downward
whatever type hint we are able in the form of an Expectation
.
Variants
NoExpectation
We know nothing about what type this expression should have.
ExpectHasType(Ty<'tcx>)
This expression should have the type given (or some subtype).
ExpectCastableToType(Ty<'tcx>)
This expression will be cast to the Ty
.
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
.
IsLast(Span)
Implementations
sourceimpl<'a, 'tcx> Expectation<'tcx>
impl<'a, 'tcx> Expectation<'tcx>
pub(super) fn adjust_for_branches(
&self,
fcx: &FnCtxt<'a, 'tcx>
) -> Expectation<'tcx>
sourcepub(super) fn rvalue_hint(
fcx: &FnCtxt<'a, 'tcx>,
ty: Ty<'tcx>
) -> Expectation<'tcx>
pub(super) 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,.
fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx>
pub(super) fn to_option(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>>
sourcepub(super) fn only_has_type(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>>
pub(super) 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.
sourcepub(super) fn coercion_target_type(
self,
fcx: &FnCtxt<'a, 'tcx>,
span: Span
) -> Ty<'tcx>
pub(super) 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
sourceimpl<'tcx> Clone for Expectation<'tcx>
impl<'tcx> Clone for Expectation<'tcx>
sourcefn clone(&self) -> Expectation<'tcx>
fn clone(&self) -> Expectation<'tcx>
1.0.0 · sourcefn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
source
. Read moresourceimpl<'tcx> Debug for Expectation<'tcx>
impl<'tcx> Debug for Expectation<'tcx>
impl<'tcx> Copy for Expectation<'tcx>
Auto Trait Implementations
impl<'tcx> !RefUnwindSafe for Expectation<'tcx>
impl<'tcx> !Send for Expectation<'tcx>
impl<'tcx> !Sync for Expectation<'tcx>
impl<'tcx> Unpin for Expectation<'tcx>
impl<'tcx> !UnwindSafe for Expectation<'tcx>
Blanket Implementations
sourceimpl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
impl<'a, T> Captures<'a> for Twhere
T: ?Sized,
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: 16 bytes
Size for each variant:
NoExpectation
: 0 bytesExpectHasType
: 12 bytesExpectCastableToType
: 12 bytesExpectRvalueLikeUnsized
: 12 bytesIsLast
: 8 bytes