# Struct rustc_pattern_analysis::usefulness::WitnessStack

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    struct WitnessStack<Cx: PatCx>(Vec<WitnessPat<Cx>>);

Expand description

A witness-tuple of non-exhaustiveness for error reporting, represented as a list of patterns (in reverse order of construction).

This mirrors `PatStack`: they function similarly, except `PatStack` contains user patterns we are inspecting, and `WitnessStack` contains witnesses we are constructing. FIXME(Nadrieril): use the same order of patterns for both.

A `WitnessStack` should have the same types and length as the `PatStack`s we are inspecting (except we store the patterns in reverse order). The same way `PatStack` starts with length 1, at the end of the algorithm this will have length 1. In the middle of the algorithm, it can contain multiple patterns.

For example, if we are constructing a witness for the match against

ⓘ

    struct Pair(Option<(u32, u32)>, bool);
    match p {
       Pair(None, _) => {}
       Pair(_, false) => {}
    }

We’ll perform the following steps (among others):

    - Start with a matrix representing the match
        `PatStack(vec![Pair(None, _)])`
        `PatStack(vec![Pair(_, false)])`
    - Specialize with `Pair`
        `PatStack(vec![None, _])`
        `PatStack(vec![_, false])`
    - Specialize with `Some`
        `PatStack(vec![_, false])`
    - Specialize with `_`
        `PatStack(vec![false])`
    - Specialize with `true`
        // no patstacks left
    - This is a non-exhaustive match: we have the empty witness stack as a witness.
        `WitnessStack(vec![])`
    - Apply `true`
        `WitnessStack(vec![true])`
    - Apply `_`
        `WitnessStack(vec![true, _])`
    - Apply `Some`
        `WitnessStack(vec![true, Some(_)])`
    - Apply `Pair`
        `WitnessStack(vec![Pair(Some(_), true)])`

The final `Pair(Some(_), true)` is then the resulting witness.

See the top of the file for more detailed explanations and examples.

## Tuple Fields§

§`0: Vec<WitnessPat<Cx>>`

## Implementations§

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### impl<Cx: PatCx> WitnessStack<Cx>

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#### fn single_pattern(self) -> WitnessPat<Cx>

Asserts that the witness contains a single pattern, and returns it.

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#### fn push_pattern(&mut self, pat: WitnessPat<Cx>)

Reverses specialization by the `Missing` constructor by pushing a whole new pattern.

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#### fn apply_constructor(&mut self, pcx: &PlaceCtxt<'_, Cx>, ctor: &Constructor<Cx>)

Reverses specialization. Given a witness obtained after specialization, this constructs a new witness valid for before specialization. See the section on `unspecialize` at the top of the file.

Examples:

    ctor: tuple of 2 elements
    pats: [false, "foo", _, true]
    result: [(false, "foo"), _, true]

    ctor: Enum::Variant { a: (bool, &'static str), b: usize}
    pats: [(false, "foo"), _, true]
    result: [Enum::Variant { a: (false, "foo"), b: _ }, true]

## Trait Implementations§

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### impl<Cx: PatCx> Clone for WitnessStack<Cx>

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

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<Cx: Debug + PatCx> Debug for WitnessStack<Cx>

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

Formats the value using the given formatter. Read more

## Auto Trait Implementations§

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### impl<Cx> DynSend for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: DynSend, <Cx as PatCx>::Ty: DynSend, <Cx as PatCx>::VariantIdx: DynSend,

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### impl<Cx> DynSync for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: DynSync, <Cx as PatCx>::Ty: DynSync, <Cx as PatCx>::VariantIdx: DynSync,

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### impl<Cx> Freeze for WitnessStack<Cx>

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### impl<Cx> RefUnwindSafe for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: RefUnwindSafe, <Cx as PatCx>::Ty: RefUnwindSafe, <Cx as PatCx>::VariantIdx: RefUnwindSafe,

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### impl<Cx> Send for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: Send, <Cx as PatCx>::Ty: Send, <Cx as PatCx>::VariantIdx: Send,

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### impl<Cx> Sync for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: Sync, <Cx as PatCx>::Ty: Sync, <Cx as PatCx>::VariantIdx: Sync,

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### impl<Cx> Unpin for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: Unpin, <Cx as PatCx>::Ty: Unpin, <Cx as PatCx>::VariantIdx: Unpin,

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### impl<Cx> UnwindSafe for WitnessStack<Cx>
where <Cx as PatCx>::StrLit: UnwindSafe, <Cx as PatCx>::Ty: UnwindSafe, <Cx as PatCx>::VariantIdx: UnwindSafe,

## 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> 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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#### fn into_query_param(self) -> 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<'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,

## 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: 24 bytes