Slice in rustc_pattern_analysis::constructor - Rust
Description: A constructor for array and slice patterns.

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## Slice

# Struct Slice

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    pub struct Slice {
        pub(crate) array_len: Option<usize>,
        pub(crate) kind: SliceKind,
    }

Expand description

A constructor for array and slice patterns.

## Fields§

§`array_len: Option<usize>`

`None` if the matched value is a slice, `Some(n)` if it is an array of size `n`.

§`kind: SliceKind`

The kind of pattern it is: fixed-length `[x, y]` or variable length `[x, .., y]`.

## Implementations§

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### impl Slice

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#### pub fn new(array_len: Option<usize>, kind: SliceKind) -> Self

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#### pub fn arity(self) -> usize

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#### fn is_covered_by(self, other: Self) -> bool

See `Constructor::is_covered_by`

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#### pub fn array_len(self) -> Option<usize>

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#### pub fn kind(self) -> SliceKind

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#### fn split( self, column_slices: impl Iterator<Item = Slice>, ) -> impl Iterator<Item = (Presence, Slice)>

This computes constructor splitting for variable-length slices, as explained at the top of the file.

A slice pattern `[x, .., y]` behaves like the infinite or-pattern `[x, y] | [x, _, y] | [x, _, _, y] | etc`. The corresponding value constructors are fixed-length array constructors of corresponding lengths. We obviously can’t list this infinitude of constructors. Thankfully, it turns out that for each finite set of slice patterns, all sufficiently large array lengths are equivalent.

Let’s look at an example, where we are trying to split the last pattern:

    match x {
        [true, true, ..] => {}
        [.., false, false] => {}
        [..] => {}
    }

Here are the results of specialization for the first few lengths:

    // length 0
    [] => {}
    // length 1
    [_] => {}
    // length 2
    [true, true] => {}
    [false, false] => {}
    [_, _] => {}
    // length 3
    [true, true,  _    ] => {}
    [_,    false, false] => {}
    [_,    _,     _    ] => {}
    // length 4
    [true, true, _,     _    ] => {}
    [_,    _,    false, false] => {}
    [_,    _,    _,     _    ] => {}
    // length 5
    [true, true, _, _,     _    ] => {}
    [_,    _,    _, false, false] => {}
    [_,    _,    _, _,     _    ] => {}

We see that above length 4, we are simply inserting columns full of wildcards in the middle. This means that specialization and witness computation with slices of length `l >= 4` will give equivalent results regardless of `l`. This applies to any set of slice patterns: there will be a length `L` above which all lengths behave the same. This is exactly what we need for constructor splitting.

A variable-length slice pattern covers all lengths from its arity up to infinity. As we just saw, we can split this in two: lengths below `L` are treated individually with a fixed-length slice each; lengths above `L` are grouped into a single variable-length slice constructor.

For each variable-length slice pattern `p` with a prefix of length `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last `slₚ` elements are examined. Therefore, as long as `L` is positive (to avoid concerns about empty types), all elements after the maximum prefix length and before the maximum suffix length are not examined by any variable-length pattern, and therefore can be ignored. This gives us a way to compute `L`.

Additionally, if fixed-length patterns exist, we must pick an `L` large enough to miss them, so we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`. `max_slice` below will be made to have this arity `L`.

If `self` is fixed-length, it is returned as-is.

Additionally, we track for each output slice whether it is covered by one of the column slices or not.

## Trait Implementations§

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### impl Clone for Slice

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

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 Slice

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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 PartialEq for Slice

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#### fn eq(&self, other: &Slice) -> 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 Copy for Slice

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### impl Eq for Slice

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### impl StructuralPartialEq for Slice

## Auto Trait Implementations§

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### impl DynSend for Slice

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### impl DynSync for Slice

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### impl Freeze for Slice

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### impl RefUnwindSafe for Slice

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### impl Send for Slice

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### impl Sync for Slice

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### impl Unpin for Slice

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### impl UnsafeUnpin for Slice

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### impl UnwindSafe for Slice

## 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<T> DynClone for T
where T: Clone,

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

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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Compare self to `key` and return `true` if they are equal.
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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<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, 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<'a, T> Captures<'a> for T
where T: ?Sized,

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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: 40 bytes