Struct rustc_pattern_analysis::constructor::Slice

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

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

impl Slice

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

pub fn arity(self) -> usize

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

See Constructor::is_covered_by

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

impl Clone for Slice

fn clone(&self) -> Slice

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Slice

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

Formats the value using the given formatter.

impl PartialEq for Slice

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

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for Slice

impl Eq for Slice

impl StructuralPartialEq for Slice

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