Struct rustc_pattern_analysis::usefulness::WitnessStack

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

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 PatStacks 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

impl<Cx: PatCx> WitnessStack<Cx>

fn single_pattern(self) -> WitnessPat<Cx>

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

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

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

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

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

fn clone(&self) -> Self

Returns a copy of the value.

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

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

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