Struct rustc_mir_build::thir::pattern::usefulness::Witness

pub(crate) struct Witness<'p, 'tcx>(Vec<DeconstructedPat<'p, 'tcx>>);

A witness of non-exhaustiveness for error reporting, represented as a list of patterns (in reverse order of construction) with wildcards inside to represent elements that can take any inhabitant of the type as a value.

A witness against a list of patterns should have the same types and length as the pattern matched against. Because Rust match is always against a single pattern, at the end the witness will have length 1, but 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) {
   Pair(None, _) => {}
   Pair(_, false) => {}
}

We’ll perform the following steps:

1. Start with an empty witness Witness(vec![])
2. Push a witness true against the false Witness(vec![true])
3. Push a witness Some(_) against the None Witness(vec![true, Some(_)])
4. Apply the Pair constructor to the witnesses Witness(vec![Pair(Some(_), true)])

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

Tuple Fields

0: Vec<DeconstructedPat<'p, 'tcx>>

Implementations

impl<'p, 'tcx> Witness<'p, 'tcx>

fn single_pattern(self) -> DeconstructedPat<'p, 'tcx>

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

fn apply_constructor(
    self,
    pcx: &PatCtxt<'_, 'p, 'tcx>,
    ctor: &Constructor<'tcx>
) -> Self

Constructs a partial witness for a pattern given a list of patterns expanded by the specialization step.

When a pattern P is discovered to be useful, this function is used bottom-up to reconstruct a complete witness, e.g., a pattern P’ that covers a subset of values, V, where each value in that set is not covered by any previously used patterns and is covered by the pattern P’. Examples:

left_ty: tuple of 3 elements pats: [10, 20, _] => (10, 20, _)

left_ty: struct X { a: (bool, &’static str), b: usize} pats: [(false, “foo”), 42] => X { a: (false, “foo”), b: 42 }

Trait Implementations

impl<'p, 'tcx> Debug for Witness<'p, 'tcx>

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