Struct rustc_mir_build::thir::pattern::usefulness::WitnessStack

pub(crate) struct WitnessStack<'tcx>(Vec<WitnessPat<'tcx>>);

A witness-tuple 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.

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). Because Rust match is always against a single pattern, at the end the stack 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.

Tuple Fields

0: Vec<WitnessPat<'tcx>>

Implementations

impl<'tcx> WitnessStack<'tcx>

fn single_pattern(self) -> WitnessPat<'tcx>

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

fn apply_constructor( self, pcx: &PatCtxt<'_, '_, '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<'tcx> Clone for WitnessStack<'tcx>

fn clone(&self) -> WitnessStack<'tcx>

Returns a copy of the value.

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

Performs copy-assignment from source.

impl<'tcx> Debug for WitnessStack<'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