Function rustc_pattern_analysis::usefulness::compute_exhaustiveness_and_usefulness

fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
    mcx: UsefulnessCtxt<'a, Cx>,
    matrix: &mut Matrix<'p, Cx>
) -> Result<WitnessMatrix<Cx>, Cx::Error>

The core of the algorithm.

This recursively computes witnesses of the non-exhaustiveness of matrix (if any). Also tracks usefulness of each row in the matrix (in row.useful). We track usefulness of each subpattern using interior mutability in DeconstructedPat.

The input Matrix and the output WitnessMatrix together match the type exhaustively.

The key steps are:

• specialization, where we dig into the rows that have a specific constructor and call ourselves recursively;
• unspecialization, where we lift the results from the previous step into results for this step (using apply_constructor and by updating row.useful for each parent row). This is all explained at the top of the file.