rust/compiler/rustc_pattern_analysis/src/lints.rs

use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
use rustc_span::ErrorGuaranteed;

use crate::errors::{NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Uncovered};
use crate::pat::PatOrWild;
use crate::rustc::{
    Constructor, DeconstructedPat, MatchArm, MatchCtxt, PlaceCtxt, RevealedTy, RustcMatchCheckCtxt,
    SplitConstructorSet, WitnessPat,
};

/// A column of patterns in the matrix, where a column is the intuitive notion of "subpatterns that
/// inspect the same subvalue/place".
/// This is used to traverse patterns column-by-column for lints. Despite similarities with the
/// algorithm in [`crate::usefulness`], this does a different traversal. Notably this is linear in
/// the depth of patterns, whereas `compute_exhaustiveness_and_usefulness` is worst-case exponential
/// (exhaustiveness is NP-complete). The core difference is that we treat sub-columns separately.
///
/// This must not contain an or-pattern. `expand_and_push` takes care to expand them.
///
/// This is not used in the usefulness algorithm; only in lints.
#[derive(Debug)]
pub(crate) struct PatternColumn<'p, 'tcx> {
    patterns: Vec<&'p DeconstructedPat<'p, 'tcx>>,
}

impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
    pub(crate) fn new(arms: &[MatchArm<'p, 'tcx>]) -> Self {
        let patterns = Vec::with_capacity(arms.len());
        let mut column = PatternColumn { patterns };
        for arm in arms {
            column.expand_and_push(PatOrWild::Pat(arm.pat));
        }
        column
    }
    /// Pushes a pattern onto the column, expanding any or-patterns into its subpatterns.
    /// Internal method, prefer [`PatternColumn::new`].
    fn expand_and_push(&mut self, pat: PatOrWild<'p, RustcMatchCheckCtxt<'p, 'tcx>>) {
        // We flatten or-patterns and skip algorithm-generated wildcards.
        if pat.is_or_pat() {
            self.patterns.extend(
                pat.flatten_or_pat().into_iter().filter_map(|pat_or_wild| pat_or_wild.as_pat()),
            )
        } else if let Some(pat) = pat.as_pat() {
            self.patterns.push(pat)
        }
    }

    fn head_ty(&self) -> Option<RevealedTy<'tcx>> {
        self.patterns.first().map(|pat| pat.ty())
    }

    /// Do constructor splitting on the constructors of the column.
    fn analyze_ctors(
        &self,
        pcx: &PlaceCtxt<'_, 'p, 'tcx>,
    ) -> Result<SplitConstructorSet<'p, 'tcx>, ErrorGuaranteed> {
        let column_ctors = self.patterns.iter().map(|p| p.ctor());
        let ctors_for_ty = &pcx.ctors_for_ty()?;
        Ok(ctors_for_ty.split(column_ctors))
    }

    /// Does specialization: given a constructor, this takes the patterns from the column that match
    /// the constructor, and outputs their fields.
    /// This returns one column per field of the constructor. They usually all have the same length
    /// (the number of patterns in `self` that matched `ctor`), except that we expand or-patterns
    /// which may change the lengths.
    fn specialize(
        &self,
        pcx: &PlaceCtxt<'_, 'p, 'tcx>,
        ctor: &Constructor<'p, 'tcx>,
    ) -> Vec<PatternColumn<'p, 'tcx>> {
        let arity = ctor.arity(pcx);
        if arity == 0 {
            return Vec::new();
        }

        // We specialize the column by `ctor`. This gives us `arity`-many columns of patterns. These
        // columns may have different lengths in the presence of or-patterns (this is why we can't
        // reuse `Matrix`).
        let mut specialized_columns: Vec<_> =
            (0..arity).map(|_| Self { patterns: Vec::new() }).collect();
        let relevant_patterns =
            self.patterns.iter().filter(|pat| ctor.is_covered_by(pcx, pat.ctor()));
        for pat in relevant_patterns {
            let specialized = pat.specialize(ctor, arity);
            for (subpat, column) in specialized.into_iter().zip(&mut specialized_columns) {
                column.expand_and_push(subpat);
            }
        }
        specialized_columns
    }
}

/// Traverse the patterns to collect any variants of a non_exhaustive enum that fail to be mentioned
/// in a given column.
#[instrument(level = "debug", skip(cx), ret)]
fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
    cx: MatchCtxt<'a, 'p, 'tcx>,
    column: &PatternColumn<'p, 'tcx>,
) -> Result<Vec<WitnessPat<'p, 'tcx>>, ErrorGuaranteed> {
    let Some(ty) = column.head_ty() else {
        return Ok(Vec::new());
    };
    let pcx = &PlaceCtxt::new_dummy(cx, ty);

    let set = column.analyze_ctors(pcx)?;
    if set.present.is_empty() {
        // We can't consistently handle the case where no constructors are present (since this would
        // require digging deep through any type in case there's a non_exhaustive enum somewhere),
        // so for consistency we refuse to handle the top-level case, where we could handle it.
        return Ok(Vec::new());
    }

    let mut witnesses = Vec::new();
    if cx.tycx.is_foreign_non_exhaustive_enum(ty) {
        witnesses.extend(
            set.missing
                .into_iter()
                // This will list missing visible variants.
                .filter(|c| !matches!(c, Constructor::Hidden | Constructor::NonExhaustive))
                .map(|missing_ctor| WitnessPat::wild_from_ctor(pcx, missing_ctor)),
        )
    }

    // Recurse into the fields.
    for ctor in set.present {
        let specialized_columns = column.specialize(pcx, &ctor);
        let wild_pat = WitnessPat::wild_from_ctor(pcx, ctor);
        for (i, col_i) in specialized_columns.iter().enumerate() {
            // Compute witnesses for each column.
            let wits_for_col_i = collect_nonexhaustive_missing_variants(cx, col_i)?;
            // For each witness, we build a new pattern in the shape of `ctor(_, _, wit, _, _)`,
            // adding enough wildcards to match `arity`.
            for wit in wits_for_col_i {
                let mut pat = wild_pat.clone();
                pat.fields[i] = wit;
                witnesses.push(pat);
            }
        }
    }
    Ok(witnesses)
}

pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
    cx: MatchCtxt<'a, 'p, 'tcx>,
    arms: &[MatchArm<'p, 'tcx>],
    pat_column: &PatternColumn<'p, 'tcx>,
    scrut_ty: RevealedTy<'tcx>,
) -> Result<(), ErrorGuaranteed> {
    let rcx: &RustcMatchCheckCtxt<'_, '_> = cx.tycx;
    if !matches!(
        rcx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, rcx.match_lint_level).0,
        rustc_session::lint::Level::Allow
    ) {
        let witnesses = collect_nonexhaustive_missing_variants(cx, pat_column)?;
        if !witnesses.is_empty() {
            // Report that a match of a `non_exhaustive` enum marked with `non_exhaustive_omitted_patterns`
            // is not exhaustive enough.
            //
            // NB: The partner lint for structs lives in `compiler/rustc_hir_analysis/src/check/pat.rs`.
            rcx.tcx.emit_spanned_lint(
                NON_EXHAUSTIVE_OMITTED_PATTERNS,
                rcx.match_lint_level,
                rcx.scrut_span,
                NonExhaustiveOmittedPattern {
                    scrut_ty: scrut_ty.inner(),
                    uncovered: Uncovered::new(rcx.scrut_span, rcx, witnesses),
                },
            );
        }
    } else {
        // We used to allow putting the `#[allow(non_exhaustive_omitted_patterns)]` on a match
        // arm. This no longer makes sense so we warn users, to avoid silently breaking their
        // usage of the lint.
        for arm in arms {
            let (lint_level, lint_level_source) =
                rcx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.arm_data);
            if !matches!(lint_level, rustc_session::lint::Level::Allow) {
                let decorator = NonExhaustiveOmittedPatternLintOnArm {
                    lint_span: lint_level_source.span(),
                    suggest_lint_on_match: rcx.whole_match_span.map(|span| span.shrink_to_lo()),
                    lint_level: lint_level.as_str(),
                    lint_name: "non_exhaustive_omitted_patterns",
                };

                use rustc_errors::DecorateLint;
                let mut err = rcx.tcx.dcx().struct_span_warn(arm.pat.data().unwrap().span, "");
                err.primary_message(decorator.msg());
                decorator.decorate_lint(&mut err);
                err.emit();
            }
        }
    }
    Ok(())
}