Repurpose MatchCtxt for usefulness only

Author: Nadrieril

compiler/rustc_pattern_analysis/src/lib.rs

@@ -126,14 +126,6 @@ pub trait TypeCx: Sized + fmt::Debug {
     }
 }
 
-/// Context that provides information global to a match.
-#[derive(derivative::Derivative)]
-#[derivative(Clone(bound = ""), Copy(bound = ""))]
-pub struct MatchCtxt<'a, Cx: TypeCx> {
-    /// The context for type information.
-    pub tycx: &'a Cx,
-}
-
 /// The arm of a match expression.
 #[derive(Debug)]
 #[derive(derivative::Derivative)]
@@ -154,9 +146,7 @@ pub fn analyze_match<'p, 'tcx>(
 ) -> Result<rustc::UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
     let scrut_ty = tycx.reveal_opaque_ty(scrut_ty);
     let scrut_validity = ValidityConstraint::from_bool(tycx.known_valid_scrutinee);
-    let cx = MatchCtxt { tycx };
-
-    let report = compute_match_usefulness(cx, arms, scrut_ty, scrut_validity)?;
+    let report = compute_match_usefulness(tycx, arms, scrut_ty, scrut_validity)?;
 
     // Run the non_exhaustive_omitted_patterns lint. Only run on refutable patterns to avoid hitting
     // `if let`s. Only run if the match is exhaustive otherwise the error is redundant.

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -718,7 +718,7 @@ use std::fmt;
 
 use crate::constructor::{Constructor, ConstructorSet, IntRange};
 use crate::pat::{DeconstructedPat, PatOrWild, WitnessPat};
-use crate::{Captures, MatchArm, MatchCtxt, TypeCx};
+use crate::{Captures, MatchArm, TypeCx};
 
 use self::ValidityConstraint::*;
 
@@ -729,12 +729,20 @@ pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
     f()
 }
 
+/// Context that provides information for usefulness checking.
+#[derive(derivative::Derivative)]
+#[derivative(Clone(bound = ""), Copy(bound = ""))]
+pub struct UsefulnessCtxt<'a, Cx: TypeCx> {
+    /// The context for type information.
+    pub tycx: &'a Cx,
+}
+
 /// Context that provides information local to a place under investigation.
 #[derive(derivative::Derivative)]
 #[derivative(Debug(bound = ""), Clone(bound = ""), Copy(bound = ""))]
 struct PlaceCtxt<'a, Cx: TypeCx> {
     #[derivative(Debug = "ignore")]
-    pub(crate) mcx: MatchCtxt<'a, Cx>,
+    pub(crate) mcx: UsefulnessCtxt<'a, Cx>,
     /// Type of the place under investigation.
     #[derivative(Clone(clone_with = "Clone::clone"))] // See rust-derivative#90
     pub(crate) ty: &'a Cx::Ty,
@@ -1322,7 +1330,7 @@ impl<Cx: TypeCx> WitnessMatrix<Cx> {
 /// We can however get false negatives because exhaustiveness does not explore all cases. See the
 /// section on relevancy at the top of the file.
 fn collect_overlapping_range_endpoints<'p, Cx: TypeCx>(
-    mcx: MatchCtxt<'_, Cx>,
+    mcx: UsefulnessCtxt<'_, Cx>,
     overlap_range: IntRange,
     matrix: &Matrix<'p, Cx>,
     specialized_matrix: &Matrix<'p, Cx>,
@@ -1395,7 +1403,7 @@ fn collect_overlapping_range_endpoints<'p, Cx: TypeCx>(
 /// This is all explained at the top of the file.
 #[instrument(level = "debug", skip(mcx, is_top_level), ret)]
 fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
-    mcx: MatchCtxt<'a, Cx>,
+    mcx: UsefulnessCtxt<'a, Cx>,
     matrix: &mut Matrix<'p, Cx>,
     is_top_level: bool,
 ) -> Result<WitnessMatrix<Cx>, Cx::Error> {
@@ -1545,13 +1553,14 @@ pub struct UsefulnessReport<'p, Cx: TypeCx> {
 }
 
 /// Computes whether a match is exhaustive and which of its arms are useful.
-#[instrument(skip(cx, arms), level = "debug")]
+#[instrument(skip(tycx, arms), level = "debug")]
 pub fn compute_match_usefulness<'p, Cx: TypeCx>(
-    cx: MatchCtxt<'_, Cx>,
+    tycx: &Cx,
     arms: &[MatchArm<'p, Cx>],
     scrut_ty: Cx::Ty,
     scrut_validity: ValidityConstraint,
 ) -> Result<UsefulnessReport<'p, Cx>, Cx::Error> {
+    let cx = UsefulnessCtxt { tycx };
     let mut matrix = Matrix::new(arms, scrut_ty, scrut_validity);
     let non_exhaustiveness_witnesses =
         compute_exhaustiveness_and_usefulness(cx, &mut matrix, true)?;