pattern_analysis: add option to get a full set of witnesses

Author: Nadrieril

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -950,9 +950,7 @@ impl<Cx: PatCx> Constructor<Cx> {
                 }
             }
             Never => write!(f, "!")?,
-            Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => {
-                write!(f, "_ : {:?}", ty)?
-            }
+            Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => write!(f, "_")?,
         }
         Ok(())
     }

compiler/rustc_pattern_analysis/src/lib.rs

@@ -56,6 +56,13 @@ pub trait PatCx: Sized + fmt::Debug {
 
     fn is_exhaustive_patterns_feature_on(&self) -> bool;
 
+    /// Whether to ensure the non-exhaustiveness witnesses we report for a complete set. This is
+    /// `false` by default to avoid some exponential blowup cases such as
+    /// https://github.com/rust-lang/rust/issues/118437.
+    fn exhaustive_witnesses(&self) -> bool {
+        false
+    }
+
     /// The number of fields for this constructor.
     fn ctor_arity(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> usize;
 

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -1747,7 +1747,9 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: PatCx>(
         // `ctor` is *irrelevant* if there's another constructor in `split_ctors` that matches
         // strictly fewer rows. In that case we can sometimes skip it. See the top of the file for
         // details.
-        let ctor_is_relevant = matches!(ctor, Constructor::Missing) || missing_ctors.is_empty();
+        let ctor_is_relevant = matches!(ctor, Constructor::Missing)
+            || missing_ctors.is_empty()
+            || mcx.tycx.exhaustive_witnesses();
         let mut spec_matrix = matrix.specialize_constructor(pcx, &ctor, ctor_is_relevant)?;
         let mut witnesses = ensure_sufficient_stack(|| {
             compute_exhaustiveness_and_usefulness(mcx, &mut spec_matrix)

compiler/rustc_pattern_analysis/tests/common/mod.rs

@@ -125,10 +125,11 @@ pub(super) fn compute_match_usefulness<'p>(
     ty: Ty,
     scrut_validity: PlaceValidity,
     complexity_limit: usize,
+    exhaustive_witnesses: bool,
 ) -> Result<UsefulnessReport<'p, Cx>, ()> {
     init_tracing();
     rustc_pattern_analysis::usefulness::compute_match_usefulness(
-        &Cx,
+        &Cx { exhaustive_witnesses },
         arms,
         ty,
         scrut_validity,
@@ -137,7 +138,9 @@ pub(super) fn compute_match_usefulness<'p>(
 }
 
 #[derive(Debug)]
-pub(super) struct Cx;
+pub(super) struct Cx {
+    exhaustive_witnesses: bool,
+}
 
 /// The context for pattern analysis. Forwards anything interesting to `Ty` methods.
 impl PatCx for Cx {
@@ -152,6 +155,10 @@ impl PatCx for Cx {
         false
     }
 
+    fn exhaustive_witnesses(&self) -> bool {
+        self.exhaustive_witnesses
+    }
+
     fn ctor_arity(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> usize {
         ty.sub_tys(ctor).len()
     }
@@ -210,16 +217,18 @@ macro_rules! pats {
     // Entrypoint
     // Parse `type; ..`
     ($ty:expr; $($rest:tt)*) => {{
-        #[allow(unused_imports)]
+        #[allow(unused)]
         use rustc_pattern_analysis::{
             constructor::{Constructor, IntRange, MaybeInfiniteInt, RangeEnd},
-            pat::DeconstructedPat,
+            pat::{DeconstructedPat, IndexedPat},
         };
         let ty = $ty;
         // The heart of the macro is designed to push `IndexedPat`s into a `Vec`, so we work around
         // that.
+        #[allow(unused)]
         let sub_tys = ::std::iter::repeat(&ty);
-        let mut vec = Vec::new();
+        #[allow(unused)]
+        let mut vec: Vec<IndexedPat<_>> = Vec::new();
         pats!(@ctor(vec:vec, sub_tys:sub_tys, idx:0) $($rest)*);
         vec.into_iter().map(|ipat| ipat.pat).collect::<Vec<_>>()
     }};
@@ -254,6 +263,8 @@ macro_rules! pats {
         let ctor = Constructor::Wildcard;
         pats!(@pat($($args)*, ctor:ctor) $($rest)*)
     }};
+    // Nothing
+    (@ctor($($args:tt)*)) => {};
 
     // Integers and int ranges
     (@ctor($($args:tt)*) $($start:literal)?..$end:literal $($rest:tt)*) => {{

compiler/rustc_pattern_analysis/tests/complexity.rs

@@ -16,7 +16,7 @@ fn check(patterns: &[DeconstructedPat<Cx>], complexity_limit: usize) -> Result<(
     let ty = *patterns[0].ty();
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
-    compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, complexity_limit)
+    compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, complexity_limit, false)
         .map(|_report| ())
 }
 

compiler/rustc_pattern_analysis/tests/exhaustiveness.rs

@@ -11,16 +11,30 @@ use rustc_pattern_analysis::usefulness::PlaceValidity;
 mod common;
 
 /// Analyze a match made of these patterns.
-fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<WitnessPat<Cx>> {
-    let ty = *patterns[0].ty();
+fn run(
+    ty: Ty,
+    patterns: Vec<DeconstructedPat<Cx>>,
+    exhaustive_witnesses: bool,
+) -> Vec<WitnessPat<Cx>> {
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
-    let report =
-        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, usize::MAX)
-            .unwrap();
+    let report = compute_match_usefulness(
+        arms.as_slice(),
+        ty,
+        PlaceValidity::ValidOnly,
+        usize::MAX,
+        exhaustive_witnesses,
+    )
+    .unwrap();
     report.non_exhaustiveness_witnesses
 }
 
+/// Analyze a match made of these patterns. Panics if there are no patterns
+fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<WitnessPat<Cx>> {
+    let ty = *patterns[0].ty();
+    run(ty, patterns, true)
+}
+
 #[track_caller]
 fn assert_exhaustive(patterns: Vec<DeconstructedPat<Cx>>) {
     let witnesses = check(patterns);
@@ -35,6 +49,26 @@ fn assert_non_exhaustive(patterns: Vec<DeconstructedPat<Cx>>) {
     assert!(!witnesses.is_empty())
 }
 
+use WhichWitnesses::*;
+enum WhichWitnesses {
+    AllOfThem,
+    OnlySome,
+}
+
+#[track_caller]
+/// We take the type as input to support empty matches.
+fn assert_witnesses(
+    which: WhichWitnesses,
+    ty: Ty,
+    patterns: Vec<DeconstructedPat<Cx>>,
+    expected: Vec<&str>,
+) {
+    let exhaustive_wit = matches!(which, AllOfThem);
+    let witnesses = run(ty, patterns, exhaustive_wit);
+    let witnesses: Vec<_> = witnesses.iter().map(|w| format!("{w:?}")).collect();
+    assert_eq!(witnesses, expected)
+}
+
 #[test]
 fn test_int_ranges() {
     let ty = Ty::U8;
@@ -59,6 +93,8 @@ fn test_int_ranges() {
 
 #[test]
 fn test_nested() {
+    // enum E { A(bool), B(bool) }
+    // ty = (E, E)
     let ty = Ty::BigStruct { arity: 2, ty: &Ty::BigEnum { arity: 2, ty: &Ty::Bool } };
     assert_non_exhaustive(pats!(ty;
         Struct(Variant.0, _),
@@ -78,6 +114,52 @@ fn test_nested() {
     ));
 }
 
+#[test]
+fn test_witnesses() {
+    // TY = Option<bool>
+    const TY: Ty = Ty::Enum(&[Ty::Bool, Ty::Tuple(&[])]);
+    // ty = (Option<bool>, Option<bool>)
+    let ty = Ty::Tuple(&[TY, TY]);
+    assert_witnesses(AllOfThem, ty, vec![], vec!["(_, _)"]);
+    assert_witnesses(
+        OnlySome,
+        ty,
+        pats!(ty;
+            (Variant.0(false), Variant.0(false)),
+        ),
+        vec!["(Enum::Variant1(_), _)"],
+    );
+    assert_witnesses(
+        AllOfThem,
+        ty,
+        pats!(ty;
+            (Variant.0(false), Variant.0(false)),
+        ),
+        vec![
+            "(Enum::Variant0(false), Enum::Variant0(true))",
+            "(Enum::Variant0(false), Enum::Variant1(_))",
+            "(Enum::Variant0(true), _)",
+            "(Enum::Variant1(_), _)",
+        ],
+    );
+    assert_witnesses(
+        OnlySome,
+        ty,
+        pats!(ty;
+            (_, Variant.0(false)),
+        ),
+        vec!["(_, Enum::Variant1(_))"],
+    );
+    assert_witnesses(
+        AllOfThem,
+        ty,
+        pats!(ty;
+            (_, Variant.0(false)),
+        ),
+        vec!["(_, Enum::Variant0(true))", "(_, Enum::Variant1(_))"],
+    );
+}
+
 #[test]
 fn test_empty() {
     // `TY = Result<bool, !>`

compiler/rustc_pattern_analysis/tests/intersection.rs

@@ -16,7 +16,7 @@ fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<Vec<usize>> {
     let arms: Vec<_> =
         patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
     let report =
-        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, usize::MAX)
+        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, usize::MAX, false)
             .unwrap();
     report.arm_intersections.into_iter().map(|bitset| bitset.iter().collect()).collect()
 }