Exhaustiveness: use an Option instead of allocating fictitious patterns

compiler/rustc_pattern_analysis/src/lints.rs

@@ -11,6 +11,7 @@ use crate::errors::{
     NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Overlap,
     OverlappingRangeEndpoints, Uncovered,
 };
+use crate::pat::PatOrWild;
 use crate::rustc::{
     Constructor, DeconstructedPat, MatchArm, MatchCtxt, PlaceCtxt, RevealedTy, RustcMatchCheckCtxt,
     SplitConstructorSet, WitnessPat,
@@ -23,30 +24,36 @@ use crate::rustc::{
 /// 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. `specialize` takes care to expand them.
+/// This must not contain an or-pattern. `expand_and_push` takes care to expand them.
 ///
-/// This is not used in the main algorithm; only in lints.
+/// 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 mut patterns = Vec::with_capacity(arms.len());
+        let patterns = Vec::with_capacity(arms.len());
+        let mut column = PatternColumn { patterns };
         for arm in arms {
-            if arm.pat.is_or_pat() {
-                patterns.extend(arm.pat.flatten_or_pat())
-            } else {
-                patterns.push(arm.pat)
-            }
+            column.expand_and_push(PatOrWild::Pat(arm.pat));
         }
-        Self { patterns }
+        column
     }
-
-    fn is_empty(&self) -> bool {
-        self.patterns.is_empty()
+    /// 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())
     }
@@ -83,23 +90,12 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
             (0..arity).map(|_| Self { patterns: Vec::new() }).collect();
         let relevant_patterns =
             self.patterns.iter().filter(|pat| ctor.is_covered_by(pcx, pat.ctor()));
-        let ctor_sub_tys = pcx.ctor_sub_tys(ctor);
         for pat in relevant_patterns {
-            let specialized = pat.specialize(pcx, ctor, ctor_sub_tys);
-            for (subpat, column) in specialized.iter().zip(&mut specialized_columns) {
-                if subpat.is_or_pat() {
-                    column.patterns.extend(subpat.flatten_or_pat())
-                } else {
-                    column.patterns.push(subpat)
-                }
+            let specialized = pat.specialize(ctor, arity);
+            for (subpat, column) in specialized.into_iter().zip(&mut specialized_columns) {
+                column.expand_and_push(subpat);
             }
         }
-
-        assert!(
-            !specialized_columns[0].is_empty(),
-            "ctor {ctor:?} was listed as present but isn't;
-            there is an inconsistency between `Constructor::is_covered_by` and `ConstructorSet::split`"
-        );
         specialized_columns
     }
 }

compiler/rustc_pattern_analysis/src/pat.rs

@@ -50,14 +50,6 @@ impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
     pub(crate) fn is_or_pat(&self) -> bool {
         matches!(self.ctor, Or)
     }
-    /// Expand this (possibly-nested) or-pattern into its alternatives.
-    pub(crate) fn flatten_or_pat(&self) -> SmallVec<[&Self; 1]> {
-        if self.is_or_pat() {
-            self.iter_fields().flat_map(|p| p.flatten_or_pat()).collect()
-        } else {
-            smallvec![self]
-        }
-    }
 
     pub fn ctor(&self) -> &Constructor<Cx> {
         &self.ctor
@@ -79,17 +71,10 @@ impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
     /// `other_ctor` can be different from `self.ctor`, but must be covered by it.
     pub(crate) fn specialize(
         &self,
-        pcx: &PlaceCtxt<'_, 'p, Cx>,
         other_ctor: &Constructor<Cx>,
-        ctor_sub_tys: &[Cx::Ty],
-    ) -> SmallVec<[&'p DeconstructedPat<'p, Cx>; 2]> {
-        let wildcard_sub_tys = || {
-            ctor_sub_tys
-                .iter()
-                .map(|ty| DeconstructedPat::wildcard(*ty))
-                .map(|pat| pcx.mcx.wildcard_arena.alloc(pat) as &_)
-                .collect()
-        };
+        ctor_arity: usize,
+    ) -> SmallVec<[PatOrWild<'p, Cx>; 2]> {
+        let wildcard_sub_tys = || (0..ctor_arity).map(|_| PatOrWild::Wild).collect();
         match (&self.ctor, other_ctor) {
             // Return a wildcard for each field of `other_ctor`.
             (Wildcard, _) => wildcard_sub_tys(),
@@ -105,14 +90,15 @@ impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
                 // Fill in the fields from both ends.
                 let new_arity = fields.len();
                 for i in 0..prefix {
-                    fields[i] = &self.fields[i];
+                    fields[i] = PatOrWild::Pat(&self.fields[i]);
                 }
                 for i in 0..suffix {
-                    fields[new_arity - 1 - i] = &self.fields[self.fields.len() - 1 - i];
+                    fields[new_arity - 1 - i] =
+                        PatOrWild::Pat(&self.fields[self.fields.len() - 1 - i]);
                 }
                 fields
             }
-            _ => self.fields.iter().collect(),
+            _ => self.fields.iter().map(PatOrWild::Pat).collect(),
         }
     }
 
@@ -153,14 +139,86 @@ impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
     }
 }
 
-/// This is mostly copied from the `Pat` impl. This is best effort and not good enough for a
-/// `Display` impl.
+/// This is best effort and not good enough for a `Display` impl.
 impl<'p, Cx: TypeCx> fmt::Debug for DeconstructedPat<'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         Cx::debug_pat(f, self)
     }
 }
 
+/// Represents either a pattern obtained from user input or a wildcard constructed during the
+/// algorithm. Do not use `Wild` to represent a wildcard pattern comping from user input.
+///
+/// This is morally `Option<&'p DeconstructedPat>` where `None` is interpreted as a wildcard.
+#[derive(derivative::Derivative)]
+#[derivative(Clone(bound = ""), Copy(bound = ""))]
+pub(crate) enum PatOrWild<'p, Cx: TypeCx> {
+    /// A non-user-provided wildcard, created during specialization.
+    Wild,
+    /// A user-provided pattern.
+    Pat(&'p DeconstructedPat<'p, Cx>),
+}
+
+impl<'p, Cx: TypeCx> PatOrWild<'p, Cx> {
+    pub(crate) fn as_pat(&self) -> Option<&'p DeconstructedPat<'p, Cx>> {
+        match self {
+            PatOrWild::Wild => None,
+            PatOrWild::Pat(pat) => Some(pat),
+        }
+    }
+    pub(crate) fn ctor(self) -> &'p Constructor<Cx> {
+        match self {
+            PatOrWild::Wild => &Wildcard,
+            PatOrWild::Pat(pat) => pat.ctor(),
+        }
+    }
+
+    pub(crate) fn is_or_pat(&self) -> bool {
+        match self {
+            PatOrWild::Wild => false,
+            PatOrWild::Pat(pat) => pat.is_or_pat(),
+        }
+    }
+
+    /// Expand this (possibly-nested) or-pattern into its alternatives.
+    pub(crate) fn flatten_or_pat(self) -> SmallVec<[Self; 1]> {
+        match self {
+            PatOrWild::Pat(pat) if pat.is_or_pat() => {
+                pat.iter_fields().flat_map(|p| PatOrWild::Pat(p).flatten_or_pat()).collect()
+            }
+            _ => smallvec![self],
+        }
+    }
+
+    /// Specialize this pattern with a constructor.
+    /// `other_ctor` can be different from `self.ctor`, but must be covered by it.
+    pub(crate) fn specialize(
+        &self,
+        other_ctor: &Constructor<Cx>,
+        ctor_arity: usize,
+    ) -> SmallVec<[PatOrWild<'p, Cx>; 2]> {
+        match self {
+            PatOrWild::Wild => (0..ctor_arity).map(|_| PatOrWild::Wild).collect(),
+            PatOrWild::Pat(pat) => pat.specialize(other_ctor, ctor_arity),
+        }
+    }
+
+    pub(crate) fn set_useful(&self) {
+        if let PatOrWild::Pat(pat) = self {
+            pat.set_useful()
+        }
+    }
+}
+
+impl<'p, Cx: TypeCx> fmt::Debug for PatOrWild<'p, Cx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            PatOrWild::Wild => write!(f, "_"),
+            PatOrWild::Pat(pat) => pat.fmt(f),
+        }
+    }
+}
+
 /// Same idea as `DeconstructedPat`, except this is a fictitious pattern built up for diagnostics
 /// purposes. As such they don't use interning and can be cloned.
 #[derive(derivative::Derivative)]

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -716,7 +716,7 @@ use smallvec::{smallvec, SmallVec};
 use std::fmt;
 
 use crate::constructor::{Constructor, ConstructorSet};
-use crate::pat::{DeconstructedPat, WitnessPat};
+use crate::pat::{DeconstructedPat, PatOrWild, WitnessPat};
 use crate::{Captures, MatchArm, MatchCtxt, TypeCx};
 
 use self::ValidityConstraint::*;
@@ -827,7 +827,7 @@ impl fmt::Display for ValidityConstraint {
 #[derivative(Clone(bound = ""))]
 struct PatStack<'p, Cx: TypeCx> {
     // Rows of len 1 are very common, which is why `SmallVec[_; 2]` works well.
-    pats: SmallVec<[&'p DeconstructedPat<'p, Cx>; 2]>,
+    pats: SmallVec<[PatOrWild<'p, Cx>; 2]>,
     /// Sometimes we know that as far as this row is concerned, the current case is already handled
     /// by a different, more general, case. When the case is irrelevant for all rows this allows us
     /// to skip a case entirely. This is purely an optimization. See at the top for details.
@@ -836,7 +836,7 @@ struct PatStack<'p, Cx: TypeCx> {
 
 impl<'p, Cx: TypeCx> PatStack<'p, Cx> {
     fn from_pattern(pat: &'p DeconstructedPat<'p, Cx>) -> Self {
-        PatStack { pats: smallvec![pat], relevant: true }
+        PatStack { pats: smallvec![PatOrWild::Pat(pat)], relevant: true }
     }
 
     fn is_empty(&self) -> bool {
@@ -847,14 +847,11 @@ impl<'p, Cx: TypeCx> PatStack<'p, Cx> {
         self.pats.len()
     }
 
-    fn head_opt(&self) -> Option<&'p DeconstructedPat<'p, Cx>> {
-        self.pats.first().copied()
-    }
-    fn head(&self) -> &'p DeconstructedPat<'p, Cx> {
-        self.head_opt().unwrap()
+    fn head(&self) -> PatOrWild<'p, Cx> {
+        self.pats[0]
     }
 
-    fn iter(&self) -> impl Iterator<Item = &'p DeconstructedPat<'p, Cx>> + Captures<'_> {
+    fn iter(&self) -> impl Iterator<Item = PatOrWild<'p, Cx>> + Captures<'_> {
         self.pats.iter().copied()
     }
 
@@ -872,14 +869,13 @@ impl<'p, Cx: TypeCx> PatStack<'p, Cx> {
     /// Only call if `ctor.is_covered_by(self.head().ctor())` is true.
     fn pop_head_constructor(
         &self,
-        pcx: &PlaceCtxt<'_, 'p, Cx>,
         ctor: &Constructor<Cx>,
-        ctor_sub_tys: &[Cx::Ty],
+        ctor_arity: usize,
         ctor_is_relevant: bool,
     ) -> PatStack<'p, Cx> {
         // We pop the head pattern and push the new fields extracted from the arguments of
         // `self.head()`.
-        let mut new_pats = self.head().specialize(pcx, ctor, ctor_sub_tys);
+        let mut new_pats = self.head().specialize(ctor, ctor_arity);
         new_pats.extend_from_slice(&self.pats[1..]);
         // `ctor` is relevant for this row if it is the actual constructor of this row, or if the
         // row has a wildcard and `ctor` is relevant for wildcards.
@@ -926,11 +922,11 @@ impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
         self.pats.len()
     }
 
-    fn head(&self) -> &'p DeconstructedPat<'p, Cx> {
+    fn head(&self) -> PatOrWild<'p, Cx> {
         self.pats.head()
     }
 
-    fn iter(&self) -> impl Iterator<Item = &'p DeconstructedPat<'p, Cx>> + Captures<'_> {
+    fn iter(&self) -> impl Iterator<Item = PatOrWild<'p, Cx>> + Captures<'_> {
         self.pats.iter()
     }
 
@@ -949,14 +945,13 @@ impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
     /// Only call if `ctor.is_covered_by(self.head().ctor())` is true.
     fn pop_head_constructor(
         &self,
-        pcx: &PlaceCtxt<'_, 'p, Cx>,
         ctor: &Constructor<Cx>,
-        ctor_sub_tys: &[Cx::Ty],
+        ctor_arity: usize,
         ctor_is_relevant: bool,
         parent_row: usize,
     ) -> MatrixRow<'p, Cx> {
         MatrixRow {
-            pats: self.pats.pop_head_constructor(pcx, ctor, ctor_sub_tys, ctor_is_relevant),
+            pats: self.pats.pop_head_constructor(ctor, ctor_arity, ctor_is_relevant),
             parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
@@ -1054,7 +1049,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
     }
 
     /// Iterate over the first pattern of each row.
-    fn heads(&self) -> impl Iterator<Item = &'p DeconstructedPat<'p, Cx>> + Clone + Captures<'_> {
+    fn heads(&self) -> impl Iterator<Item = PatOrWild<'p, Cx>> + Clone + Captures<'_> {
         self.rows().map(|r| r.head())
     }
 
@@ -1066,11 +1061,12 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
         ctor_is_relevant: bool,
     ) -> Matrix<'p, Cx> {
         let ctor_sub_tys = pcx.ctor_sub_tys(ctor);
+        let arity = ctor_sub_tys.len();
         let specialized_place_ty =
             ctor_sub_tys.iter().chain(self.place_ty[1..].iter()).copied().collect();
         let ctor_sub_validity = self.place_validity[0].specialize(ctor);
         let specialized_place_validity = std::iter::repeat(ctor_sub_validity)
-            .take(ctor.arity(pcx))
+            .take(arity)
             .chain(self.place_validity[1..].iter().copied())
             .collect();
         let mut matrix = Matrix {
@@ -1081,8 +1077,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
         };
         for (i, row) in self.rows().enumerate() {
             if ctor.is_covered_by(pcx, row.head().ctor()) {
-                let new_row =
-                    row.pop_head_constructor(pcx, ctor, ctor_sub_tys, ctor_is_relevant, i);
+                let new_row = row.pop_head_constructor(ctor, arity, ctor_is_relevant, i);
                 matrix.expand_and_push(new_row);
             }
         }