Auto merge of #119688 - Nadrieril:dont-alloc-custom-wildcards, r=compiler-errors

Exhaustiveness: use an `Option` instead of allocating fictitious patterns

In the process of exhaustiveness checking, `Matrix` stores a 2D array of patterns. Those are subpatterns of the patterns we were provided as input, _except_ sometimes we allocate some extra wildcard patterns to fill a hole during specialization.

Morally though, we could store `Option<&'p DeconstructedPat>` in the matrix, where `None` signifies a wildcard. That way we'd only have "real" patterns in the matrix and we wouldn't need the arena to allocate these wildcards. This is what this PR does.

This is part of me splitting up https://github.com/rust-lang/rust/pull/119581 for ease of review.

r? `@compiler-errors`

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,
@@ -23,9 +24,9 @@ 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>>,
@@ -33,20 +34,26 @@ pub(crate) struct PatternColumn<'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() {
+            column.expand_and_push(PatOrWild::Pat(arm.pat));
-                patterns.extend(arm.pat.flatten_or_pat())
+        }
-            } else {
+        column
-                patterns.push(arm.pat)
+    }
-            }
+    /// 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)
         }
-        Self { patterns }
     }
 
-    fn is_empty(&self) -> bool {
-        self.patterns.is_empty()
-    }
     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);
+            let specialized = pat.specialize(ctor, arity);
-            for (subpat, column) in specialized.iter().zip(&mut specialized_columns) {
+            for (subpat, column) in specialized.into_iter().zip(&mut specialized_columns) {
-                if subpat.is_or_pat() {
+                column.expand_and_push(subpat);
-                    column.patterns.extend(subpat.flatten_or_pat())
-                } else {
-                    column.patterns.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],
+        ctor_arity: usize,
-    ) -> SmallVec<[&'p DeconstructedPat<'p, Cx>; 2]> {
+    ) -> SmallVec<[PatOrWild<'p, Cx>; 2]> {
-        let wildcard_sub_tys = || {
+        let wildcard_sub_tys = || (0..ctor_arity).map(|_| PatOrWild::Wild).collect();
-            ctor_sub_tys
-                .iter()
-                .map(|ty| DeconstructedPat::wildcard(*ty))
-                .map(|pat| pcx.mcx.wildcard_arena.alloc(pat) as &_)
-                .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
+/// This is best effort and not good enough for a `Display` impl.
-/// `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>> {
+    fn head(&self) -> PatOrWild<'p, Cx> {
-        self.pats.first().copied()
+        self.pats[0]
-    }
-    fn head(&self) -> &'p DeconstructedPat<'p, Cx> {
-        self.head_opt().unwrap()
     }
 
-    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 =
+                let new_row = row.pop_head_constructor(ctor, arity, ctor_is_relevant, i);
-                    row.pop_head_constructor(pcx, ctor, ctor_sub_tys, ctor_is_relevant, i);
                 matrix.expand_and_push(new_row);
             }
         }