pattern analysis: Store field indices in DeconstructedPat to avoid virtual wildcards

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -420,9 +420,9 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             {
                 let mut redundant_subpats = redundant_subpats.clone();
                 // Emit lints in the order in which they occur in the file.
-                redundant_subpats.sort_unstable_by_key(|pat| pat.data().unwrap().span);
+                redundant_subpats.sort_unstable_by_key(|pat| pat.data().span);
                 for pat in redundant_subpats {
-                    report_unreachable_pattern(cx, arm.arm_data, pat.data().unwrap().span, None)
+                    report_unreachable_pattern(cx, arm.arm_data, pat.data().span, None)
                 }
             }
         }
@@ -905,10 +905,10 @@ fn report_arm_reachability<'p, 'tcx>(
     let mut catchall = None;
     for (arm, is_useful) in report.arm_usefulness.iter() {
         if matches!(is_useful, Usefulness::Redundant) {
-            report_unreachable_pattern(cx, arm.arm_data, arm.pat.data().unwrap().span, catchall)
+            report_unreachable_pattern(cx, arm.arm_data, arm.pat.data().span, catchall)
         }
         if !arm.has_guard && catchall.is_none() && pat_is_catchall(arm.pat) {
-            catchall = Some(arm.pat.data().unwrap().span);
+            catchall = Some(arm.pat.data().span);
         }
     }
 }
@@ -917,7 +917,9 @@ fn report_arm_reachability<'p, 'tcx>(
 fn pat_is_catchall(pat: &DeconstructedPat<'_, '_>) -> bool {
     match pat.ctor() {
         Constructor::Wildcard => true,
-        Constructor::Struct | Constructor::Ref => pat.iter_fields().all(|pat| pat_is_catchall(pat)),
+        Constructor::Struct | Constructor::Ref => {
+            pat.iter_fields().all(|ipat| pat_is_catchall(&ipat.pat))
+        }
         _ => false,
     }
 }

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -423,7 +423,7 @@ pub enum SliceKind {
 }
 
 impl SliceKind {
-    fn arity(self) -> usize {
+    pub fn arity(self) -> usize {
         match self {
             FixedLen(length) => length,
             VarLen(prefix, suffix) => prefix + suffix,
@@ -462,7 +462,7 @@ impl Slice {
         Slice { array_len, kind }
     }
 
-    pub(crate) fn arity(self) -> usize {
+    pub fn arity(self) -> usize {
         self.kind.arity()
     }
 

compiler/rustc_pattern_analysis/src/lints.rs

@@ -98,7 +98,7 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
                 };
 
                 use rustc_errors::LintDiagnostic;
-                let mut err = rcx.tcx.dcx().struct_span_warn(arm.pat.data().unwrap().span, "");
+                let mut err = rcx.tcx.dcx().struct_span_warn(arm.pat.data().span, "");
                 err.primary_message(decorator.msg());
                 decorator.decorate_lint(&mut err);
                 err.emit();

compiler/rustc_pattern_analysis/src/pat.rs

@@ -20,32 +20,42 @@ impl PatId {
     }
 }
 
+/// A pattern with an index denoting which field it corresponds to.
+pub struct IndexedPat<Cx: TypeCx> {
+    pub idx: usize,
+    pub pat: DeconstructedPat<Cx>,
+}
+
 /// Values and patterns can be represented as a constructor applied to some fields. This represents
 /// a pattern in this form. A `DeconstructedPat` will almost always come from user input; the only
 /// exception are some `Wildcard`s introduced during pattern lowering.
 pub struct DeconstructedPat<Cx: TypeCx> {
     ctor: Constructor<Cx>,
-    fields: Vec<DeconstructedPat<Cx>>,
+    fields: Vec<IndexedPat<Cx>>,
+    /// The number of fields in this pattern. E.g. if the pattern is `SomeStruct { field12: true, ..
+    /// }` this would be the total number of fields of the struct.
+    /// This is also the same as `self.ctor.arity(self.ty)`.
+    arity: usize,
     ty: Cx::Ty,
-    /// Extra data to store in a pattern. `None` if the pattern is a wildcard that does not
-    /// correspond to a user-supplied pattern.
-    data: Option<Cx::PatData>,
+    /// Extra data to store in a pattern.
+    data: Cx::PatData,
     /// Globally-unique id used to track usefulness at the level of subpatterns.
     pub(crate) uid: PatId,
 }
 
 impl<Cx: TypeCx> DeconstructedPat<Cx> {
-    pub fn wildcard(ty: Cx::Ty) -> Self {
-        DeconstructedPat { ctor: Wildcard, fields: Vec::new(), ty, data: None, uid: PatId::new() }
-    }
-
     pub fn new(
         ctor: Constructor<Cx>,
-        fields: Vec<DeconstructedPat<Cx>>,
+        fields: Vec<IndexedPat<Cx>>,
+        arity: usize,
         ty: Cx::Ty,
         data: Cx::PatData,
     ) -> Self {
-        DeconstructedPat { ctor, fields, ty, data: Some(data), uid: PatId::new() }
+        DeconstructedPat { ctor, fields, arity, ty, data, uid: PatId::new() }
+    }
+
+    pub fn at_index(self, idx: usize) -> IndexedPat<Cx> {
+        IndexedPat { idx, pat: self }
     }
 
     pub(crate) fn is_or_pat(&self) -> bool {
@@ -58,13 +68,15 @@ impl<Cx: TypeCx> DeconstructedPat<Cx> {
     pub fn ty(&self) -> &Cx::Ty {
         &self.ty
     }
-    /// Returns the extra data stored in a pattern. Returns `None` if the pattern is a wildcard that
-    /// does not correspond to a user-supplied pattern.
-    pub fn data(&self) -> Option<&Cx::PatData> {
-        self.data.as_ref()
+    /// Returns the extra data stored in a pattern.
+    pub fn data(&self) -> &Cx::PatData {
+        &self.data
+    }
+    pub fn arity(&self) -> usize {
+        self.arity
     }
 
-    pub fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'a DeconstructedPat<Cx>> {
+    pub fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'a IndexedPat<Cx>> {
         self.fields.iter()
     }
 
@@ -73,36 +85,40 @@ impl<Cx: TypeCx> DeconstructedPat<Cx> {
     pub(crate) fn specialize<'a>(
         &'a self,
         other_ctor: &Constructor<Cx>,
-        ctor_arity: usize,
+        other_ctor_arity: usize,
     ) -> SmallVec<[PatOrWild<'a, 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(),
+        if matches!(other_ctor, PrivateUninhabited) {
             // Skip this column.
-            (_, PrivateUninhabited) => smallvec![],
-            // The only non-trivial case: two slices of different arity. `other_slice` is
-            // guaranteed to have a larger arity, so we fill the middle part with enough
-            // wildcards to reach the length of the new, larger slice.
-            (
-                &Slice(self_slice @ Slice { kind: SliceKind::VarLen(prefix, suffix), .. }),
-                &Slice(other_slice),
-            ) if self_slice.arity() != other_slice.arity() => {
-                // Start with a slice of wildcards of the appropriate length.
-                let mut fields: SmallVec<[_; 2]> = wildcard_sub_tys();
-                // Fill in the fields from both ends.
-                let new_arity = fields.len();
-                for i in 0..prefix {
-                    fields[i] = PatOrWild::Pat(&self.fields[i]);
+            return smallvec![];
+        }
+
+        // Start with a slice of wildcards of the appropriate length.
+        let mut fields: SmallVec<[_; 2]> = (0..other_ctor_arity).map(|_| PatOrWild::Wild).collect();
+        // Fill `fields` with our fields. The arities are known to be compatible.
+        match self.ctor {
+            // The only non-trivial case: two slices of different arity. `other_ctor` is guaranteed
+            // to have a larger arity, so we adjust the indices of the patterns in the suffix so
+            // that they are correctly positioned in the larger slice.
+            Slice(Slice { kind: SliceKind::VarLen(prefix, _), .. })
+                if self.arity != other_ctor_arity =>
+            {
+                for ipat in &self.fields {
+                    let new_idx = if ipat.idx < prefix {
+                        ipat.idx
+                    } else {
+                        // Adjust the indices in the suffix.
+                        ipat.idx + other_ctor_arity - self.arity
+                    };
+                    fields[new_idx] = PatOrWild::Pat(&ipat.pat);
                 }
-                for i in 0..suffix {
-                    fields[new_arity - 1 - i] =
-                        PatOrWild::Pat(&self.fields[self.fields.len() - 1 - i]);
+            }
+            _ => {
+                for ipat in &self.fields {
+                    fields[ipat.idx] = PatOrWild::Pat(&ipat.pat);
                 }
-                fields
             }
-            _ => self.fields.iter().map(PatOrWild::Pat).collect(),
         }
+        fields
     }
 
     /// Walk top-down and call `it` in each place where a pattern occurs
@@ -114,7 +130,7 @@ impl<Cx: TypeCx> DeconstructedPat<Cx> {
         }
 
         for p in self.iter_fields() {
-            p.walk(it)
+            p.pat.walk(it)
         }
     }
 }
@@ -134,14 +150,19 @@ impl<Cx: TypeCx> fmt::Debug for DeconstructedPat<Cx> {
         };
         let mut start_or_comma = || start_or_continue(", ");
 
+        let mut fields: Vec<_> = (0..self.arity).map(|_| PatOrWild::Wild).collect();
+        for ipat in self.iter_fields() {
+            fields[ipat.idx] = PatOrWild::Pat(&ipat.pat);
+        }
+
         match pat.ctor() {
             Struct | Variant(_) | UnionField => {
                 Cx::write_variant_name(f, pat)?;
                 // Without `cx`, we can't know which field corresponds to which, so we can't
                 // get the names of the fields. Instead we just display everything as a tuple
                 // struct, which should be good enough.
                 write!(f, "(")?;
-                for p in pat.iter_fields() {
+                for p in fields {
                     write!(f, "{}", start_or_comma())?;
                     write!(f, "{p:?}")?;
                 }
@@ -151,25 +172,23 @@ impl<Cx: TypeCx> fmt::Debug for DeconstructedPat<Cx> {
             // be careful to detect strings here. However a string literal pattern will never
             // be reported as a non-exhaustiveness witness, so we can ignore this issue.
             Ref => {
-                let subpattern = pat.iter_fields().next().unwrap();
-                write!(f, "&{:?}", subpattern)
+                write!(f, "&{:?}", &fields[0])
             }
             Slice(slice) => {
-                let mut subpatterns = pat.iter_fields();
                 write!(f, "[")?;
                 match slice.kind {
                     SliceKind::FixedLen(_) => {
-                        for p in subpatterns {
+                        for p in fields {
                             write!(f, "{}{:?}", start_or_comma(), p)?;
                         }
                     }
                     SliceKind::VarLen(prefix_len, _) => {
-                        for p in subpatterns.by_ref().take(prefix_len) {
+                        for p in &fields[..prefix_len] {
                             write!(f, "{}{:?}", start_or_comma(), p)?;
                         }
                         write!(f, "{}", start_or_comma())?;
                         write!(f, "..")?;
-                        for p in subpatterns {
+                        for p in &fields[prefix_len..] {
                             write!(f, "{}{:?}", start_or_comma(), p)?;
                         }
                     }
@@ -184,7 +203,7 @@ impl<Cx: TypeCx> fmt::Debug for DeconstructedPat<Cx> {
             Str(value) => write!(f, "{value:?}"),
             Opaque(..) => write!(f, "<constant pattern>"),
             Or => {
-                for pat in pat.iter_fields() {
+                for pat in fields {
                     write!(f, "{}{:?}", start_or_continue(" | "), pat)?;
                 }
                 Ok(())
@@ -242,9 +261,10 @@ impl<'p, Cx: TypeCx> PatOrWild<'p, Cx> {
     /// 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()
-            }
+            PatOrWild::Pat(pat) if pat.is_or_pat() => pat
+                .iter_fields()
+                .flat_map(|ipat| PatOrWild::Pat(&ipat.pat).flatten_or_pat())
+                .collect(),
             _ => smallvec![self],
         }
     }