ayy one pass

Author: Nadrieril

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

@@ -37,10 +37,10 @@
 //! skipping some constructors as long as it doesn't change whether the resulting list of witnesses
 //! is empty of not. We use this in the wildcard `_` case.
 //!
-//! Splitting is implemented in the [`Constructor::split`] function. We don't do splitting for
+//! Splitting is implemented in the [`ConstructorSet::split`] function. We don't do splitting for
 //! or-patterns; instead we just try the alternatives one-by-one. For details on splitting
-//! wildcards, see [`Constructor::split`]; for integer ranges, see
-//! [`IntRange::split`]; for slices, see [`Slice::split`].
+//! wildcards, see [`ConstructorSet::split`]; for integer ranges, see [`IntRange::split`]; for
+//! slices, see [`Slice::split`].
 //!
 //! ## Opaque patterns
 //!
@@ -678,8 +678,8 @@ pub(super) enum Constructor<'tcx> {
     /// We use this for variants behind an unstable gate as well as
     /// `#[doc(hidden)]` ones.
     Hidden,
-    /// Fake extra constructor for constructors that are not seen in the matrix, as explained in the
-    /// code for [`Constructor::split`].
+    /// Fake extra constructor for constructors that are not seen in the matrix, as explained at the
+    /// top of the file.
     Missing,
 }
 
@@ -761,104 +761,12 @@ impl<'tcx> Constructor<'tcx> {
         }
     }
 
-    /// Some constructors (namely `Wildcard`, `IntRange` and `Slice`) actually stand for a set of
-    /// actual constructors (like variants, integers or fixed-sized slices). When specializing for
-    /// these constructors, we want to be specialising for the actual underlying constructors.
-    /// Naively, we would simply return the list of constructors they correspond to. We instead are
-    /// more clever: if there are constructors that we know will behave the same w.r.t. the current
-    /// matrix, we keep them grouped. For example, all slices of a sufficiently large length will
-    /// either be all useful or all non-useful with a given matrix.
-    ///
-    /// See the branches for details on how the splitting is done.
-    ///
-    /// This function may discard some irrelevant constructors if this preserves behavior. Eg. for
-    /// the `_` case, we ignore the constructors already present in the column, unless all of them
-    /// are.
-    pub(super) fn split<'a>(
-        &self,
-        pcx: &PatCtxt<'_, '_, 'tcx>,
-        ctors: impl Iterator<Item = &'a Constructor<'tcx>> + Clone,
-    ) -> SmallVec<[Self; 1]>
-    where
-        'tcx: 'a,
-    {
-        match self {
-            Wildcard => {
-                let split_set = ConstructorSet::for_ty(pcx.cx, pcx.ty).split(pcx, ctors);
-                if !split_set.missing.is_empty() {
-                    // We are splitting a wildcard in order to compute its usefulness. Some constructors are
-                    // not present in the column. The first thing we note is that specializing with any of
-                    // the missing constructors would select exactly the rows with wildcards. Moreover, they
-                    // would all return equivalent results. We can therefore group them all into a
-                    // fictitious `Missing` constructor.
-                    //
-                    // As an important optimization, this function will skip all the present constructors.
-                    // This is correct because specializing with any of the present constructors would
-                    // select a strict superset of the wildcard rows, and thus would only find witnesses
-                    // already found with the `Missing` constructor.
-                    // This does mean that diagnostics are incomplete: in
-                    // ```
-                    // match x {
-                    //   Some(true) => {}
-                    // }
-                    // ```
-                    // we report `None` as missing but not `Some(false)`.
-                    //
-                    // When all the constructors are missing we can equivalently return the `Wildcard`
-                    // constructor on its own. The difference between `Wildcard` and `Missing` will then
-                    // only be in diagnostics.
-
-                    // If some constructors are missing, we typically want to report those constructors,
-                    // e.g.:
-                    // ```
-                    //     enum Direction { N, S, E, W }
-                    //     let Direction::N = ...;
-                    // ```
-                    // we can report 3 witnesses: `S`, `E`, and `W`.
-                    //
-                    // However, if the user didn't actually specify a constructor
-                    // in this arm, e.g., in
-                    // ```
-                    //     let x: (Direction, Direction, bool) = ...;
-                    //     let (_, _, false) = x;
-                    // ```
-                    // we don't want to show all 16 possible witnesses `(<direction-1>, <direction-2>,
-                    // true)` - we are satisfied with `(_, _, true)`. So if all constructors are missing we
-                    // prefer to report just a wildcard `_`.
-                    //
-                    // The exception is: if we are at the top-level, for example in an empty match, we
-                    // usually prefer to report the full list of constructors.
-                    let all_missing = split_set.present.is_empty();
-                    let report_when_all_missing =
-                        pcx.is_top_level && !IntRange::is_integral(pcx.ty);
-                    let ctor =
-                        if all_missing && !report_when_all_missing { Wildcard } else { Missing };
-                    smallvec![ctor]
-                } else {
-                    split_set.present
-                }
-            }
-            // Fast-track if the range is trivial.
-            IntRange(this_range) if !this_range.is_singleton() => {
-                let column_ranges = ctors.filter_map(|ctor| ctor.as_int_range()).cloned();
-                this_range.split(column_ranges).map(|(_, range)| IntRange(range)).collect()
-            }
-            Slice(this_slice @ Slice { kind: VarLen(..), .. }) => {
-                let column_slices = ctors.filter_map(|c| c.as_slice());
-                this_slice.split(column_slices).map(|(_, slice)| Slice(slice)).collect()
-            }
-            // Any other constructor can be used unchanged.
-            _ => smallvec![self.clone()],
-        }
-    }
-
     /// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
     /// For the simple cases, this is simply checking for equality. For the "grouped" constructors,
     /// this checks for inclusion.
     // We inline because this has a single call site in `Matrix::specialize_constructor`.
     #[inline]
     pub(super) fn is_covered_by<'p>(&self, pcx: &PatCtxt<'_, 'p, 'tcx>, other: &Self) -> bool {
-        // This must be kept in sync with `is_covered_by_any`.
         match (self, other) {
             // Wildcards cover anything
             (_, Wildcard) => true,
@@ -1244,19 +1152,6 @@ impl ConstructorSet {
 
         SplitConstructorSet { present, missing }
     }
-
-    /// Compute the set of constructors missing from this column.
-    /// This is only used for reporting to the user.
-    pub(super) fn compute_missing<'a, 'tcx>(
-        &self,
-        pcx: &PatCtxt<'_, '_, 'tcx>,
-        ctors: impl Iterator<Item = &'a Constructor<'tcx>> + Clone,
-    ) -> Vec<Constructor<'tcx>>
-    where
-        'tcx: 'a,
-    {
-        self.split(pcx, ctors).missing
-    }
 }
 
 /// A value can be decomposed into a constructor applied to some fields. This struct represents
@@ -1697,7 +1592,17 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
         self.reachable.set(true)
     }
     pub(super) fn is_reachable(&self) -> bool {
-        self.reachable.get()
+        if self.reachable.get() {
+            true
+        } else if self.is_or_pat() && self.iter_fields().any(|f| f.is_reachable()) {
+            // We always expand or patterns in the matrix, so we will never see the actual
+            // or-pattern (the one with constructor `Or`) in the column. As such, it will not be
+            // marked as reachable itself, only its children will. We recover this information here.
+            self.set_reachable();
+            true
+        } else {
+            false
+        }
     }
 
     /// Report the spans of subpatterns that were not reachable, if any.
@@ -1706,7 +1611,6 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
         self.collect_unreachable_spans(&mut spans);
         spans
     }
-
     fn collect_unreachable_spans(&self, spans: &mut Vec<Span>) {
         // We don't look at subpatterns if we already reported the whole pattern as unreachable.
         if !self.is_reachable() {