mir_build: Simplify length-determination and indexing for array/slice patterns

compiler/rustc_middle/src/mir/syntax.rs

@@ -1221,25 +1221,32 @@ pub enum ProjectionElem<V, T> {
     /// thing is true of the `ConstantIndex` and `Subslice` projections below.
     Index(V),
 
-    /// These indices are generated by slice patterns. Easiest to explain
-    /// by example:
+    /// These endpoint-relative indices are generated by slice/array patterns.
+    ///
+    /// For array types, `offset` is always relative to the start of the array.
+    /// For slice types, `from_end` determines whether `offset` is relative to
+    /// the start or the end of the slice being inspected.
+    ///
+    /// Slice-pattern indices are easiest to explain by the position of `X` in
+    /// these examples:
     ///
     /// ```ignore (illustrative)
-    /// [X, _, .._, _, _] => { offset: 0, min_length: 4, from_end: false },
-    /// [_, X, .._, _, _] => { offset: 1, min_length: 4, from_end: false },
-    /// [_, _, .._, X, _] => { offset: 2, min_length: 4, from_end: true },
-    /// [_, _, .._, _, X] => { offset: 1, min_length: 4, from_end: true },
+    /// [X, _, .., _, _] => { offset: 0, min_length: 4, from_end: false },
+    /// [_, X, .., _, _] => { offset: 1, min_length: 4, from_end: false },
+    /// [_, _, .., X, _] => { offset: 2, min_length: 4, from_end: true },
+    /// [_, _, .., _, X] => { offset: 1, min_length: 4, from_end: true },
     /// ```
     ConstantIndex {
-        /// index or -index (in Python terms), depending on from_end
+        /// - If `from_end == false`, this is a 0-based offset from the start of the array/slice.
+        /// - If `from_end == true`, this is a 1-based offset from the end of the slice.
         offset: u64,
         /// The thing being indexed must be at least this long -- otherwise, the
         /// projection is UB.
         ///
         /// For arrays this is always the exact length.
         min_length: u64,
-        /// Counting backwards from end? This is always false when indexing an
-        /// array.
+        /// If `true`, `offset` is a 1-based offset from the end of the slice.
+        /// Always false when indexing an array.
         from_end: bool,
     },
 

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -40,58 +40,44 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         match_pairs: &mut Vec<MatchPairTree<'tcx>>,
         extra_data: &mut PatternExtraData<'tcx>,
         place: &PlaceBuilder<'tcx>,
+        array_len: Option<u64>,
         prefix: &[Pat<'tcx>],
         opt_slice: &Option<Box<Pat<'tcx>>>,
         suffix: &[Pat<'tcx>],
     ) {
-        let tcx = self.tcx;
-        let (min_length, exact_size) = if let Some(place_resolved) = place.try_to_place(self) {
-            let place_ty = place_resolved.ty(&self.local_decls, tcx).ty;
-            match place_ty.kind() {
-                ty::Array(_, length) => {
-                    if let Some(length) = length.try_to_target_usize(tcx) {
-                        (length, true)
-                    } else {
-                        // This can happen when the array length is a generic const
-                        // expression that couldn't be evaluated (e.g., due to an error).
-                        // Since there's already a compilation error, we use a fallback
-                        // to avoid an ICE.
-                        tcx.dcx().span_delayed_bug(
-                            tcx.def_span(self.def_id),
-                            "array length in pattern couldn't be evaluated",
-                        );
-                        ((prefix.len() + suffix.len()).try_into().unwrap(), false)
-                    }
-                }
-                _ => ((prefix.len() + suffix.len()).try_into().unwrap(), false),
-            }
-        } else {
-            ((prefix.len() + suffix.len()).try_into().unwrap(), false)
+        let prefix_len = u64::try_from(prefix.len()).unwrap();
+        let suffix_len = u64::try_from(suffix.len()).unwrap();
+
+        // For slice patterns with a `..` followed by 0 or more suffix subpatterns,
+        // the actual slice index of those subpatterns isn't statically known, so
+        // we have to index them relative to the end of the slice.
+        //
+        // For array patterns, all subpatterns are indexed relative to the start.
+        let (min_length, is_array) = match array_len {
+            Some(len) => (len, true),
+            None => (prefix_len + suffix_len, false),
         };
 
-        for (idx, subpattern) in prefix.iter().enumerate() {
-            let elem =
-                ProjectionElem::ConstantIndex { offset: idx as u64, min_length, from_end: false };
+        for (offset, subpattern) in (0u64..).zip(prefix) {
+            let elem = ProjectionElem::ConstantIndex { offset, min_length, from_end: false };
             let place = place.clone_project(elem);
             MatchPairTree::for_pattern(place, subpattern, self, match_pairs, extra_data)
         }
 
         if let Some(subslice_pat) = opt_slice {
-            let suffix_len = suffix.len() as u64;
             let subslice = place.clone_project(PlaceElem::Subslice {
-                from: prefix.len() as u64,
-                to: if exact_size { min_length - suffix_len } else { suffix_len },
-                from_end: !exact_size,
+                from: prefix_len,
+                to: if is_array { min_length - suffix_len } else { suffix_len },
+                from_end: !is_array,
             });
             MatchPairTree::for_pattern(subslice, subslice_pat, self, match_pairs, extra_data);
         }
 
-        for (idx, subpattern) in suffix.iter().rev().enumerate() {
-            let end_offset = (idx + 1) as u64;
+        for (end_offset, subpattern) in (1u64..).zip(suffix.iter().rev()) {
             let elem = ProjectionElem::ConstantIndex {
-                offset: if exact_size { min_length - end_offset } else { end_offset },
+                offset: if is_array { min_length - end_offset } else { end_offset },
                 min_length,
-                from_end: !exact_size,
+                from_end: !is_array,
             };
             let place = place.clone_project(elem);
             MatchPairTree::for_pattern(place, subpattern, self, match_pairs, extra_data)
@@ -256,21 +242,44 @@ impl<'tcx> MatchPairTree<'tcx> {
             }
 
             PatKind::Array { ref prefix, ref slice, ref suffix } => {
-                cx.prefix_slice_suffix(
-                    &mut subpairs,
-                    extra_data,
-                    &place_builder,
-                    prefix,
-                    slice,
-                    suffix,
-                );
+                // Determine the statically-known length of the array type being matched.
+                // This should always succeed for legal programs, but could fail for
+                // erroneous programs (e.g. the type is `[u8; const { panic!() }]`),
+                // so take care not to ICE if this fails.
+                let array_len = match pattern.ty.kind() {
+                    ty::Array(_, len) => len.try_to_target_usize(cx.tcx),
+                    _ => None,
+                };
+                if let Some(array_len) = array_len {
+                    cx.prefix_slice_suffix(
+                        &mut subpairs,
+                        extra_data,
+                        &place_builder,
+                        Some(array_len),
+                        prefix,
+                        slice,
+                        suffix,
+                    );
+                } else {
+                    // If the array length couldn't be determined, ignore the
+                    // subpatterns and delayed-assert that compilation will fail.
+                    cx.tcx.dcx().span_delayed_bug(
+                        pattern.span,
+                        format!(
+                            "array length in pattern couldn't be determined for ty={:?}",
+                            pattern.ty
+                        ),
+                    );
+                }
+
                 None
             }
             PatKind::Slice { ref prefix, ref slice, ref suffix } => {
                 cx.prefix_slice_suffix(
                     &mut subpairs,
                     extra_data,
                     &place_builder,
+                    None,
                     prefix,
                     slice,
                     suffix,