identify subslicing as a problem

Author: xacrimon

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

@@ -56,13 +56,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ((prefix.len() + suffix.len()).try_into().unwrap(), false)
         };
 
-        if self.subslice_work_optimizing(prefix) {
+        if self.subslice_work_optimizing(prefix) && opt_slice.is_none() && suffix.is_empty() {
             let elem_ty = prefix[0].ty;
             let prefix_valtree = self.simplify_const_pattern_slice_into_valtree(prefix);
 
             let project = PlaceElem::Subslice {
                 from: 0 as u64,
-                to: prefix.len() as u64,
+                to: prefix.len() as u64, // TODO: look at this
                 from_end: !exact_size,
             };
 
@@ -81,7 +81,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let elem = ProjectionElem::ConstantIndex {
                     offset: idx as u64,
                     min_length,
-                    from_end: !exact_size,
+                    from_end: false,
                 };
                 MatchPairTree::for_pattern(place.clone_project(elem), subpattern, self)
             }));
@@ -91,7 +91,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             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 },
+                to: if exact_size { min_length - suffix_len } else { suffix_len }, // TODO: look at this
                 from_end: !exact_size,
             });
             match_pairs.push(MatchPairTree::for_pattern(subslice, subslice_pat, self));