fix midsection offset

Author: xacrimon

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

@@ -59,11 +59,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             let elem_ty = prefix[0].ty;
             let prefix_valtree = self.simplify_const_pattern_slice_into_valtree(prefix);
             let prefix_len = prefix.len() as u64;
+
             //let suffix_len = suffix.len() as u64;
+            let midsection_offset = if opt_slice.is_some() {1} else {0};
+
+            // TODO: handle suffix offset
 
             let src_path_subslice = place.clone_project(PlaceElem::Subslice {
                 from: 0 as u64,
-                to: if exact_size { prefix_len } else { min_length - prefix_len }, // TODO: Is this correct?
+                to: if exact_size { prefix_len } else { min_length - prefix_len + midsection_offset }, // TODO: Is this correct?
                 from_end: !exact_size,
             });