Use a set for obvious performance and correctness reasons

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -678,14 +678,15 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, '_, 'infcx, 'tcx> {
         let inits = &self.move_data.init_path_map[mpi];
         let move_path = &self.move_data.move_paths[mpi];
         let decl_span = self.body.local_decls[move_path.place.local].source_info.span;
-        let mut spans = vec![];
+        let mut spans_set = FxIndexSet::default();
         for init_idx in inits {
             let init = &self.move_data.inits[*init_idx];
             let span = init.span(self.body);
-            if !span.is_dummy() && !spans.contains(&span) {
-                spans.push(span);
+            if !span.is_dummy() {
+                spans_set.insert(span);
             }
         }
+        let spans: Vec<_> = spans_set.into_iter().collect();
 
         let (name, desc) = match self.describe_place_with_options(
             moved_place,