Rollup merge of rust-lang#96916 - matthiaskrgr:simpl_count, r=compiler-errors

simplify length count

Author: JohnTitor

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -485,7 +485,7 @@ impl<'a, 'tcx> CoverageSpans<'a, 'tcx> {
             }) {
                 let merged_prefix_len = self.curr_original_span.lo() - self.curr().span.lo();
                 let after_macro_bang =
-                    merged_prefix_len + BytePos(visible_macro.as_str().bytes().count() as u32 + 1);
+                    merged_prefix_len + BytePos(visible_macro.as_str().len() as u32 + 1);
                 let mut macro_name_cov = self.curr().clone();
                 self.curr_mut().span =
                     self.curr().span.with_lo(self.curr().span.lo() + after_macro_bang);