Rollup merge of #76567 - matthiaskrgr:clone_on_copy, r=varkor

use push(char) to add chars (single-char &strs) to strings instead of push_str(&str)

compiler/rustc_mir/src/transform/instrument_coverage.rs

@@ -353,7 +353,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
                             if !INCLUDE_COVERAGE_STATEMENTS {
                                 continue;
                             }
-                            format!("unreachable")
+                            String::from("unreachable")
                         }
                     },
                     _ => format!("{:?}", statement),