Avoid naming variables str

This renames variables named `str` to other names, to make sure `str`
always refers to a type.

It's confusing to read code where `str` (or another standard type name)
is used as an identifier. It also produces misleading syntax
highlighting.

compiler/rustc_log/src/lib.rs

@@ -130,11 +130,11 @@ pub fn init_logger(cfg: LoggerConfig) -> Result<(), Error> {
 
     let subscriber = tracing_subscriber::Registry::default().with(filter).with(layer);
     match cfg.backtrace {
-        Ok(str) => {
+        Ok(backtrace_target) => {
             let fmt_layer = tracing_subscriber::fmt::layer()
                 .with_writer(io::stderr)
                 .without_time()
-                .event_format(BacktraceFormatter { backtrace_target: str });
+                .event_format(BacktraceFormatter { backtrace_target });
             let subscriber = subscriber.with(fmt_layer);
             tracing::subscriber::set_global_default(subscriber).unwrap();
         }