Fix clippy warnings

Fixes clippy::{cone_on_copy, filter_next, redundant_closure, single_char_pattern, len_zero,redundant_field_names, useless_format, identity_conversion, map_clone, into_iter_on_ref, needless_return, option_as_ref_deref, unused_unit, unnecessary_mut_passed}

src/librustdoc/config.rs

@@ -447,7 +447,7 @@ impl Options {
             None => return Err(3),
         };
 
-        match matches.opt_str("r").as_ref().map(|s| &**s) {
+        match matches.opt_str("r").as_deref() {
             Some("rust") | None => {}
             Some(s) => {
                 diag.struct_err(&format!("unknown input format: {}", s)).emit();