some low hanging clippy::perf fixes

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -2090,7 +2090,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                                 self.is_unnamed(),
                                 self.is_underscore(),
                                 self.is_named(),
-                                sugg.starts_with("&"),
+                                sugg.starts_with('&'),
                             ) {
                                 (true, _, _, false) => (self.span_unnamed_borrow(), sugg),
                                 (true, _, _, true) => {

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -2605,8 +2605,10 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                     binding.ident,
                 );
                 self.with(scope, |_, this| {
-                    let scope =
-                        Scope::Supertrait { lifetimes: lifetimes.unwrap_or(vec![]), s: this.scope };
+                    let scope = Scope::Supertrait {
+                        lifetimes: lifetimes.unwrap_or_default(),
+                        s: this.scope,
+                    };
                     this.with(scope, |_, this| this.visit_assoc_type_binding(binding));
                 });
             } else {

compiler/rustc_target/src/spec/mod.rs

@@ -664,7 +664,7 @@ impl ToJson for SanitizerSet {
         self.into_iter()
             .map(|v| Some(v.as_str()?.to_json()))
             .collect::<Option<Vec<_>>>()
-            .unwrap_or(Vec::new())
+            .unwrap_or_default()
             .to_json()
     }
 }

compiler/rustc_typeck/src/check/expr.rs

@@ -165,7 +165,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.tcx().sess.verbose() {
             // make this code only run with -Zverbose because it is probably slow
             if let Ok(lint_str) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
-                if !lint_str.contains("\n") {
+                if !lint_str.contains('\n') {
                     debug!("expr text: {}", lint_str);
                 } else {
                     let mut lines = lint_str.lines();

compiler/rustc_typeck/src/check/pat.rs

@@ -1008,7 +1008,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let mut err = struct_span_err!(
             self.tcx.sess,
-            MultiSpan::from_spans(subpat_spans.clone()),
+            MultiSpan::from_spans(subpat_spans),
             E0023,
             "this pattern has {} field{}, but the corresponding {} has {} field{}",
             subpats.len(),

src/librustdoc/html/sources.rs

@@ -72,10 +72,9 @@ impl LocalSourcesCollector<'_, '_> {
             href.push('/');
         });
 
-        let src_fname = p.file_name().expect("source has no filename").to_os_string();
-        let mut fname = src_fname.clone();
-        fname.push(".html");
-        href.push_str(&fname.to_string_lossy());
+        let mut src_fname = p.file_name().expect("source has no filename").to_os_string();
+        src_fname.push(".html");
+        href.push_str(&src_fname.to_string_lossy());
         self.local_sources.insert(p, href);
     }
 }