couple of clippy::perf fixes

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/cpp_like.rs

@@ -372,7 +372,6 @@ fn build_union_fields_for_direct_tag_generator<'ll, 'tcx>(
 
     // Build the type node for each field.
     let variant_field_infos: SmallVec<VariantFieldInfo<'ll>> = variant_range
-        .clone()
         .map(|variant_index| {
             let variant_struct_type_di_node = super::build_generator_variant_struct_type_di_node(
                 cx,

compiler/rustc_errors/src/lib.rs

@@ -1208,7 +1208,7 @@ impl HandlerInner {
             (0, 0) => return,
             (0, _) => self.emitter.emit_diagnostic(&Diagnostic::new(
                 Level::Warning,
-                DiagnosticMessage::Str(warnings.to_owned()),
+                DiagnosticMessage::Str(warnings),
             )),
             (_, 0) => {
                 let _ = self.fatal(&errors);

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -589,7 +589,7 @@ impl TtParser {
                 (_, 0) => {
                     // Dump all possible `next_mps` into `cur_mps` for the next iteration. Then
                     // process the next token.
-                    self.cur_mps.extend(self.next_mps.drain(..));
+                    self.cur_mps.append(&mut self.next_mps);
                     parser.to_mut().bump();
                 }
 

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -844,7 +844,7 @@ fn non_exhaustive_match<'p, 'tcx>(
                     format!(
                         "{}{}{} => todo!()",
                         comma,
-                        snippet.strip_prefix(",").unwrap_or(&snippet),
+                        snippet.strip_prefix(',').unwrap_or(&snippet),
                         pattern
                     ),
                 ));

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2724,9 +2724,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 &format!(
                     "consider annotating `{}` with `#[derive({})]`",
                     trait_pred.skip_binder().self_ty(),
-                    diagnostic_name.to_string(),
+                    diagnostic_name,
                 ),
-                format!("#[derive({})]\n", diagnostic_name.to_string()),
+                format!("#[derive({})]\n", diagnostic_name),
                 Applicability::MaybeIncorrect,
             );
         }

src/librustdoc/passes/html_tags.rs

@@ -215,7 +215,7 @@ impl<'a, 'tcx> DocVisitor for InvalidHtmlTagsLinter<'a, 'tcx> {
                     // We don't try to detect stuff `<like, this>` because that's not valid HTML,
                     // and we don't try to detect stuff `<like this>` because that's not valid Rust.
                     if let Some(Some(generics_start)) = (is_open_tag
-                        && dox[..range.end].ends_with(">"))
+                        && dox[..range.end].ends_with('>'))
                     .then(|| extract_path_backwards(&dox, range.start))
                     {
                         let generics_sp = match super::source_span_for_markdown_range(