rustc: minor changes suggested by clippy perf lints.

compiler/rustc_errors/src/emitter.rs

@@ -2362,11 +2362,7 @@ impl FileWithAnnotatedLines {
 
                 let label = label.as_ref().map(|m| {
                     normalize_whitespace(
-                        &emitter
-                            .translate_message(m, &args)
-                            .map_err(Report::new)
-                            .unwrap()
-                            .to_string(),
+                        &emitter.translate_message(m, &args).map_err(Report::new).unwrap(),
                     )
                 });
 

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -274,7 +274,7 @@ pub fn suggest_constraining_type_params<'a>(
                 span,
                 if span_to_replace.is_some() {
                     constraint.clone()
-                } else if constraint.starts_with("<") {
+                } else if constraint.starts_with('<') {
                     constraint.to_string()
                 } else if bound_list_non_empty {
                     format!(" + {constraint}")

compiler/rustc_resolve/src/diagnostics.rs

@@ -1545,7 +1545,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         let (span, sugg, post) = if let SuggestionTarget::SimilarlyNamed = suggestion.target
             && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
             && let Some(span) = suggestion.span
-            && let Some(candidate) = suggestion.candidate.as_str().strip_prefix("_")
+            && let Some(candidate) = suggestion.candidate.as_str().strip_prefix('_')
             && snippet == candidate
         {
             // When the suggested binding change would be from `x` to `_x`, suggest changing the

compiler/rustc_session/src/config.rs

@@ -2477,7 +2477,7 @@ pub fn parse_externs(
             let mut error = handler.early_struct_error(format!(
                 "crate name `{name}` passed to `--extern` is not a valid ASCII identifier"
             ));
-            let adjusted_name = name.replace("-", "_");
+            let adjusted_name = name.replace('-', "_");
             if crate::utils::is_ascii_ident(&adjusted_name) {
                 error.help(format!(
                     "consider replacing the dashes with underscores: `{adjusted_name}`"