remove a couple of redundant String to String conversion

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -706,7 +706,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 suggestions.push((
                     pat_span,
                     format!("consider removing the {to_remove}"),
-                    suggestion.to_string(),
+                    suggestion,
                 ));
             }
         }

compiler/rustc_codegen_llvm/src/asm.rs

@@ -154,7 +154,7 @@ impl<'ll, 'tcx> AsmBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                     // We prefer the latter because it matches the behavior of
                     // Clang.
                     if late && matches!(reg, InlineAsmRegOrRegClass::Reg(_)) {
-                        constraints.push(reg_to_llvm(reg, Some(&in_value.layout)).to_string());
+                        constraints.push(reg_to_llvm(reg, Some(&in_value.layout)));
                     } else {
                         constraints.push(format!("{}", op_idx[&idx]));
                     }

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -325,7 +325,7 @@ pub fn suggest_constraining_type_params<'a>(
             let suggestion = if span_to_replace.is_some() {
                 constraint.clone()
             } else if constraint.starts_with('<') {
-                constraint.to_string()
+                constraint.clone()
             } else if bound_list_non_empty {
                 format!(" + {constraint}")
             } else {

compiler/rustc_session/src/utils.rs

@@ -137,7 +137,7 @@ pub fn extra_compiler_flags() -> Option<(Vec<String>, bool)> {
             let content = if arg.len() == a.len() {
                 // A space-separated option, like `-C incremental=foo` or `--crate-type rlib`
                 match args.next() {
-                    Some(arg) => arg.to_string(),
+                    Some(arg) => arg,
                     None => continue,
                 }
             } else if arg.get(a.len()..a.len() + 1) == Some("=") {