Auto merge of #88262 - klensy:pprust-cow, r=nagisa

Cow'ify some pprust methods

Reduce number of potential needless de/allocations by using `Cow<'static, str>` instead of explicit `String` type.

compiler/rustc_parse/src/parser/expr.rs

@@ -1528,7 +1528,7 @@ impl<'a> Parser<'a> {
             .span_suggestion(
                 token.span,
                 "must have an integer part",
-                pprust::token_to_string(token),
+                pprust::token_to_string(token).into(),
                 Applicability::MachineApplicable,
             )
             .emit();