Rollup merge of #56336 - nnethercote:clean-up-pp, r=nikomatsakis

Clean up and streamline the pretty-printer

Some minor improvements.

src/libsyntax/parse/parser.rs

@@ -2791,7 +2791,7 @@ impl<'a> Parser<'a> {
                             s.print_usize(float.trunc() as usize)?;
                             s.pclose()?;
                             s.s.word(".")?;
-                            s.s.word(fstr.splitn(2, ".").last().unwrap())
+                            s.s.word(fstr.splitn(2, ".").last().unwrap().to_string())
                         });
                         err.span_suggestion_with_applicability(
                             lo.to(self.prev_span),