Write char::DebugEscape sequences using write_str

Instead of writing each `char` of an escape sequence one by one, this delegates to `Display`, which uses `write_str` internally in order to write the whole escape sequence at once.

library/core/benches/str/debug.rs

@@ -44,7 +44,7 @@ fn ascii_escapes(b: &mut Bencher) {
  assert_fmt(
  s,
  r#""some\tmore\tascii\ttext\nthis time with some \"escapes\", also 64 byte""#,
- 21,
+ 15,
  );
  b.iter(|| {
  black_box(format!("{:?}", black_box(s)));
@@ -72,7 +72,7 @@ fn mostly_unicode(b: &mut Bencher) {
 #[bench]
 fn mixed(b: &mut Bencher) {
  let s = "\"❤️\"\n\"hűha ez betű\"\n\"кириллических букв\".";
- assert_fmt(s, r#""\"❤\u{fe0f}\"\n\"hűha ez betű\"\n\"кириллических букв\".""#, 36);
+ assert_fmt(s, r#""\"❤\u{fe0f}\"\n\"hűha ez betű\"\n\"кириллических букв\".""#, 21);
  b.iter(|| {
  black_box(format!("{:?}", black_box(s)));
  });

library/core/src/fmt/mod.rs

@@ -2397,9 +2397,7 @@ impl Debug for str {
  // If char needs escaping, flush backlog so far and write, else skip
  if esc.len() != 1 {
  f.write_str(&self[from..i])?;
- for c in esc {
- f.write_char(c)?;
- }
+ Display::fmt(&esc, f)?;
  from = i + c.len_utf8();
  }
  }
@@ -2419,13 +2417,12 @@ impl Display for str {
 impl Debug for char {
  fn fmt(&self, f: &mut Formatter<'_>) -> Result {
  f.write_char('\'')?;
- for c in self.escape_debug_ext(EscapeDebugExtArgs {
+ let esc = self.escape_debug_ext(EscapeDebugExtArgs {
  escape_grapheme_extended: true,
  escape_single_quote: true,
  escape_double_quote: false,
- }) {
- f.write_char(c)?
- }
+ });
+ Display::fmt(&esc, f)?;
  f.write_char('\'')
  }
 }