Add benchmarks for impl Debug for str

In order to inform future perf improvements and prevent regressions,
lets add some benchmarks that stress `impl Debug for str`.

Author: Swatinem

library/core/benches/str.rs

@@ -3,6 +3,7 @@ use test::{black_box, Bencher};
 
 mod char_count;
 mod corpora;
+mod debug;
 mod iter;
 
 #[bench]

library/core/benches/str/debug.rs

@@ -0,0 +1,76 @@
+use std::fmt::{self, Write};
+use test::{black_box, Bencher};
+
+#[derive(Default)]
+struct CountingWriter {
+    buf: String,
+    write_calls: usize,
+}
+
+impl Write for CountingWriter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.buf.push_str(s);
+        self.write_calls += 1;
+        Ok(())
+    }
+}
+
+fn run_fmt(s: &str, expected: &str, expected_write_calls: usize) {
+    let mut w = CountingWriter::default();
+
+    write!(&mut w, "{:?}", black_box(s)).unwrap();
+    assert_eq!(s.len(), 64);
+    assert_eq!(w.buf, expected);
+    assert_eq!(w.write_calls, expected_write_calls);
+}
+
+#[bench]
+fn ascii_only(b: &mut Bencher) {
+    b.iter(|| {
+        run_fmt(
+            "just a bit of ascii text that has no escapes. 64 bytes exactly!!",
+            r#""just a bit of ascii text that has no escapes. 64 bytes exactly!!""#,
+            3,
+        );
+    });
+}
+
+#[bench]
+fn ascii_escapes(b: &mut Bencher) {
+    b.iter(|| {
+        run_fmt(
+            "some\tmore\tascii\ttext\nthis time with some \"escapes\", also 64 byte",
+            r#""some\tmore\tascii\ttext\nthis time with some \"escapes\", also 64 byte""#,
+            21,
+        );
+    });
+}
+
+#[bench]
+fn some_unicode(b: &mut Bencher) {
+    b.iter(|| {
+        run_fmt(
+            "egy kis szöveg néhány unicode betűvel. legyen ez is 64 byte.",
+            r#""egy kis szöveg néhány unicode betűvel. legyen ez is 64 byte.""#,
+            3,
+        );
+    });
+}
+
+#[bench]
+fn mostly_unicode(b: &mut Bencher) {
+    b.iter(|| {
+        run_fmt("предложение из кириллических букв.", r#""предложение из кириллических букв.""#, 3);
+    });
+}
+
+#[bench]
+fn mixed(b: &mut Bencher) {
+    b.iter(|| {
+        run_fmt(
+            "\"❤️\"\n\"hűha ez betű\"\n\"кириллических букв\".",
+            r#""\"❤\u{fe0f}\"\n\"hűha ez betű\"\n\"кириллических букв\".""#,
+            36,
+        );
+    });
+}