Auto merge of #34485 - tbu-:pr_unicode_debug_str, r=alexcrichton

Escape fewer Unicode codepoints in `Debug` impl of `str`

Use the same procedure as Python to determine whether a character is
printable, described in [PEP 3138]. In particular, this means that the
following character classes are escaped:

- Cc (Other, Control)
- Cf (Other, Format)
- Cs (Other, Surrogate), even though they can't appear in Rust strings
- Co (Other, Private Use)
- Cn (Other, Not Assigned)
- Zl (Separator, Line)
- Zp (Separator, Paragraph)
- Zs (Separator, Space), except for the ASCII space `' '` `0x20`

This allows for user-friendly inspection of strings that are not
English (e.g. compare `"\u{e9}\u{e8}\u{ea}"` to `"éèê"`).

Fixes #34318.
CC #34422.

[PEP 3138]: https://www.python.org/dev/peps/pep-3138/

src/librustc_unicode/char.rs

@@ -36,7 +36,7 @@ use tables::{conversions, derived_property, general_category, property};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::char::{MAX, from_digit, from_u32, from_u32_unchecked};
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use core::char::{EncodeUtf16, EncodeUtf8, EscapeDefault, EscapeUnicode};
+pub use core::char::{EncodeUtf16, EncodeUtf8, EscapeDebug, EscapeDefault, EscapeUnicode};
 
 // unstable reexports
 #[unstable(feature = "decode_utf8", issue = "33906")]
@@ -267,6 +267,41 @@ impl char {
         C::escape_unicode(self)
     }
 
+    /// Returns an iterator that yields the literal escape code of a `char`.
+    ///
+    /// This will escape the characters similar to the `Debug` implementations
+    /// of `str` or `char`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// for i in '\n'.escape_default() {
+    ///     println!("{}", i);
+    /// }
+    /// ```
+    ///
+    /// This prints:
+    ///
+    /// ```text
+    /// \
+    /// n
+    /// ```
+    ///
+    /// Collecting into a `String`:
+    ///
+    /// ```
+    /// let quote: String = '\n'.escape_default().collect();
+    ///
+    /// assert_eq!(quote, "\\n");
+    /// ```
+    #[unstable(feature = "char_escape_debug", issue = "35068")]
+    #[inline]
+    pub fn escape_debug(self) -> EscapeDebug {
+        C::escape_debug(self)
+    }
+
     /// Returns an iterator that yields the literal escape code of a `char`.
     ///
     /// The default is chosen with a bias toward producing literals that are

src/librustc_unicode/lib.rs

@@ -32,6 +32,7 @@
 #![cfg_attr(not(stage0), deny(warnings))]
 #![no_std]
 
+#![feature(char_escape_debug)]
 #![feature(core_char_ext)]
 #![feature(decode_utf8)]
 #![feature(lang_items)]