Escape combining characters in escape_debug

Although combining characters are technically printable, they make little sense to print on their own with `Debug`: it'd be better to escape them like non-printable characters.

Author: varkor

src/libcore/char.rs

@@ -363,6 +363,17 @@ pub fn from_digit(num: u32, radix: u32) -> Option<char> {
     }
 }
 
+fn is_combining_char(c: char) -> bool {
+    match c as u32 {
+        0x0300..=0x036f |
+        0x1ab0..=0x1aff |
+        0x1dc0..=0x1dff |
+        0x20d0..=0x20ff |
+        0xfe20..=0xfe2f => true,
+        _ => false
+    }
+}
+
 // NB: the stabilization and documentation for this trait is in
 // unicode/char.rs, not here
 #[allow(missing_docs)] // docs in libunicode/u_char.rs
@@ -451,8 +462,8 @@ impl CharExt for char {
             '\r' => EscapeDefaultState::Backslash('r'),
             '\n' => EscapeDefaultState::Backslash('n'),
             '\\' | '\'' | '"' => EscapeDefaultState::Backslash(self),
-            c if is_printable(c) => EscapeDefaultState::Char(c),
-            c => EscapeDefaultState::Unicode(c.escape_unicode()),
+            _ if is_printable(self) && !is_combining_char(self) => EscapeDefaultState::Char(self),
+            _ => EscapeDefaultState::Unicode(self.escape_unicode()),
         };
         EscapeDebug(EscapeDefault { state: init_state })
     }

src/libcore/lib.rs

@@ -76,6 +76,7 @@
 #![feature(custom_attribute)]
 #![feature(doc_cfg)]
 #![feature(doc_spotlight)]
+#![cfg_attr(stage0, feature(dotdoteq_in_patterns))]
 #![feature(fn_must_use)]
 #![feature(fundamental)]
 #![feature(i128_type)]

src/libcore/tests/char.rs

@@ -181,6 +181,7 @@ fn test_escape_debug() {
     assert_eq!(string('\u{ff}'), "\u{ff}");
     assert_eq!(string('\u{11b}'), "\u{11b}");
     assert_eq!(string('\u{1d4b6}'), "\u{1d4b6}");
+    assert_eq!(string('\u{301}'), "\\u{301}");       // combining character
     assert_eq!(string('\u{200b}'),"\\u{200b}");      // zero width space
     assert_eq!(string('\u{e000}'), "\\u{e000}");     // private use 1
     assert_eq!(string('\u{100000}'), "\\u{100000}"); // private use 2