Auto merge of #30624 - Ticki:specialization, r=alexcrichton

Part of #30520. Completes #24214

Author: bors

src/libcore/char.rs

@@ -537,4 +537,49 @@ impl Iterator for EscapeDefault {
             EscapeDefaultState::Done => (0, Some(0)),
         }
     }
+
+    fn count(self) -> usize {
+        match self.state {
+            EscapeDefaultState::Char(_) => 1,
+            EscapeDefaultState::Unicode(iter) => iter.count(),
+            EscapeDefaultState::Done => 0,
+            EscapeDefaultState::Backslash(_) => 2,
+        }
+    }
+
+    fn nth(&mut self, n: usize) -> Option<char> {
+        match self.state {
+            EscapeDefaultState::Backslash(c) if n == 0 => {
+                self.state = EscapeDefaultState::Char(c);
+                Some('\\')
+            },
+            EscapeDefaultState::Backslash(c) if n == 1 => {
+                self.state = EscapeDefaultState::Done;
+                Some(c)
+            },
+            EscapeDefaultState::Backslash(_) => {
+                self.state = EscapeDefaultState::Done;
+                None
+            },
+            EscapeDefaultState::Char(c) => {
+                self.state = EscapeDefaultState::Done;
+
+                if n == 0 {
+                    Some(c)
+                } else {
+                    None
+                }
+            },
+            EscapeDefaultState::Done => return None,
+            EscapeDefaultState::Unicode(ref mut i) => return i.nth(n),
+        }
+    }
+
+    fn last(self) -> Option<char> {
+        match self.state {
+            EscapeDefaultState::Unicode(iter) => iter.last(),
+            EscapeDefaultState::Done => None,
+            EscapeDefaultState::Backslash(c) | EscapeDefaultState::Char(c) => Some(c),
+        }
+    }
 }

src/libcoretest/char.rs

@@ -216,3 +216,43 @@ fn test_decode_utf16() {
     check(&[0xD800, 0x41, 0x42], &[Err(0xD800), Ok('A'), Ok('B')]);
     check(&[0xD800, 0], &[Err(0xD800), Ok('\0')]);
 }
+
+#[test]
+fn ed_iterator_specializations() {
+    // Check counting
+    assert_eq!('\n'.escape_default().count(), 2);
+    assert_eq!('c'.escape_default().count(), 1);
+    assert_eq!(' '.escape_default().count(), 1);
+    assert_eq!('\\'.escape_default().count(), 2);
+    assert_eq!('\''.escape_default().count(), 2);
+
+    // Check nth
+
+    // Check that OoB is handled correctly
+    assert_eq!('\n'.escape_default().nth(2), None);
+    assert_eq!('c'.escape_default().nth(1), None);
+    assert_eq!(' '.escape_default().nth(1), None);
+    assert_eq!('\\'.escape_default().nth(2), None);
+    assert_eq!('\''.escape_default().nth(2), None);
+
+    // Check the first char
+    assert_eq!('\n'.escape_default().nth(0), Some('\\'));
+    assert_eq!('c'.escape_default().nth(0), Some('c'));
+    assert_eq!(' '.escape_default().nth(0), Some(' '));
+    assert_eq!('\\'.escape_default().nth(0), Some('\\'));
+    assert_eq!('\''.escape_default().nth(0), Some('\\'));
+
+    // Check the second char
+    assert_eq!('\n'.escape_default().nth(1), Some('n'));
+    assert_eq!('\\'.escape_default().nth(1), Some('\\'));
+    assert_eq!('\''.escape_default().nth(1), Some('\''));
+
+    // Check the last char
+    assert_eq!('\n'.escape_default().last(), Some('n'));
+    assert_eq!('c'.escape_default().last(), Some('c'));
+    assert_eq!(' '.escape_default().last(), Some(' '));
+    assert_eq!('\\'.escape_default().last(), Some('\\'));
+    assert_eq!('\''.escape_default().last(), Some('\''));
+}
+
+