Unify computation of length in EscapeUnicode

The `offset` value was computed both in `next` and in `size_hint`;
computing it in a single place ensures consistency and makes it easier
to apply improvements. The value is now computed as soon as the
iterator is constructed. This means that the time to compute it is
spent immediately and cannot be avoided, but it also guarantees that
it is only spent once.

src/libcore/char.rs

@@ -299,7 +299,15 @@ impl CharExt for char {
 
     #[inline]
     fn escape_unicode(self) -> EscapeUnicode {
-        EscapeUnicode { c: self, state: EscapeUnicodeState::Backslash }
+        let mut n = 0;
+        while (self as u32) >> (4 * (n + 1)) != 0 {
+            n += 1;
+        }
+        EscapeUnicode {
+            c: self,
+            state: EscapeUnicodeState::Backslash,
+            offset: n,
+        }
     }
 
     #[inline]
@@ -420,15 +428,16 @@ pub fn encode_utf16_raw(mut ch: u32, dst: &mut [u16]) -> Option<usize> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct EscapeUnicode {
     c: char,
-    state: EscapeUnicodeState
+    state: EscapeUnicodeState,
+    offset: usize,
 }
 
 #[derive(Clone)]
 enum EscapeUnicodeState {
     Backslash,
     Type,
     LeftBrace,
-    Value(usize),
+    Value,
     RightBrace,
     Done,
 }
@@ -448,19 +457,15 @@ impl Iterator for EscapeUnicode {
                 Some('u')
             }
             EscapeUnicodeState::LeftBrace => {
-                let mut n = 0;
-                while (self.c as u32) >> (4 * (n + 1)) != 0 {
-                    n += 1;
-                }
-                self.state = EscapeUnicodeState::Value(n);
+                self.state = EscapeUnicodeState::Value;
                 Some('{')
             }
-            EscapeUnicodeState::Value(offset) => {
-                let c = from_digit(((self.c as u32) >> (offset * 4)) & 0xf, 16).unwrap();
-                if offset == 0 {
+            EscapeUnicodeState::Value => {
+                let c = from_digit(((self.c as u32) >> (self.offset * 4)) & 0xf, 16).unwrap();
+                if self.offset == 0 {
                     self.state = EscapeUnicodeState::RightBrace;
                 } else {
-                    self.state = EscapeUnicodeState::Value(offset - 1);
+                    self.offset -= 1;
                 }
                 Some(c)
             }
@@ -473,18 +478,15 @@ impl Iterator for EscapeUnicode {
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let mut n = 0;
-        while (self.c as usize) >> (4 * (n + 1)) != 0 {
-            n += 1;
-        }
         let n = match self.state {
-            EscapeUnicodeState::Backslash => n + 5,
-            EscapeUnicodeState::Type => n + 4,
-            EscapeUnicodeState::LeftBrace => n + 3,
-            EscapeUnicodeState::Value(offset) => offset + 2,
+            EscapeUnicodeState::Backslash => 5,
+            EscapeUnicodeState::Type => 4,
+            EscapeUnicodeState::LeftBrace => 3,
+            EscapeUnicodeState::Value => 2,
             EscapeUnicodeState::RightBrace => 1,
             EscapeUnicodeState::Done => 0,
         };
+        let n = n + self.offset;
         (n, Some(n))
     }
 }