Add Buffer::resize and Buffer::truncate.

These methods enable users to use `Buffer` without having an intermediate
array.

For example:

```
let mut buffer = Buffer::with_capacity(1024);
buffer.resize(1024, 0);
let len = fill_some_data(buffer.buf_mut());
buffer.truncate(len);
```

Author: oblique

src/buffer/mod.rs

@@ -100,6 +100,8 @@ impl BufImpl {
 
         pub fn reserve(&mut self, additional: usize)[additional] -> bool;
 
+        pub fn resize(&mut self, new_len: usize, value: u8)[new_len, value] -> bool;
+
         pub fn make_room(&mut self)[];
 
         pub fn buf(&self)[] -> &[u8];
@@ -111,5 +113,7 @@ impl BufImpl {
         pub unsafe fn bytes_written(&mut self, add: usize)[add];
 
         pub fn consume(&mut self, amt: usize)[amt];
+
+        pub fn truncate(&mut self, len: usize)[len];
     }
 }

src/buffer/slice_deque_buf.rs

@@ -48,6 +48,11 @@ impl SliceDequeBuf {
         true
     }
 
+    pub fn resize(&mut self, new_len: usize, value: u8) -> bool {
+        self.deque.resize(new_len, value);
+        true
+    }
+
     /// This method is a no-op.
     pub fn make_room(&mut self) {}
 
@@ -80,4 +85,8 @@ impl SliceDequeBuf {
             self.deque.move_head(offset);
         }
     }
+
+    pub fn truncate(&mut self, len: usize) {
+        self.deque.truncate_back(len);
+    }
 }

src/buffer/std_buf.rs

@@ -63,6 +63,25 @@ impl StdBuf {
         self.buf.reserve(additional - usable_space)
     }
 
+    pub fn resize(&mut self, new_len: usize, value: u8) -> bool {
+        if new_len <= self.len() {
+            self.truncate(new_len);
+            false
+        } else {
+            let additional = new_len - self.len();
+            let res = self.reserve(additional);
+
+            unsafe {
+                for x in &mut self.buf.as_mut_slice()[self.end .. self.end + additional] {
+                    *x = value;
+                }
+            }
+
+            self.end += additional;
+            res
+        }
+    }
+
     pub fn make_room(&mut self) {
         self.check_cursors();
 
@@ -104,6 +123,11 @@ impl StdBuf {
         self.check_cursors();
     }
 
+    pub fn truncate(&mut self, len: usize) {
+        self.end = cmp::min(self.pos + len, self.end);
+        self.check_cursors();
+    }
+
     pub fn check_cursors(&mut self) -> bool {
         if self.pos == self.end {
             self.pos = 0;
@@ -233,3 +257,94 @@ fn read_into_full() {
     assert_eq!(buffer.read_from(&mut bytes).unwrap(), 1);
     assert_eq!(buffer.read_from(&mut bytes).unwrap(), 0);
 }
+
+#[test]
+fn test_truncate() {
+    use Buffer;
+    let mut buffer = Buffer::with_capacity(32);
+
+    buffer.truncate(5);
+    assert_eq!(buffer.len(), 0);
+
+    buffer.push_bytes(&[1, 2, 3, 4, 5, 6, 7]);
+    assert_eq!(buffer.len(), 7);
+
+    buffer.truncate(10);
+    assert_eq!(buffer.len(), 7);
+
+    buffer.truncate(7);
+    assert_eq!(buffer.len(), 7);
+
+    buffer.truncate(5);
+    assert_eq!(buffer.len(), 5);
+    assert_eq!(buffer.buf(), &[1, 2, 3, 4, 5]);
+
+    buffer.consume(1);
+    buffer.truncate(3);
+    assert_eq!(buffer.len(), 3);
+    assert_eq!(buffer.buf(), &[2, 3, 4]);
+
+    buffer.truncate(0);
+    assert_eq!(buffer.len(), 0);
+}
+
+#[test]
+fn test_resize() {
+    use Buffer;
+    let mut buffer = Buffer::with_capacity(10);
+
+    buffer.resize(10, 0);
+    assert_eq!(buffer.len(), 10);
+    assert_eq!(buffer.capacity(), 10);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+    buffer.resize(11, 1);
+    assert_eq!(buffer.len(), 11);
+    assert_eq!(buffer.capacity(), 11);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1]);
+
+    buffer.resize(5, 2);
+    assert_eq!(buffer.len(), 5);
+    assert_eq!(buffer.capacity(), 11);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 0, 0]);
+
+    buffer.resize(7, 3);
+    assert_eq!(buffer.len(), 7);
+    assert_eq!(buffer.capacity(), 11);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 0, 0, 3, 3]);
+    assert_eq!(buffer.usable_space(), 4);
+    assert_eq!(buffer.free_space(), 4);
+
+    buffer.consume(2);
+    buffer.resize(7, 4);
+    assert_eq!(buffer.len(), 7);
+    assert_eq!(buffer.capacity(), 11);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 3, 3, 4, 4]);
+    assert_eq!(buffer.usable_space(), 2);
+    assert_eq!(buffer.free_space(), 4);
+
+    buffer.resize(11, 5);
+    assert_eq!(buffer.len(), 11);
+    assert_eq!(buffer.capacity(), 13);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 3, 3, 4, 4, 5, 5, 5, 5]);
+    assert_eq!(buffer.usable_space(), 0);
+    assert_eq!(buffer.free_space(), 2);
+
+    buffer.make_room();
+    assert_eq!(buffer.usable_space(), 2);
+    assert_eq!(buffer.free_space(), 2);
+
+    buffer.resize(13, 6);
+    assert_eq!(buffer.len(), 13);
+    assert_eq!(buffer.capacity(), 13);
+    assert_eq!(buffer.buf(), &[0, 0, 0, 3, 3, 4, 4, 5, 5, 5, 5, 6, 6]);
+    assert_eq!(buffer.usable_space(), 0);
+    assert_eq!(buffer.free_space(), 0);
+
+    buffer.resize(0, 7);
+    assert_eq!(buffer.len(), 0);
+    assert_eq!(buffer.capacity(), 13);
+    assert_eq!(buffer.buf(), &[]);
+    assert_eq!(buffer.usable_space(), 13);
+    assert_eq!(buffer.free_space(), 13);
+}

src/lib.rs

@@ -1012,6 +1012,31 @@ impl Buffer {
         }
     }
 
+    /// Resizes the buffer in-place sto that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is greater than `len`, the buffer is extended by the difference,
+    /// with each additional slot filled with `value`. If `new_len` is less than `len`,
+    /// the buffer is simply truncated.
+    ///
+    /// Note that this will reallocate even if there is enough free space at the head of
+    /// the buffer if `len() + usable_space() < new_len`.
+    ///
+    /// If you prefer copying data down in the buffer before attempting to reallocate
+    /// you may wish to call `.make_room()` first.
+    pub fn resize(&mut self, new_len: usize, value: u8) {
+        // Returns `true` if we reallocated out-of-place and thus need to re-zero.
+        if self.buf.resize(new_len, value) {
+            self.zeroed = 0;
+        }
+    }
+
+    /// Resizes the buffer in-place sto that `len` is equal to `new_len`.
+    ///
+    /// Same as calling `.resize(new_len, 0)`.
+    pub fn resize_default(&mut self, new_len: usize) {
+        self.buf.resize(new_len, 0);
+    }
+
     /// Get an immutable slice of the available bytes in this buffer.
     ///
     /// Call `.consume()` to remove bytes from the beginning of this slice.
@@ -1175,6 +1200,13 @@ impl Buffer {
         self.buf.consume(amt);
     }
 
+    /// Shortens the buffer, keeping the first `len`.
+    ///
+    /// If `len` is greater then buffer's length, this has no effect.
+    pub fn truncate(&mut self, len: usize) {
+        self.buf.truncate(len);
+    }
+
     /// Empty this buffer by consuming all bytes.
     pub fn clear(&mut self) {
         let buf_len = self.len();