add encoded_size function

src/lib.rs

@@ -66,3 +66,15 @@ pub fn decode_from<R: Buffer, T: Decodable>(r: &mut R, size_limit: SizeLimit) ->
 DecodingResult<T> {
     Decodable::decode(&mut reader::DecoderReader::new(r, size_limit))
 }
+
+
+/// Returns the size that an object would be if encoded using bincode.
+///
+/// This is used internally as part of the check for encode_into, but it can
+/// be useful for preallocating buffers if thats your style.
+pub fn encoded_size<T: Encodable>(t: &T) -> u64 {
+    use std::u64::MAX;
+    let mut size_checker = SizeChecker::new(MAX);
+    t.encode(&mut size_checker).ok();
+    size_checker.written
+}

src/test.rs

@@ -14,6 +14,7 @@ use super::{
     encode,
     decode,
     decode_from,
+    encoded_size,
     DecodingError,
     DecodingResult
 };
@@ -220,3 +221,13 @@ fn too_big_encode() {
     assert!(encode(&"abcde", UpperBound(4)).is_err());
     assert!(encode(&"abcde", UpperBound(5)).is_ok());
 }
+
+#[test]
+fn test_encoded_size() {
+    assert!(encoded_size(&0u8) == 1);
+    assert!(encoded_size(&0u16) == 2);
+    assert!(encoded_size(&0u32) == 4);
+    assert!(encoded_size(&0u64) == 8);
+
+    assert!(encoded_size(&vec![0u32, 1u32, 2u32]) == 8 + 3 * (4))
+}

src/writer.rs

@@ -20,8 +20,8 @@ pub struct EncoderWriter<'a, W: 'a> {
 }
 
 pub struct SizeChecker {
-    size_limit: u64,
+    pub size_limit: u64,
-    written: u64
+    pub written: u64
 }
 
 fn wrap_io(err: IoError) -> EncodingError {