Add VarInt from implementations by way of macro

Throughout the codebase we cast values to `u64` when constructing a
`VarInt`. We can make the code marginally cleaner by adding `From<T>`
impls for all unsigned integer types less than or equal to 64 bits.
Also allows us to (possibly unnecessarily) comment the cast in a single
place.

bitcoin/src/bip152.rs

@@ -74,7 +74,7 @@ impl convert::AsRef<Transaction> for PrefilledTransaction {
 impl Encodable for PrefilledTransaction {
     #[inline]
     fn consensus_encode<S: io::Write + ?Sized>(&self, mut s: &mut S) -> Result<usize, io::Error> {
-        Ok(VarInt(self.idx as u64).consensus_encode(&mut s)? + self.tx.consensus_encode(&mut s)?)
+        Ok(VarInt::from(self.idx).consensus_encode(&mut s)? + self.tx.consensus_encode(&mut s)?)
     }
 }
 

bitcoin/src/bip158.rs

@@ -391,7 +391,7 @@ impl<'a, W: io::Write> GcsFilterWriter<'a, W> {
         mapped.sort_unstable();
 
         // write number of elements as varint
-        let mut wrote = VarInt(mapped.len() as u64).consensus_encode(&mut self.writer)?;
+        let mut wrote = VarInt::from(mapped.len()).consensus_encode(&mut self.writer)?;
 
         // write out deltas of sorted values into a Golonb-Rice coded bit stream
         let mut writer = BitStreamWriter::new(self.writer);

bitcoin/src/blockdata/block.rs

@@ -276,7 +276,7 @@ impl Block {
     }
 
     /// base_size == size of header + size of encoded transaction count.
-    fn base_size(&self) -> usize { 80 + VarInt(self.txdata.len() as u64).len() }
+    fn base_size(&self) -> usize { 80 + VarInt::from(self.txdata.len()).len() }
 
     /// Returns the size of the block.
     ///

bitcoin/src/blockdata/transaction.rs

@@ -227,7 +227,7 @@ impl TxIn {
         // Size in vbytes:
         // previous_output (36) + script_sig varint len + script_sig push + sequence (4)
         Weight::from_non_witness_data_size(
-            (36 + VarInt(script_sig_size as u64).len() + script_sig_size + 4) as u64,
+            (36 + VarInt::from(script_sig_size).len() + script_sig_size + 4) as u64,
         )
     }
 
@@ -501,9 +501,7 @@ impl TxOut {
         let script_len = self.script_pubkey.len();
         // In vbytes:
         // value (8) + script varint len + script push
-        Weight::from_non_witness_data_size(
-            (8 + VarInt(script_len as u64).len() + script_len) as u64,
-        )
+        Weight::from_non_witness_data_size((8 + VarInt::from(script_len).len() + script_len) as u64)
     }
 
     /// Creates a `TxOut` with given script and the smallest possible `value` that is **not** dust

bitcoin/src/blockdata/witness.rs

@@ -213,7 +213,7 @@ fn resize_if_needed(vec: &mut Vec<u8>, required_len: usize) {
 
 impl Encodable for Witness {
     fn consensus_encode<W: Write + ?Sized>(&self, w: &mut W) -> Result<usize, io::Error> {
-        let len = VarInt(self.witness_elements as u64);
+        let len = VarInt::from(self.witness_elements);
         len.consensus_encode(w)?;
         let content_with_indices_len = self.content.len();
         let indices_size = self.witness_elements * 4;
@@ -233,14 +233,14 @@ impl Witness {
         let index_size = witness_elements * 4;
         let content_size = slice
             .iter()
-            .map(|elem| elem.as_ref().len() + VarInt(elem.as_ref().len() as u64).len())
+            .map(|elem| elem.as_ref().len() + VarInt::from(elem.as_ref().len()).len())
             .sum();
 
         let mut content = vec![0u8; content_size + index_size];
         let mut cursor = 0usize;
         for (i, elem) in slice.iter().enumerate() {
             encode_cursor(&mut content, content_size, i, cursor);
-            let elem_len_varint = VarInt(elem.as_ref().len() as u64);
+            let elem_len_varint = VarInt::from(elem.as_ref().len());
             elem_len_varint
                 .consensus_encode(&mut &mut content[cursor..cursor + elem_len_varint.len()])
                 .expect("writers on vec don't errors, space granted by content_size");
@@ -268,8 +268,8 @@ impl Witness {
 
     /// Returns the bytes required when this Witness is consensus encoded.
     pub fn serialized_len(&self) -> usize {
-        self.iter().map(|el| VarInt(el.len() as u64).len() + el.len()).sum::<usize>()
-            + VarInt(self.witness_elements as u64).len()
+        self.iter().map(|el| VarInt::from(el.len()).len() + el.len()).sum::<usize>()
+            + VarInt::from(self.witness_elements).len()
     }
 
     /// Clear the witness.
@@ -288,7 +288,7 @@ impl Witness {
     fn push_slice(&mut self, new_element: &[u8]) {
         self.witness_elements += 1;
         let previous_content_end = self.indices_start;
-        let element_len_varint = VarInt(new_element.len() as u64);
+        let element_len_varint = VarInt::from(new_element.len());
         let current_content_len = self.content.len();
         let new_item_total_len = element_len_varint.len() + new_element.len();
         self.content.resize(current_content_len + new_item_total_len + 4, 0);

bitcoin/src/consensus/encode.rs

@@ -399,6 +399,21 @@ impl VarInt {
     }
 }
 
+/// Implements `From<T> for VarInt`.
+///
+/// `VarInt`s are consensus encoded as `u64`s so we store them as such. Casting from any integer size smaller than or equal to `u64` is always safe and the cast value is correctly handled by `consensus_encode`.
+macro_rules! impl_var_int_from {
+    ($($ty:tt),*) => {
+        $(
+            /// Creates a `VarInt` from a `usize` by casting the to a `u64`.
+            impl From<$ty> for VarInt {
+                fn from(x: $ty) -> Self { VarInt(x as u64) }
+            }
+        )*
+    }
+}
+impl_var_int_from!(u8, u16, u32, u64, usize);
+
 impl Encodable for VarInt {
     #[inline]
     fn consensus_encode<W: io::Write + ?Sized>(&self, w: &mut W) -> Result<usize, io::Error> {
@@ -436,7 +451,7 @@ impl Decodable for VarInt {
                 if x < 0x100000000 {
                     Err(self::Error::NonMinimalVarInt)
                 } else {
-                    Ok(VarInt(x))
+                    Ok(VarInt::from(x))
                 }
             }
             0xFE => {
@@ -444,7 +459,7 @@ impl Decodable for VarInt {
                 if x < 0x10000 {
                     Err(self::Error::NonMinimalVarInt)
                 } else {
-                    Ok(VarInt(x as u64))
+                    Ok(VarInt::from(x))
                 }
             }
             0xFD => {
@@ -452,10 +467,10 @@ impl Decodable for VarInt {
                 if x < 0xFD {
                     Err(self::Error::NonMinimalVarInt)
                 } else {
-                    Ok(VarInt(x as u64))
+                    Ok(VarInt::from(x))
                 }
             }
-            n => Ok(VarInt(n as u64)),
+            n => Ok(VarInt::from(n)),
         }
     }
 }

bitcoin/src/merkle_tree/block.rs

@@ -436,7 +436,7 @@ impl Encodable for PartialMerkleTree {
         ret += self.hashes.consensus_encode(w)?;
 
         let nb_bytes_for_bits = (self.bits.len() + 7) / 8;
-        ret += encode::VarInt(nb_bytes_for_bits as u64).consensus_encode(w)?;
+        ret += encode::VarInt::from(nb_bytes_for_bits).consensus_encode(w)?;
         for chunk in self.bits.chunks(8) {
             let mut byte = 0u8;
             for (i, bit) in chunk.iter().enumerate() {

bitcoin/src/p2p/address.rs

@@ -148,7 +148,7 @@ impl Encodable for AddrV2 {
             bytes: &[u8],
         ) -> Result<usize, io::Error> {
             let len = network.consensus_encode(w)?
-                + VarInt(bytes.len() as u64).consensus_encode(w)?
+                + VarInt::from(bytes.len()).consensus_encode(w)?
                 + bytes.len();
             w.emit_slice(bytes)?;
             Ok(len)

bitcoin/src/p2p/message.rs

@@ -335,7 +335,7 @@ impl<'a> Encodable for HeaderSerializationWrapper<'a> {
     #[inline]
     fn consensus_encode<W: io::Write + ?Sized>(&self, w: &mut W) -> Result<usize, io::Error> {
         let mut len = 0;
-        len += VarInt(self.0.len() as u64).consensus_encode(w)?;
+        len += VarInt::from(self.0.len()).consensus_encode(w)?;
         for header in self.0.iter() {
             len += header.consensus_encode(w)?;
             len += 0u8.consensus_encode(w)?;

bitcoin/src/psbt/raw.rs

@@ -104,7 +104,7 @@ impl Key {
 impl Serialize for Key {
     fn serialize(&self) -> Vec<u8> {
         let mut buf = Vec::new();
-        VarInt((self.key.len() + 1) as u64)
+        VarInt::from(self.key.len() + 1)
             .consensus_encode(&mut buf)
             .expect("in-memory writers don't error");
 

bitcoin/src/psbt/serialize.rs

@@ -342,7 +342,7 @@ impl Serialize for TapTree {
         let capacity = self
             .script_leaves()
             .map(|l| {
-                l.script().len() + VarInt(l.script().len() as u64).len() // script version
+                l.script().len() + VarInt::from(l.script().len()).len() // script version
             + 1 // merkle branch
             + 1 // leaf version
             })

bitcoin/src/sign_message.rs

@@ -198,7 +198,7 @@ mod message_signing {
 pub fn signed_msg_hash(msg: &str) -> sha256d::Hash {
     let mut engine = sha256d::Hash::engine();
     engine.input(BITCOIN_SIGNED_MSG_PREFIX);
-    let msg_len = encode::VarInt(msg.len() as u64);
+    let msg_len = encode::VarInt::from(msg.len());
     msg_len.consensus_encode(&mut engine).expect("engines don't error");
     engine.input(msg.as_bytes());
     sha256d::Hash::from_engine(engine)