docs: add comment about safety of u8 -> u64 cast in signature (#1704)

* docs: add comment about u8 -> u64 cast in signature

* fix: format

Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Unreleased
 
+- Add comment about safety of u8 -> u64 cast in `ethers_core::types::Signature`
 - Stop defaulting to the `"latest"` block in `eth_estimateGas` params [#1657](https://github.com/gakonst/ethers-rs/pull/1657)
 - Fix geth trace types for debug_traceTransaction rpc
 - Fix RLP decoding of legacy `Transaction`

ethers-core/src/types/signature.rs

@@ -215,6 +215,11 @@ impl From<&Signature> for [u8; 65] {
         sig[32..64].copy_from_slice(&s_bytes);
         // TODO: What if we try to serialize a signature where
         // the `v` is not normalized?
+
+        // The u64 to u8 cast is safe because `sig.v` can only ever be 27 or 28
+        // here. Regarding EIP-155, the modification to `v` happens during tx
+        // creation only _after_ the transaction is signed using
+        // `ethers_signers::to_eip155_v`.
         sig[64] = src.v as u8;
         sig
     }