Avoid an unnecessary allocation in `TaggedHash`

A well-formed tag is always a constant, so allocating to store it
is unnecessary when we can just make the tag a `&'static str`.

diff --git a/lightning/src/offers/merkle.rs b/lightning/src/offers/merkle.rs
index a86f0b4e6fbff57c4b7ac59403bb357d894c3b45..7330541220f625240dee1db5e13539a73aaea824 100644 (file)
--- a/lightning/src/offers/merkle.rs
+++ b/lightning/src/offers/merkle.rs
@@ -32,7 +32,7 @@ tlv_stream!(SignatureTlvStream, SignatureTlvStreamRef, SIGNATURE_TYPES, {
 /// [BOLT 12]: https://github.com/rustyrussell/lightning-rfc/blob/guilt/offers/12-offer-encoding.md#signature-calculation
 #[derive(Clone, Debug, PartialEq)]
 pub struct TaggedHash {
-       tag: String,
+       tag: &'static str,
        merkle_root: sha256::Hash,
        digest: Message,
 }
@@ -41,12 +41,12 @@ impl TaggedHash {
        /// Creates a tagged hash with the given parameters.
        ///
        /// Panics if `tlv_stream` is not a well-formed TLV stream containing at least one TLV record.
-       pub(super) fn new(tag: &str, tlv_stream: &[u8]) -> Self {
+       pub(super) fn new(tag: &'static str, tlv_stream: &[u8]) -> Self {
                let tag_hash = sha256::Hash::hash(tag.as_bytes());
                let merkle_root = root_hash(tlv_stream);
                let digest = Message::from_slice(&tagged_hash(tag_hash, merkle_root)).unwrap();
                Self {
-                       tag: tag.to_owned(),
+                       tag,
                        merkle_root,
                        digest,
                }