Implement Display for block::Header

Not all the fields within `block::Header` implement `Display` however
a block header can reasonably be displayed by using 160 hex characters.

Implement `Display` for `block::Header` by printing the header in hex in
the same layout as we hash it in `block_hash`.

Cargo-minimal.lock

@@ -111,6 +111,7 @@ name = "bitcoin-primitives"
 version = "0.101.0"
 dependencies = [
  "arbitrary",
+ "arrayvec",
  "bincode",
  "bitcoin-internals",
  "bitcoin-units",

Cargo-recent.lock

@@ -110,6 +110,7 @@ name = "bitcoin-primitives"
 version = "0.101.0"
 dependencies = [
  "arbitrary",
+ "arrayvec",
  "bincode",
  "bitcoin-internals",
  "bitcoin-units",

primitives/Cargo.toml

@@ -16,7 +16,7 @@ exclude = ["tests", "contrib"]
 
 [features]
 default = ["std", "hex"]
-std = ["alloc", "hashes/std", "hex?/std", "internals/std", "units/std"]
+std = ["alloc", "hashes/std", "hex?/std", "internals/std", "units/std", "arrayvec/std"]
 alloc = ["hashes/alloc", "hex?/alloc", "internals/alloc", "units/alloc"]
 serde = ["dep:serde", "hashes/serde", "hex?/serde", "internals/serde", "units/serde", "alloc", "hex"]
 arbitrary = ["dep:arbitrary", "units/arbitrary"]
@@ -26,6 +26,7 @@ hex = ["dep:hex", "hashes/hex", "internals/hex"]
 hashes = { package = "bitcoin_hashes", path = "../hashes", default-features = false }
 internals = { package = "bitcoin-internals", path = "../internals" }
 units = { package = "bitcoin-units", path = "../units", default-features = false }
+arrayvec = { version = "0.7", default-features = false }
 
 arbitrary = { version = "1.4", optional = true }
 hex = { package = "hex-conservative", version = "0.3.0", default-features = false, optional = true }

primitives/src/block.rs

@@ -207,6 +207,27 @@ impl Header {
     }
 }
 
+#[cfg(feature = "hex")]
+impl fmt::Display for Header {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use fmt::Write as _;
+        use hex::DisplayHex as _;
+
+        let mut buf = arrayvec::ArrayString::<160>::new();
+        write!(
+            &mut buf,
+            "{}{}{}{}{}{}",
+            self.version.to_consensus().to_le_bytes().as_hex(),
+            self.prev_blockhash.as_byte_array().as_hex(),
+            self.merkle_root.as_byte_array().as_hex(),
+            self.time.to_u32().to_le_bytes().as_hex(),
+            self.bits.to_consensus().to_le_bytes().as_hex(),
+            self.nonce.to_le_bytes().as_hex(),
+        )?;
+        fmt::Display::fmt(&buf, f)
+    }
+}
+
 impl fmt::Debug for Header {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_struct("Header")
@@ -535,4 +556,35 @@ mod tests {
         );
         assert_eq!(format!("{:?}", header), expected);
     }
+
+    #[test]
+    #[cfg(feature = "hex")]
+    fn header_display() {
+        let seconds: u32 = 1_653_195_600; // Arbitrary timestamp: May 22nd, 5am UTC.
+
+        let header = Header {
+            version: Version::TWO,
+            prev_blockhash: BlockHash::from_byte_array([0xab; 32]),
+            merkle_root: TxMerkleNode::from_byte_array([0xcd; 32]),
+            time: BlockTime::from(seconds),
+            bits: CompactTarget::from_consensus(0xbeef),
+            nonce: 0xcafe,
+        };
+
+        let want = concat!(
+            "02000000",                                                         // version
+            "abababababababababababababababababababababababababababababababab", // prev_blockhash
+            "cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd", // merkle_root
+            "50c38962",                                                         // time
+            "efbe0000",                                                         // bits
+            "feca0000",                                                         // nonce
+        );
+        assert_eq!(want.len(), 160);
+        assert_eq!(format!("{}", header), want);
+
+        // Check how formatting options are handled.
+        let want = format!("{:.20}", want);
+        let got = format!("{:.20}", header);
+        assert_eq!(got, want);
+    }
 }