Make constructors const

Audit the codebase for any function that starts with `/// Creates` and
see if we can make it const. Inline them at the same time.

bitcoin/src/blockdata/block.rs

@@ -169,7 +169,8 @@ impl Version {
     /// Creates a [`Version`] from a signed 32 bit integer value.
     ///
     /// This is the data type used in consensus code in Bitcoin Core.
-    pub fn from_consensus(v: i32) -> Self { Version(v) }
+    #[inline]
+    pub const fn from_consensus(v: i32) -> Self { Version(v) }
 
     /// Returns the inner `i32` value.
     ///

bitcoin/src/blockdata/script/builder.rs

@@ -18,7 +18,8 @@ pub struct Builder(ScriptBuf, Option<Opcode>);
 
 impl Builder {
     /// Creates a new empty script.
-    pub fn new() -> Self { Builder(ScriptBuf::new(), None) }
+    #[inline]
+    pub const fn new() -> Self { Builder(ScriptBuf::new(), None) }
 
     /// Returns the length in bytes of the script.
     pub fn len(&self) -> usize { self.0.len() }

bitcoin/src/blockdata/script/owned.rs

@@ -33,6 +33,7 @@ pub struct ScriptBuf(pub(in crate::blockdata::script) Vec<u8>);
 
 impl ScriptBuf {
     /// Creates a new empty script.
+    #[inline]
     pub const fn new() -> Self { ScriptBuf(Vec::new()) }
 
     /// Creates a new empty script with pre-allocated capacity.

bitcoin/src/blockdata/script/push_bytes.rs

@@ -194,7 +194,8 @@ mod primitive {
 
     impl PushBytesBuf {
         /// Creates a new empty `PushBytesBuf`.
-        pub fn new() -> Self { PushBytesBuf(Vec::new()) }
+        #[inline]
+        pub const fn new() -> Self { PushBytesBuf(Vec::new()) }
 
         /// Creates a new empty `PushBytesBuf` with reserved capacity.
         pub fn with_capacity(capacity: usize) -> Self { PushBytesBuf(Vec::with_capacity(capacity)) }

bitcoin/src/blockdata/transaction.rs

@@ -78,7 +78,7 @@ impl OutPoint {
 
     /// Creates a new [`OutPoint`].
     #[inline]
-    pub fn new(txid: Txid, vout: u32) -> OutPoint { OutPoint { txid, vout } }
+    pub const fn new(txid: Txid, vout: u32) -> OutPoint { OutPoint { txid, vout } }
 
     /// Creates a "null" `OutPoint`.
     ///

bitcoin/src/blockdata/witness.rs

@@ -27,7 +27,7 @@ use crate::{Script, VarInt};
 /// saving some allocations.
 ///
 /// [segwit upgrade]: <https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki>
-#[derive(Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Witness {
     /// Contains the witness `Vec<Vec<u8>>` serialization without the initial varint indicating the
     /// number of elements (which is stored in `witness_elements`).
@@ -233,7 +233,14 @@ impl Encodable for Witness {
 
 impl Witness {
     /// Creates a new empty [`Witness`].
-    pub fn new() -> Self { Witness::default() }
+    #[inline]
+    pub const fn new() -> Self {
+        Witness {
+            content: Vec::new(),
+            witness_elements: 0,
+            indices_start: 0,
+        }
+    }
 
     /// Creates a witness required to spend a P2WPKH output.
     ///
@@ -543,6 +550,10 @@ impl From<Vec<&[u8]>> for Witness {
     fn from(vec: Vec<&[u8]>) -> Self { Witness::from_slice(&vec) }
 }
 
+impl Default for Witness {
+    fn default() -> Self { Self::new() }
+}
+
 #[cfg(test)]
 mod test {
     use hex::{test_hex_unwrap as hex};

hashes/src/siphash24.rs

@@ -90,7 +90,8 @@ pub struct HashEngine {
 
 impl HashEngine {
     /// Creates a new SipHash24 engine with keys.
-    pub fn with_keys(k0: u64, k1: u64) -> HashEngine {
+    #[inline]
+    pub const fn with_keys(k0: u64, k1: u64) -> HashEngine {
         HashEngine {
             k0,
             k1,
@@ -107,7 +108,8 @@ impl HashEngine {
     }
 
     /// Creates a new SipHash24 engine.
-    pub fn new() -> HashEngine { HashEngine::with_keys(0, 0) }
+    #[inline]
+    pub const fn new() -> HashEngine { HashEngine::with_keys(0, 0) }
 
     /// Retrieves the keys of this engine.
     pub fn keys(&self) -> (u64, u64) { (self.k0, self.k1) }