Merge rust-bitcoin/rust-bitcoin#3421: Const locktime constructors

de319670ae feat: Create relative lock times at compile time (Christian Lewe)
53e1fb6b0c feat: Create absolute lock time at compile time (Christian Lewe)

Pull request description:

  I noticed that I cannot create lock times as compile-time constants. This PR tries to remedy this issue by marking lock time constructors as `const`.

  Because the `internals` crate depends on the `units` crate in a way that I don't fully understand yet, this PR updates the `units` crate only.

  If `from_consensus` is being kept non-`const` by design, to keep the API flexible to future changes, then please close this PR. In this case, I overlooked existing discussions.

ACKs for top commit:
  apoelstra:
    ACK de319670ae successfully ran local tests; good start; should backport
  tcharding:
    ACK de319670ae

Tree-SHA512: 69f9147707bcf8f91f755dd6d1be5ed08945e775ee46918e33d77a9d07ce474047a80ed1226134a3914ead51d1ddbbc657552ca934dc3c079b92ad3d50b13152

units/src/locktime/absolute.rs

@@ -62,7 +62,7 @@ impl Height {
     /// assert_eq!(height.to_consensus_u32(), h);
     /// ```
     #[inline]
-    pub fn from_consensus(n: u32) -> Result<Height, ConversionError> {
+    pub const fn from_consensus(n: u32) -> Result<Height, ConversionError> {
         if is_block_height(n) {
             Ok(Self(n))
         } else {
@@ -72,7 +72,7 @@ impl Height {
 
     /// Converts this [`Height`] to its inner `u32` value.
     #[inline]
-    pub fn to_consensus_u32(self) -> u32 { self.0 }
+    pub const fn to_consensus_u32(self) -> u32 { self.0 }
 }
 
 impl fmt::Display for Height {
@@ -158,7 +158,7 @@ impl Time {
     /// assert_eq!(time.to_consensus_u32(), t);
     /// ```
     #[inline]
-    pub fn from_consensus(n: u32) -> Result<Time, ConversionError> {
+    pub const fn from_consensus(n: u32) -> Result<Time, ConversionError> {
         if is_block_time(n) {
             Ok(Self(n))
         } else {
@@ -168,7 +168,7 @@ impl Time {
 
     /// Converts this [`Time`] to its inner `u32` value.
     #[inline]
-    pub fn to_consensus_u32(self) -> u32 { self.0 }
+    pub const fn to_consensus_u32(self) -> u32 { self.0 }
 }
 
 impl fmt::Display for Time {
@@ -244,10 +244,10 @@ where
 }
 
 /// Returns true if `n` is a block height i.e., less than 500,000,000.
-pub fn is_block_height(n: u32) -> bool { n < LOCK_TIME_THRESHOLD }
+pub const fn is_block_height(n: u32) -> bool { n < LOCK_TIME_THRESHOLD }
 
 /// Returns true if `n` is a UNIX timestamp i.e., greater than or equal to 500,000,000.
-pub fn is_block_time(n: u32) -> bool { n >= LOCK_TIME_THRESHOLD }
+pub const fn is_block_time(n: u32) -> bool { n >= LOCK_TIME_THRESHOLD }
 
 /// An error that occurs when converting a `u32` to a lock time variant.
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -261,10 +261,10 @@ pub struct ConversionError {
 
 impl ConversionError {
     /// Constructs a `ConversionError` from an invalid `n` when expecting a height value.
-    fn invalid_height(n: u32) -> Self { Self { unit: LockTimeUnit::Blocks, input: n } }
+    const fn invalid_height(n: u32) -> Self { Self { unit: LockTimeUnit::Blocks, input: n } }
 
     /// Constructs a `ConversionError` from an invalid `n` when expecting a time value.
-    fn invalid_time(n: u32) -> Self { Self { unit: LockTimeUnit::Seconds, input: n } }
+    const fn invalid_time(n: u32) -> Self { Self { unit: LockTimeUnit::Seconds, input: n } }
 }
 
 impl fmt::Display for ConversionError {

units/src/locktime/relative.rs

@@ -28,12 +28,14 @@ impl Height {
 
     /// Returns the inner `u16` value.
     #[inline]
-    pub fn value(self) -> u16 { self.0 }
+    pub const fn value(self) -> u16 { self.0 }
 
     /// Returns the `u32` value used to encode this locktime in an nSequence field or
     /// argument to `OP_CHECKSEQUENCEVERIFY`.
     #[inline]
-    pub fn to_consensus_u32(&self) -> u32 { self.0.into() }
+    pub const fn to_consensus_u32(&self) -> u32 {
+        self.0 as u32 // cast safety: u32 is wider than u16 on all architectures
+    }
 }
 
 impl From<u16> for Height {
@@ -106,12 +108,14 @@ impl Time {
 
     /// Returns the inner `u16` value.
     #[inline]
-    pub fn value(self) -> u16 { self.0 }
+    pub const fn value(self) -> u16 { self.0 }
 
     /// Returns the `u32` value used to encode this locktime in an nSequence field or
     /// argument to `OP_CHECKSEQUENCEVERIFY`.
     #[inline]
-    pub fn to_consensus_u32(&self) -> u32 { (1u32 << 22) | u32::from(self.0) }
+    pub const fn to_consensus_u32(&self) -> u32 {
+        (1u32 << 22) | self.0 as u32 // cast safety: u32 is wider than u16 on all architectures
+    }
 }
 
 crate::impl_parse_str_from_int_infallible!(Time, u16, from_512_second_intervals);