Merge rust-bitcoin#3605: Mark function as const

1db4b72 Mark function as const (yancy)

Pull request description:

  We discussed marking from_vb as const here: rust-bitcoin#3602

  However, from what I can tell, map() isn't const and I don't see a tracking issue for changing it.  Also, the question mark operator isn't available in const context either, although there is a tracking issue for that: rust-lang/rust#74935.  It will be a long while before that makes it into this projects MSRV if/when it lands.

  There are some other functions in this module that could use the same re-write to make them const as well it looks like.

ACKs for top commit:
  tcharding:
    ACK 1db4b72
  apoelstra:
    ACK 1db4b72; successfully ran local tests

Tree-SHA512: 62b612791dd3ce2f6ebf27f586a1262633a46566b9fd3583984171f62209714ad979439345fe86d8ef87d2f78a2cee21d619e2eb3621689faf59d81640e9f77c

units/src/weight.rs

@@ -55,8 +55,12 @@ impl Weight {
     pub fn from_kwu(wu: u64) -> Option<Self> { wu.checked_mul(1000).map(Weight) }
 
     /// Constructs a new [`Weight`] from virtual bytes, returning [`None`] if an overflow occurred.
-    pub fn from_vb(vb: u64) -> Option<Self> {
-        vb.checked_mul(Self::WITNESS_SCALE_FACTOR).map(Weight::from_wu)
+    pub const fn from_vb(vb: u64) -> Option<Self> {
+        // No `map()` in const context.
+        match vb.checked_mul(Self::WITNESS_SCALE_FACTOR) {
+            Some(wu) => Some(Weight::from_wu(wu)),
+            None => None,
+        }
     }
 
     /// Constructs a new [`Weight`] from virtual bytes panicking if an overflow occurred.