cherrypick bitfield changes from #112

bitfield/src/pack.rs

@@ -286,7 +286,7 @@ macro_rules! make_packers {
                 const fn mk_mask(n: u32) -> $Bits {
                     if n == 0 {
                         return 0
-                    };
+                    }
                     let one: $Bits = 1; // lolmacros
                     let shift = one.wrapping_shl(n - 1);
                     shift | (shift.saturating_sub(1))
@@ -589,33 +589,42 @@ macro_rules! make_packers {
                 ///
                 /// The packing pair can be used to pack bits from one location
                 /// into another location, and vice versa.
-                pub const fn pair_at(&self, at: u32) -> $Pair<T> {
-                    let dst = $Pack::<$Bits, ()>::starting_at(at, self.bits()).typed();
-                    let at = at.saturating_sub(1);
-                    // TODO(eliza): validate that `at + self.bits() < N_BITS` in
+                pub const fn pair_at(&self, at: u32) -> $Pair {
+                    let dst = $Pack::starting_at(at, self.bits());
+                    self.pair_with(dst.typed())
+                }
+
+                /// Returns a pair type for packing bits from the range
+                /// specified by `self` after the specified packing spec.
+                pub const fn pair_after(&self, after: &Self) -> $Pair {
+                    self.pair_at(after.shift_next())
+                }
+
+                /// Returns a pair type for packing bits from the range
+                /// specified by `self` into the range specified by `with`.
+                ///
+                /// # Note
+                /// The two ranges must be the same size. This can be asserted
+                /// by the `assert_valid` method on the returned pair type.
+                pub const fn pair_with(&self, dst: Self) -> $Pair {
+                    // TODO(eliza): validate that `dst.shift + self.bits() < N_BITS` in
                     // const fn somehow lol
-                    let (dst_shl, dst_shr) = if at > self.shift {
+                    let (dst_shl, dst_shr) = if dst.shift > self.shift {
                         // If the destination is greater than `self`, we need to
                         // shift left.
-                        ((at - self.shift) as $Bits, 0)
+                        ((dst.shift - self.shift) as $Bits, 0)
                     } else {
                         // Otherwise, shift down.
-                        (0, (self.shift - at) as $Bits)
+                        (0, (self.shift - dst.shift) as $Bits)
                     };
                     $Pair {
                         src: self.typed(),
-                        dst,
+                        dst: dst.typed(),
                         dst_shl,
                         dst_shr,
                     }
                 }
 
-                /// Returns a pair type for packing bits from the range
-                /// specified by `self` after the specified packing spec.
-                pub const fn pair_after(&self, after: &Self) -> $Pair<T> {
-                    self.pair_at(after.shift_next())
-                }
-
                 /// Pack the [`self.bits()`] least-significant bits from `value` into `base`.
                 ///
                 /// # Panics
@@ -794,6 +803,25 @@ macro_rules! make_packers {
                     Self(packer.pack_truncating(value, self.0))
                 }
 
+                /// Pack bits from `src` into `self`, using the packing pair
+                /// specified by `pair`, with `self` serving as the "destination" member
+                /// of the pair, and `src` serving as the "source" member of the
+                /// pair.
+                #[inline]
+                pub const fn pack_from_src(self, value: $Bits, pair: &$Pair) -> Self {
+                    Self(pair.pack_from_src(self.0, value))
+                }
+
+                /// Pack bits from `dst` into `self`, using the packing pair
+                /// specified by `pair`, with `self` serving as the "siyrce" member
+                /// of the pair, and `dst` serving as the "destination" member of the
+                /// pair.
+                #[inline]
+                pub const fn pack_from_dst(self, value: $Bits, pair: &$Pair) -> Self {
+                    Self(pair.pack_from_dst(value, self.0))
+                }
+
+
                 /// Pack bits from `value` into `self`, using the range
                 /// specified by `packer`.
                 ///