Customizable lazy fused broadcasting in pure Julia

NEWS.md

@@ -388,11 +388,6 @@ This section lists changes that do not have deprecation warnings.
     Its return value has been removed. Use the `process_running` function
     to determine if a process has already exited.
 
-  * Broadcasting has been redesigned with an extensible public interface. The new API is
-    documented at https://docs.julialang.org/en/latest/manual/interfaces/#Interfaces-1.
-    `AbstractArray` types that specialized broadcasting using the old internal API will
-    need to switch to the new API. ([#20740])
-
   * The logging system has been redesigned - `info` and `warn` are deprecated
     and replaced with the logging macros `@info`, `@warn`, `@debug` and
     `@error`.  The `logging` function is also deprecated and replaced with
@@ -418,6 +413,15 @@ This section lists changes that do not have deprecation warnings.
   * `findn(x::AbstractArray)` has been deprecated in favor of `findall(!iszero, x)`, which
     now returns cartesian indices for multidimensional arrays (see below, [#25532]).
 
+  * Broadcasting operations are no longer fused into a single operation by Julia's parser.
+    Instead, a lazy `Broadcasted` object is created to represent the fused expression and
+    then realized with `copy(bc::Broadcasted)` or `copyto!(dest, bc::Broadcasted)`
+    to evaluate the wrapper. Consequently, package authors generally need to specialize
+    `copy` and `copyto!` methods rather than `broadcast` and `broadcast!`. This also allows
+    for more customization and control of fused broadcasts. See the
+    [Interfaces chapter](https://docs.julialang.org/en/latest/manual/interfaces/#man-interfaces-broadcasting-1)
+    for more information.
+
   * `find` has been renamed to `findall`. `findall`, `findfirst`, `findlast`, `findnext`
     now take and/or return the same type of indices as `keys`/`pairs` for `AbstractArray`,
     `AbstractDict`, `AbstractString`, `Tuple` and `NamedTuple` objects ([#24774], [#25545]).

base/bitarray.jl

@@ -1095,20 +1095,6 @@ function (-)(B::BitArray)
     end
     return A
 end
-broadcast(::typeof(sign), B::BitArray) = copy(B)
-
-function broadcast(::typeof(~), B::BitArray)
-    C = similar(B)
-    Bc = B.chunks
-    if !isempty(Bc)
-        Cc = C.chunks
-        for i = 1:length(Bc)
-            Cc[i] = ~Bc[i]
-        end
-        Cc[end] &= _msk_end(B)
-    end
-    return C
-end
 
 """
     flipbits!(B::BitArray{N}) -> BitArray{N}
@@ -1166,33 +1152,6 @@ end
 (/)(B::BitArray, x::Number) = (/)(Array(B), x)
 (/)(x::Number, B::BitArray) = (/)(x, Array(B))
 
-# broadcast specializations for &, |, and xor/⊻
-broadcast(::typeof(&), B::BitArray, x::Bool) = x ? copy(B) : falses(size(B))
-broadcast(::typeof(&), x::Bool, B::BitArray) = broadcast(&, B, x)
-broadcast(::typeof(|), B::BitArray, x::Bool) = x ? trues(size(B)) : copy(B)
-broadcast(::typeof(|), x::Bool, B::BitArray) = broadcast(|, B, x)
-broadcast(::typeof(xor), B::BitArray, x::Bool) = x ? .~B : copy(B)
-broadcast(::typeof(xor), x::Bool, B::BitArray) = broadcast(xor, B, x)
-for f in (:&, :|, :xor)
-    @eval begin
-        function broadcast(::typeof($f), A::BitArray, B::BitArray)
-            F = BitArray(undef, promote_shape(size(A),size(B))...)
-            Fc = F.chunks
-            Ac = A.chunks
-            Bc = B.chunks
-            (isempty(Ac) || isempty(Bc)) && return F
-            for i = 1:length(Fc)
-                Fc[i] = ($f)(Ac[i], Bc[i])
-            end
-            Fc[end] &= _msk_end(F)
-            return F
-        end
-        broadcast(::typeof($f), A::DenseArray{Bool}, B::BitArray) = broadcast($f, BitArray(A), B)
-        broadcast(::typeof($f), B::BitArray, A::DenseArray{Bool}) = broadcast($f, B, BitArray(A))
-    end
-end
-
-
 ## promotion to complex ##
 
 # TODO?