RFC: Deprecate implicit scalar broadcasting in setindex! (#26347)

The current `setindex!` function (the `A[i] = b` syntax) does three very different operations:
* Given an index `i` that refers to only one location (scalar indexing), `A[i] = b` modifies `A` in that location to hold `b`.
* Given an index `I` that refers to more than one location (non-scalar indexing), `A[I] = b` can mean one of two things:
    * If `b` is an AbstractArray (multi-value), assign the values it contains to those locations `I` in `A`. That is, essentially, `[A[i] = bᵢ for (i, bᵢ) in zip(I, b)]`.
    * If `b` is not an AbstractArray (scalar-value), then broadcast its value to every location selected by `I` in `A`.

These two different behaviors in the non-scalar indexing case basically make using this function in a generic way impossible.  If you want to _always_ set the value `b` to many indices of `A`, you _cannot_ use this syntax because `b` might happen to be an array in some cases, radically changing the meaning of the expression.  You need to manually iterate over the indices, using scalar setindex methods.  But we now also have the new `broadcast!` syntax, `A[I] .= B`, which uses the usual broadcasting semantics to determine how `B` should fill into the values of `A`.

This PR deprecates the implicit broadcasting of scalar values in non-scalar indexing in favor of an explicit `.=` broadcast syntax, leaving multi-value non-scalar indexing intact.  This is the _exact opposite_ of PR #24368.

NEWS.md

@@ -697,6 +697,18 @@ Deprecated or removed
     `Matrix{Int}(undef, (2, 4))`, and `Array{Float32,3}(11, 13, 17)` is now
     `Array{Float32,3}(undef, 11, 13, 17)` ([#24781]).
 
+  * Previously `setindex!(A, x, I...)` (and the syntax `A[I...] = x`) supported two
+    different modes of operation when supplied with a set of non-scalar indices `I`
+    (e.g., at least one index is an `AbstractArray`) depending upon the value of `x`
+    on the right hand side. If `x` is an `AbstractArray`, its _contents_ are copied
+    elementwise into the locations in `A` selected by `I` and it must have the same
+    number of elements as `I` selects locations. Otherwise, if `x` is not an
+    `AbstractArray`, then its _value_ is implicitly broadcast to all locations to
+    all locations in `A` selected by `I`. This latter behavior—implicitly broadcasting
+    "scalar"-like values across many locations—is now deprecated in favor of explicitly
+    using the broadcasted assignment syntax `A[I...] .= x` or `fill!(view(A, I...), x)`
+    ([#26347]).
+
   * `LinAlg.fillslots!` has been renamed `LinAlg.fillstored!` ([#25030]).
 
   * `fill!(A::Diagonal, x)` and `fill!(A::AbstractTriangular, x)` have been deprecated

base/abstractarray.jl

@@ -1181,8 +1181,8 @@ function get!(X::AbstractArray{T}, A::AbstractArray, I::Union{AbstractRange,Abst
     # Linear indexing
     ind = findall(in(1:length(A)), I)
     X[ind] = A[I[ind]]
-    X[1:first(ind)-1] = default
-    X[last(ind)+1:length(X)] = default
+    fill!(view(X, 1:first(ind)-1), default)
+    fill!(view(X, last(ind)+1:length(X)), default)
     X
 end
 
@@ -1380,7 +1380,11 @@ function _cat(A, shape::NTuple{N}, catdims, X...) where N
             end
         end
         I::NTuple{N, UnitRange{Int}} = (inds...,)
-        A[I...] = x
+        if x isa AbstractArray
+            A[I...] = x
+        else
+            fill!(view(A, I...), x)
+        end
     end
     return A
 end
@@ -1930,27 +1934,27 @@ function mapslices(f, A::AbstractArray, dims::AbstractVector)
         ridx[d] = axes(R,d)
     end
 
-    R[ridx...] = r1
+    concatenate_setindex!(R, r1, ridx...)
 
     nidx = length(otherdims)
-    indices = Iterators.drop(CartesianIndices(itershape), 1)
+    indices = Iterators.drop(CartesianIndices(itershape), 1) # skip the first element, we already handled it
     inner_mapslices!(safe_for_reuse, indices, nidx, idx, otherdims, ridx, Aslice, A, f, R)
 end
 
 @noinline function inner_mapslices!(safe_for_reuse, indices, nidx, idx, otherdims, ridx, Aslice, A, f, R)
     if safe_for_reuse
         # when f returns an array, R[ridx...] = f(Aslice) line copies elements,
         # so we can reuse Aslice
-        for I in indices # skip the first element, we already handled it
+        for I in indices
             replace_tuples!(nidx, idx, ridx, otherdims, I)
             _unsafe_getindex!(Aslice, A, idx...)
-            R[ridx...] = f(Aslice)
+            concatenate_setindex!(R, f(Aslice), ridx...)
         end
     else
         # we can't guarantee safety (#18524), so allocate new storage for each slice
         for I in indices
             replace_tuples!(nidx, idx, ridx, otherdims, I)
-            R[ridx...] = f(A[idx...])
+            concatenate_setindex!(R, f(A[idx...]), ridx...)
         end
     end
 
@@ -1963,6 +1967,8 @@ function replace_tuples!(nidx, idx, ridx, otherdims, I)
     end
 end
 
+concatenate_setindex!(R, v, I...) = (R[I...] .= (v,); R)
+concatenate_setindex!(R, X::AbstractArray, I...) = (R[I...] = X)
 
 ## 1 argument
 

base/abstractarraymath.jl

@@ -363,10 +363,6 @@ _rshps(shp, shp_i, sz, i, ::Tuple{}) =
 _reperr(s, n, N) = throw(ArgumentError("number of " * s * " repetitions " *
     "($n) cannot be less than number of dimensions of input ($N)"))
 
-# We need special handling when repeating arrays of arrays
-cat_fill!(R, X, inds) = (R[inds...] = X)
-cat_fill!(R, X::AbstractArray, inds) = fill!(view(R, inds...), X)
-
 @noinline function _repeat(A::AbstractArray, inner, outer)
     shape, inner_shape = rep_shapes(A, inner, outer)
 
@@ -385,7 +381,7 @@ cat_fill!(R, X::AbstractArray, inds) = fill!(view(R, inds...), X)
                 n = inner[i]
                 inner_indices[i] = (1:n) .+ ((c[i] - 1) * n)
             end
-            cat_fill!(R, A[c], inner_indices)
+            fill!(view(R, inner_indices...), A[c])
         end
     end
 

base/array.jl

@@ -706,29 +706,6 @@ function setindex! end
 @eval setindex!(A::Array{T}, x, i1::Int, i2::Int, I::Int...) where {T} =
     (@_inline_meta; arrayset($(Expr(:boundscheck)), A, convert(T,x)::T, i1, i2, I...))
 
-# These are redundant with the abstract fallbacks but needed for bootstrap
-function setindex!(A::Array, x, I::AbstractVector{Int})
-    @_propagate_inbounds_meta
-    I′ = unalias(A, I)
-    for i in I′
-        A[i] = x
-    end
-    return A
-end
-function setindex!(A::Array, X::AbstractArray, I::AbstractVector{Int})
-    @_propagate_inbounds_meta
-    @boundscheck setindex_shape_check(X, length(I))
-    X′ = unalias(A, X)
-    I′ = unalias(A, I)
-    count = 1
-    for i in I′
-        @inbounds x = X′[count]
-        A[i] = x
-        count += 1
-    end
-    return A
-end
-
 # Faster contiguous setindex! with copyto!
 function setindex!(A::Array{T}, X::Array{T}, I::UnitRange{Int}) where T
     @_inline_meta
@@ -750,9 +727,6 @@ function setindex!(A::Array{T}, X::Array{T}, c::Colon) where T
     return A
 end
 
-setindex!(A::Array, x::Number, ::Colon) = fill!(A, x)
-setindex!(A::Array{T, N}, x::Number, ::Vararg{Colon, N}) where {T, N} = fill!(A, x)
-
 # efficiently grow an array
 
 _growbeg!(a::Vector, delta::Integer) =

base/bitarray.jl

@@ -584,7 +584,7 @@ function gen_bitarray_from_itr(itr, st)
         end
     end
     if ind > 1
-        @inbounds C[ind:bitcache_size] = false
+        @inbounds C[ind:bitcache_size] .= false
         resize!(B, length(B) + ind - 1)
         dumpbitcache(Bc, cind, C)
     end
@@ -608,7 +608,7 @@ function fill_bitarray_from_itr!(B::BitArray, itr, st)
         end
     end
     if ind > 1
-        @inbounds C[ind:bitcache_size] = false
+        @inbounds C[ind:bitcache_size] .= false
         dumpbitcache(Bc, cind, C)
     end
     return B
@@ -653,44 +653,10 @@ end
 indexoffset(i) = first(i)-1
 indexoffset(::Colon) = 0
 
-@inline function setindex!(B::BitArray, x, J0::Union{Colon,UnitRange{Int}})
-    I0 = to_indices(B, (J0,))[1]
-    @boundscheck checkbounds(B, I0)
-    y = Bool(x)
-    l0 = length(I0)
-    l0 == 0 && return B
-    f0 = indexoffset(I0)+1
-    fill_chunks!(B.chunks, y, f0, l0)
-    return B
-end
 @propagate_inbounds function setindex!(B::BitArray, X::AbstractArray, J0::Union{Colon,UnitRange{Int}})
     _setindex!(IndexStyle(B), B, X, to_indices(B, (J0,))[1])
 end
 
-# logical indexing
-
-# When indexing with a BitArray, we can operate whole chunks at a time for a ~100x gain
-@inline function setindex!(B::BitArray, x, I::BitArray)
-    @boundscheck checkbounds(B, I)
-    _unsafe_setindex!(B, x, I)
-end
-function _unsafe_setindex!(B::BitArray, x, I::BitArray)
-    y = convert(Bool, x)
-    Bc = B.chunks
-    Ic = I.chunks
-    length(Bc) == length(Ic) || throw_boundserror(B, I)
-    @inbounds if y
-        for i = 1:length(Bc)
-            Bc[i] |= Ic[i]
-        end
-    else
-        for i = 1:length(Bc)
-            Bc[i] &= ~Ic[i]
-        end
-    end
-    return B
-end
-
 # Assigning an array of bools is more complicated, but we can still do some
 # work on chunks by combining X and I 64 bits at a time to improve perf by ~40%
 @inline function setindex!(B::BitArray, X::AbstractArray, I::BitArray)

base/bitset.jl

@@ -114,12 +114,16 @@ end
 @inline function _growend0!(b::Bits, nchunks::Int)
     len = length(b)
     _growend!(b, nchunks)
-    @inbounds b[len+1:end] = CHK0 # resize! gives dirty memory
+    for i in len+1:length(b)
+        @inbounds b[i] = CHK0 # resize! gives dirty memory
+    end
 end
 
 @inline function _growbeg0!(b::Bits, nchunks::Int)
     _growbeg!(b, nchunks)
-    @inbounds b[1:nchunks] = CHK0
+    for i in 1:nchunks
+        @inbounds b[i] = CHK0
+    end
 end
 
 function _matched_map!(f, s1::BitSet, s2::BitSet)

base/broadcast.jl

@@ -832,7 +832,7 @@ end
         end
     end
     if ind > 1
-        @inbounds tmp[ind:bitcache_size] = false
+        @inbounds tmp[ind:bitcache_size] .= false
         dumpbitcache(destc, cind, tmp)
     end
     return dest
@@ -1108,6 +1108,39 @@ See [`broadcast_getindex`](@ref) for examples of the treatment of `inds`.
     end
 end
 
+## In specific instances, we can broadcast masked BitArrays whole chunks at a time
+# Very intentionally do not support much functionality here: scalar indexing would be O(n)
+struct BitMaskedBitArray{N,M}
+    parent::BitArray{N}
+    mask::BitArray{M}
+    BitMaskedBitArray{N,M}(parent, mask) where {N,M} = new(parent, mask)
+end
+@inline function BitMaskedBitArray(parent::BitArray{N}, mask::BitArray{M}) where {N,M}
+    @boundscheck checkbounds(parent, mask)
+    BitMaskedBitArray{N,M}(parent, mask)
+end
+Base.@propagate_inbounds dotview(B::BitArray, i::BitArray) = BitMaskedBitArray(B, i)
+Base.show(io::IO, B::BitMaskedBitArray) = foreach(arg->show(io, arg), (typeof(B), (B.parent, B.mask)))
+# Override materialize! to prevent the BitMaskedBitArray from escaping to an overrideable method
+@inline materialize!(B::BitMaskedBitArray, bc::Broadcasted{<:Any,<:Any,typeof(identity),Tuple{Bool}}) = fill!(B, bc.args[1])
+@inline materialize!(B::BitMaskedBitArray, bc::Broadcasted{<:Any}) = materialize!(SubArray(B.parent, to_indices(B.parent, (B.mask,))), bc)
+function Base.fill!(B::BitMaskedBitArray, b::Bool)
+    Bc = B.parent.chunks
+    Ic = B.mask.chunks
+    @inbounds if b
+        for i = 1:length(Bc)
+            Bc[i] |= Ic[i]
+        end
+    else
+        for i = 1:length(Bc)
+            Bc[i] &= ~Ic[i]
+        end
+    end
+    return B
+end
+
+
+
 ############################################################
 
 # x[...] .= f.(y...) ---> broadcast!(f, dotview(x, ...), y...).

base/char.jl

@@ -215,7 +215,10 @@ widen(::Type{T}) where {T<:AbstractChar} = T
 print(io::IO, c::Char) = (write(io, c); nothing)
 print(io::IO, c::AbstractChar) = print(io, Char(c)) # fallback: convert to output UTF-8
 
-const hex_chars = UInt8['0':'9';'a':'z']
+const hex_chars = UInt8['0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
+                        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
+                        'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r',
+                        's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
 
 function show_invalid(io::IO, c::Char)
     write(io, 0x27)

base/compiler/ssair/ir.jl

@@ -588,7 +588,9 @@ function resize!(compact::IncrementalCompact, nnewnodes)
     resize!(compact.result_lines, nnewnodes)
     resize!(compact.result_flags, nnewnodes)
     resize!(compact.used_ssas, nnewnodes)
-    compact.used_ssas[(old_length+1):nnewnodes] = 0
+    for i in (old_length+1):nnewnodes
+        compact.used_ssas[i] = 0
+    end
     nothing
 end
 

base/compiler/ssair/slot2ssa.jl

@@ -377,8 +377,8 @@ function domsort_ssa!(ir::IRCode, domtree::DomTree)
         end
         old_inst_range = ir.cfg.blocks[bb].stmts
         inst_range = (bb_start_off+1):(bb_start_off+length(old_inst_range))
-        inst_rename[old_inst_range] = Any[SSAValue(x) for x in inst_range]
         for (nidx, idx) in zip(inst_range, old_inst_range)
+            inst_rename[idx] = SSAValue(nidx)
             stmt = ir.stmts[idx]
             if isa(stmt, PhiNode)
                 result_stmts[nidx] = rename_phinode_edges(stmt, bb, result_order, bb_rename)

base/deprecated.jl

@@ -1491,6 +1491,27 @@ function slicedim(A::AbstractVector, d::Integer, i::Number)
     end
 end
 
+# PR #26347: Deprecate implicit scalar broadcasting in setindex!
+_axes(::Ref) = ()
+_axes(x) = axes(x)
+function deprecate_scalar_setindex_broadcast_message(v, I...)
+    value = (_axes(Base.Broadcast.broadcastable(v)) == () ? "x" : "(x,)")
+    "using `A[I...] = x` to implicitly broadcast `x` across many locations is deprecated. Use `A[I...] .= $value` instead."
+end
+deprecate_scalar_setindex_broadcast_message(v, ::Colon, ::Vararg{Colon}) =
+    "using `A[:] = x` to implicitly broadcast `x` across many locations is deprecated. Use `fill!(A, x)` instead."
+
+function _iterable(v, I...)
+    depwarn(deprecate_scalar_setindex_broadcast_message(v, I...), :setindex!)
+    Iterators.repeated(v)
+end
+function setindex!(B::BitArray, x, I0::Union{Colon,UnitRange{Int}}, I::Union{Int,UnitRange{Int},Colon}...)
+    depwarn(deprecate_scalar_setindex_broadcast_message(x, I0, I...), :setindex!)
+    B[I0, I...] .= (x,)
+    B
+end
+
+
 # PR #26283
 @deprecate contains(haystack, needle) occursin(needle, haystack)
 @deprecate contains(s::AbstractString, r::Regex, offset::Integer) occursin(r, s, offset=offset)

base/iobuffer.jl

@@ -117,7 +117,7 @@ function IOBuffer(;
         append=flags.append,
         truncate=flags.truncate,
         maxsize=maxsize)
-    buf.data[:] = 0
+    fill!(buf.data, 0)
     return buf
 end
 
@@ -246,7 +246,7 @@ function truncate(io::GenericIOBuffer, n::Integer)
     if n > length(io.data)
         resize!(io.data, n)
     end
-    io.data[io.size+1:n] = 0
+    io.data[io.size+1:n] .= 0
     io.size = n
     io.ptr = min(io.ptr, n+1)
     ismarked(io) && io.mark > n && unmark(io)