Deprecate fallback elsize for AbstractArray

Just like the deprecation of strides (#25321), the default implementation for `elsize` is generally not true. This deprecates the fallback, restructures it to be defined on the type (and not the value), and implements it where possible for the builtin array types and wrappers.

Author: mbauman

base/abstractarray.jl

@@ -128,7 +128,7 @@ prevind(::AbstractArray, i::Integer) = Int(i)-1
 nextind(::AbstractArray, i::Integer) = Int(i)+1
 
 eltype(::Type{<:AbstractArray{E}}) where {E} = @isdefined(E) ? E : Any
-elsize(::AbstractArray{T}) where {T} = sizeof(T)
+elsize(A::AbstractArray) = elsize(typeof(A))
 
 """
     ndims(A::AbstractArray) -> Integer

base/array.jl

@@ -135,7 +135,7 @@ function bitsunionsize(u::Union)
 end
 
 length(a::Array) = arraylen(a)
-elsize(a::Array{T}) where {T} = isbits(T) ? sizeof(T) : (isbitsunion(T) ? bitsunionsize(T) : sizeof(Ptr))
+elsize(::Type{<:Array{T}}) where {T} = isbits(T) ? sizeof(T) : (isbitsunion(T) ? bitsunionsize(T) : sizeof(Ptr))
 sizeof(a::Array) = Core.sizeof(a)
 
 function isassigned(a::Array, i::Int...)

base/deprecated.jl

@@ -1146,8 +1146,8 @@ workspace() = error("`workspace()` is discontinued, consider Revise.jl for an al
 # Issues #17812 Remove default stride implementation
 function strides(a::AbstractArray)
     depwarn("""
-    `strides(a::AbstractArray)` is deprecated for general arrays.
-    Specialize `strides` for custom array types that have the appropriate representation in memory.
+    The default `strides(a::AbstractArray)` implementation is deprecated for general arrays.
+    Specialize `strides(::$(typeof(a).name))` if `$(typeof(a).name)` indeed uses a strided representation in memory.
     Warning: inappropriately implementing this method for an array type that does not use strided
     storage may lead to incorrect results or segfaults.
     """, :strides)
@@ -1156,6 +1156,18 @@ end
 
 @deprecate substrides(s, parent, dim, I::Tuple) substrides(parent, strides(parent), I)
 
+# Issue #26072 Also remove default Base.elsize implementation
+function elsize(t::Type{<:AbstractArray{T}}) where T
+    depwarn("""
+    The default `Base.elsize(::Type{<:AbstractArray})` implementation is deprecated for general arrays.
+    Specialize `Base.elsize(::Type{<:$(t.name)})` if `$(t.name)` indeed has a known representation
+    in memory such that it represents the distance between two contiguous elements.
+    Warning: inappropriately implementing this method may lead to incorrect results or segfaults.
+    """, :elsize)
+    sizeof(T)
+end
+
+
 @deprecate lexcmp(x::AbstractArray, y::AbstractArray) cmp(x, y)
 @deprecate lexcmp(x::Real, y::Real)                   cmp(isless, x, y)
 @deprecate lexcmp(x::Complex, y::Complex)             cmp((real(x),imag(x)), (real(y),imag(y)))

base/reinterpretarray.jl

@@ -44,6 +44,7 @@ function size(a::ReinterpretArray{T,N,S} where {N}) where {T,S}
     tuple(size1, tail(psize)...)
 end
 
+elsize(::Type{<:ReinterpretArray{T}}) where {T} = sizeof(T)
 unsafe_convert(::Type{Ptr{T}}, a::ReinterpretArray{T,N,S} where N) where {T,S} = Ptr{T}(unsafe_convert(Ptr{S},a.parent))
 
 @inline @propagate_inbounds getindex(a::ReinterpretArray{T,0}) where {T} = reinterpret(T, a.parent[])

base/reshapedarray.jl

@@ -179,6 +179,7 @@ IndexStyle(::Type{<:ReshapedArrayLF}) = IndexLinear()
 parent(A::ReshapedArray) = A.parent
 parentindices(A::ReshapedArray) = map(s->1:s, size(parent(A)))
 reinterpret(::Type{T}, A::ReshapedArray, dims::Dims) where {T} = reinterpret(T, parent(A), dims)
+elsize(::Type{<:ReshapedArray{<:Any,<:Any,P}}) where {P} = elsize(P)
 
 unaliascopy(A::ReshapedArray) = typeof(A)(unaliascopy(A.parent), A.dims, A.mi)
 dataids(A::ReshapedArray) = dataids(A.parent)

base/strings/basic.jl

@@ -648,6 +648,7 @@ length(s::CodeUnits) = ncodeunits(s.s)
 sizeof(s::CodeUnits{T}) where {T} = ncodeunits(s.s) * sizeof(T)
 size(s::CodeUnits) = (length(s),)
 strides(s::CodeUnits) = (1,)
+elsize(s::CodeUnits{T}) where {T} = sizeof(T)
 @propagate_inbounds getindex(s::CodeUnits, i::Int) = codeunit(s.s, i)
 IndexStyle(::Type{<:CodeUnits}) = IndexLinear()
 start(s::CodeUnits) = 1

base/subarray.jl

@@ -266,6 +266,8 @@ compute_stride1(s, inds, I::Tuple{AbstractRange, Vararg{Any}}) = s*step(I[1])
 compute_stride1(s, inds, I::Tuple{Slice, Vararg{Any}}) = s
 compute_stride1(s, inds, I::Tuple{Any, Vararg{Any}}) = throw(ArgumentError("invalid strided index type $(typeof(I[1]))"))
 
+elsize(::Type{<:SubArray{<:Any,<:Any,P}}) where {P} = elsize(P)
+
 iscontiguous(A::SubArray) = iscontiguous(typeof(A))
 iscontiguous(::Type{<:SubArray}) = false
 iscontiguous(::Type{<:FastContiguousSubArray}) = true

stdlib/Random/src/RNGs.jl

@@ -371,6 +371,7 @@ Base.getindex(a::UnsafeView, i::Int) = unsafe_load(a.ptr, i)
 Base.setindex!(a::UnsafeView, x, i::Int) = unsafe_store!(a.ptr, x, i)
 Base.pointer(a::UnsafeView) = a.ptr
 Base.size(a::UnsafeView) = (a.len,)
+Base.elsize(::UnsafeView{T}) where {T} = sizeof(T)
 
 # this is essentially equivalent to rand!(r, ::AbstractArray{Float64}, I) above, but due to
 # optimizations which can't be done currently when working with pointers, we have to re-order

test/abstractarray.jl

@@ -772,7 +772,6 @@ end
 @testset "ndims and friends" begin
     @test ndims(Diagonal(rand(1:5,5))) == 2
     @test ndims(Diagonal{Float64}) == 2
-    @test Base.elsize(Diagonal(rand(1:5,5))) == sizeof(Int)
 end
 
 @testset "Issue #17811" begin