Add _unsetindex! methods for SubArrays and CartesianIndexes (JuliaLang#53383)

With this, the following (and equivalent calls) work:
```julia
julia> copyto!(view(zeros(BigInt, 2), 1:2), Vector{BigInt}(undef,2))
2-element view(::Vector{BigInt}, 1:2) with eltype BigInt:
 #undef
 #undef

julia> copyto!(view(zeros(BigInt, 2), 1:2), view(Vector{BigInt}(undef,2), 1:2))
2-element view(::Vector{BigInt}, 1:2) with eltype BigInt:
 #undef
 #undef
```

Close JuliaLang#53098. With this, all
the `_unsetindex!` branches in `copyto_unaliased!` work for
`Array`-views, and this makes certain indexing operations vectorize and
speed-up:
```julia
julia> using BenchmarkTools

julia> a = view(rand(100,100), 1:100, 1:100); b = view(similar(a), axes(a)...);

julia> @Btime copyto!($b, $a);
  16.427 μs (0 allocations: 0 bytes) # master
  2.308 μs (0 allocations: 0 bytes) # PR
``` 

Improves (but doesn't resolve)
JuliaLang#40962 and
JuliaLang#53158

```julia
julia> a = rand(40,40); b = rand(40,40);

julia> @Btime $a[1:end,1:end] .= $b;
  5.383 μs (0 allocations: 0 bytes) # v"1.12.0-DEV.16"
  3.194 μs (0 allocations: 0 bytes) # PR
```
ƒ
Co-authored-by: Jameson Nash <[email protected]>

Author: jishnub

base/abstractarray.jl

@@ -1472,7 +1472,20 @@ function _setindex!(::IndexCartesian, A::AbstractArray, v, I::Vararg{Int,M}) whe
     r
 end
 
-_unsetindex!(A::AbstractArray, i::Integer) = _unsetindex!(A, to_index(i))
+function _unsetindex!(A::AbstractArray, i::Integer...)
+    @_propagate_inbounds_meta
+    _unsetindex!(A, map(to_index, i)...)
+end
+
+function _unsetindex!(A::AbstractArray{T}, i::Int...) where T
+    # this provides a fallback method which is a no-op if the element is already unassigned
+    # such that copying into an uninitialized object generally always will work,
+    # even if the specific custom array type has not implemented `_unsetindex!`
+    @inline
+    @boundscheck checkbounds(A, i...)
+    allocatedinline(T) || @inbounds(!isassigned(A, i...)) || throw(MethodError(_unsetindex!, (A, i...)))
+    return A
+end
 
 """
     parent(A)

base/array.jl

@@ -219,7 +219,12 @@ function _unsetindex!(A::Array, i::Int)
     @inbounds _unsetindex!(GenericMemoryRef(A.ref, i))
     return A
 end
-
+function _unsetindex!(A::Array, i::Int...)
+    @inline
+    @boundscheck checkbounds(A, i...)
+    @inbounds _unsetindex!(A, _to_linear_index(A, i...))
+    return A
+end
 
 # TODO: deprecate this (aligned_sizeof and/or elsize and/or sizeof(Some{T}) are more correct)
 elsize(::Type{A}) where {T,A<:Array{T}} = aligned_sizeof(T)

base/multidimensional.jl

@@ -1610,6 +1610,12 @@ end
     end
 end
 
+# _unsetindex
+@propagate_inbounds function Base._unsetindex!(A::AbstractArray, i::CartesianIndex)
+    Base._unsetindex!(A, to_indices(A, (i,))...)
+    return A
+end
+
 ## permutedims
 
 ## Permute array dims ##

base/subarray.jl

@@ -410,6 +410,25 @@ function isassigned(V::FastSubArray{<:Any, 1}, i::Int)
     r
 end
 
+function _unsetindex!(V::FastSubArray, i::Int)
+    @inline
+    @boundscheck checkbounds(Bool, V, i)
+    @inbounds _unsetindex!(V.parent, _reindexlinear(V, i))
+    return V
+end
+function _unsetindex!(V::FastSubArray{<:Any,1}, i::Int)
+    @inline
+    @boundscheck checkbounds(Bool, V, i)
+    @inbounds _unsetindex!(V.parent, _reindexlinear(V, i))
+    return V
+end
+function _unsetindex!(V::SubArray{T,N}, i::Vararg{Int,N}) where {T,N}
+    @inline
+    @boundscheck checkbounds(Bool, V, i...)
+    @inbounds _unsetindex!(V.parent, reindex(V.indices, i)...)
+    return V
+end
+
 IndexStyle(::Type{<:FastSubArray}) = IndexLinear()
 IndexStyle(::Type{<:SubArray}) = IndexCartesian()
 

test/abstractarray.jl

@@ -2019,3 +2019,34 @@ end
         @test B == A
     end
 end
+
+@testset "_unsetindex!" begin
+    struct MyMatrixUnsetIndexCartInds{T,A<:AbstractMatrix{T}} <: AbstractMatrix{T}
+        data :: A
+    end
+    Base.size(A::MyMatrixUnsetIndexCartInds) = size(A.data)
+    Base.getindex(M::MyMatrixUnsetIndexCartInds, i::Int, j::Int) = M.data[i,j]
+    Base.setindex!(M::MyMatrixUnsetIndexCartInds, v, i::Int, j::Int) = setindex!(M.data, v, i, j)
+    struct MyMatrixUnsetIndexLinInds{T,A<:AbstractMatrix{T}} <: AbstractMatrix{T}
+        data :: A
+    end
+    Base.size(A::MyMatrixUnsetIndexLinInds) = size(A.data)
+    Base.getindex(M::MyMatrixUnsetIndexLinInds, i::Int) = M.data[i]
+    Base.setindex!(M::MyMatrixUnsetIndexLinInds, v, i::Int) = setindex!(M.data, v, i)
+    Base.IndexStyle(::Type{<:MyMatrixUnsetIndexLinInds}) = IndexLinear()
+
+    function test_unsetindex(MT)
+        M = MT(ones(2,2))
+        M2 = MT(Matrix{BigFloat}(undef, 2,2))
+        copyto!(M, M2)
+        @test all(==(1), M)
+        M3 = MT(Matrix{BigFloat}(undef, 2,2))
+        for i in eachindex(M3)
+            @test !isassigned(M3, i)
+        end
+        M3 .= 1
+        @test_throws MethodError copyto!(M3, M2)
+    end
+    test_unsetindex(MyMatrixUnsetIndexCartInds)
+    test_unsetindex(MyMatrixUnsetIndexLinInds)
+end

test/subarray.jl

@@ -1055,4 +1055,46 @@ end
         @test !isassigned(v, 1, 2) # inbounds but not assigned
         @test !isassigned(v, 3, 3) # out-of-bounds
     end
+
+    @testset "_unsetindex!" begin
+        function test_unsetindex(A, B)
+            copyto!(A, B)
+            for i in eachindex(A)
+                @test !isassigned(A, i)
+            end
+        end
+        @testset "dest IndexLinear, src IndexLinear" begin
+            for p in (fill(BigInt(2)), BigInt[1, 2], BigInt[1 2; 3 4])
+                A = view(copy(p), ntuple(_->:, ndims(p))...)
+                B = view(similar(A), ntuple(_->:, ndims(p))...)
+                test_unsetindex(A, B)
+                test_unsetindex(p, B)
+            end
+        end
+
+        @testset "dest IndexLinear, src IndexCartesian" begin
+            for p in (fill(BigInt(2)), BigInt[1, 2], BigInt[1 2; 3 4])
+                A = view(copy(p), ntuple(_->:, ndims(p))...)
+                B = view(similar(A), axes(A)...)
+                test_unsetindex(A, B)
+                test_unsetindex(p, B)
+            end
+        end
+
+        @testset "dest IndexCartesian, src IndexLinear" begin
+            for p in (fill(BigInt(2)), BigInt[1, 2], BigInt[1 2; 3 4])
+                A = view(p, axes(p)...)
+                B = similar(A)
+                test_unsetindex(A, B)
+            end
+        end
+
+        @testset "dest IndexCartesian, src IndexCartesian" begin
+            for p in (fill(BigInt(2)), BigInt[1, 2], BigInt[1 2; 3 4])
+                A = view(p, axes(p)...)
+                B = view(similar(A), axes(A)...)
+                test_unsetindex(A, B)
+            end
+        end
+    end
 end