Expose Core.memoryrefoffset as Base.memoryindex (#59600)

This exposes Core.memoryrefoffset as API through a generic function -
currently the public, unexported `Base.memoryindex`

This closes #59574.

Author: jakobnissen

base/genericmemory.jl

@@ -65,6 +65,27 @@ IndexStyle(::Type{<:GenericMemory}) = IndexLinear()
 
 parent(ref::GenericMemoryRef) = ref.mem
 
+"""
+    memoryindex(ref::GenericMemoryRef)::Int
+
+Get the 1-based index of `ref` in its `GenericMemory`.
+
+# Examples
+```jldoctest
+julia> mem = Memory{String}(undef, 10);
+
+julia> ref = Base.memoryindex(memoryref(mem, 3))
+3
+
+julia> Base.memoryindex(memoryref(Memory{Nothing}(undef, 10), 8))
+8
+```
+
+!!! compat "Julia 1.13"
+    This function requires at least Julia 1.13.
+"""
+memoryindex(ref::GenericMemoryRef) = memoryrefoffset(ref)
+
 pointer(mem::GenericMemoryRef) = unsafe_convert(Ptr{Cvoid}, mem) # no bounds check, even for empty array
 
 _unsetindex!(A::Memory, i::Int) =  (@_propagate_inbounds_meta; _unsetindex!(memoryref(A, i)); A)

base/public.jl

@@ -34,6 +34,7 @@ public
 # arrays
     has_offset_axes,
     require_one_based_indexing,
+    memoryindex,
 
 # collections
     IteratorEltype,

doc/src/base/arrays.md

@@ -34,6 +34,7 @@ Base.GenericMemory
 Base.Memory
 Base.Memory(::UndefInitializer, ::Int)
 Base.memoryref
+Base.memoryindex
 Base.Slices
 Base.RowSlices
 Base.ColumnSlices

test/arrayops.jl

@@ -3403,4 +3403,11 @@ end
     mem = Memory{Float32}(undef, 3)
     ref = memoryref(mem, 2)
     @test parent(ref) === mem
+    @test Base.memoryindex(ref) === 2
+
+    # Test for zero-sized structs
+    mem = Memory{Nothing}(undef, 10)
+    ref = memoryref(mem, 8)
+    @test parent(ref) === mem
+    @test Base.memoryindex(ref) === 8
 end