Switch supertype of CodeUnits from DenseVector to AbstractVector

base/strings/basic.jl

@@ -786,7 +786,7 @@ iterate(r::Iterators.Reverse{<:EachStringIndex}, i=lastindex(r.itr.s)) = i < fir
 Wrap a string (without copying) in an immutable vector-like object that accesses the code units
 of the string's representation.
 """
-struct CodeUnits{T,S<:AbstractString} <: DenseVector{T}
+struct CodeUnits{T,S<:AbstractString} <: AbstractVector{T}
     s::S
     CodeUnits(s::S) where {S<:AbstractString} = new{codeunit(s),S}(s)
 end

base/strings/substring.jl

@@ -72,6 +72,13 @@ convert(::Type{Union{String, SubString{String}}}, s::String) = s
 convert(::Type{Union{String, SubString{String}}}, s::SubString{String}) = s
 convert(::Type{Union{String, SubString{String}}}, s::AbstractString) = convert(String, s)::String
 
+# This allows CodeUnits of known, memory-backed String types to act like a dense array
+# See issue #53996
+function _memory_offset(x::CodeUnits{<:Any, <:Union{String, SubString{String}}}, I::Vararg{Any,N}) where {N}
+    (_to_linear_index(x, I...) - first(LinearIndices(x)))*elsize(x)
+end
+strides(x::CodeUnits{<:Any, <:Union{String, SubString{String}}}) = (1,)
+
 function String(s::SubString{String})
     parent = s.string
     copy = GC.@preserve parent unsafe_string(pointer(parent, s.offset+1), s.ncodeunits)