struct Vec{T} <: AbstractVector{T}
    mem::Memory{T}
end
Base.IndexStyle(::Type{<:Vec}) = IndexLinear()
Base.@propagate_inbounds Base.getindex(A::Vec, i::Int) = A.mem[i]
Base.size(a::Vec) = size(a.mem)

function f(v)
    z = eltype(v)(false)
    for x ∈ v
        z += x
    end
    z
end

using BenchmarkTools
using Random: rand!

const vec = let m = Memory{Int8}(undef, 4096)
    rand!(m)
    Vec(m)
end

@btime f(vec) setup=(rand!(vec.mem);)
# 91.502 ns (0 allocations: 0 bytes)

# redefine the methods: placebo
function Base.iterate(A::AbstractArray, state=(eachindex(A),))
    y = iterate(state...)
    y === nothing && return nothing
    A[y[1]], (state[1], Base.tail(y)...)
end

@btime f(vec) setup=(rand!(vec.mem);)
# 91.764 ns (0 allocations: 0 bytes)

# redefine the methods: add `@inbounds`
function Base.iterate(A::AbstractArray, state=(eachindex(A),))
    y = iterate(state...)
    y === nothing && return nothing
    @inbounds A[y[1]], (state[1], Base.tail(y)...)
end

@btime f(vec) setup=(rand!(vec.mem);)
# 31.134 ns (0 allocations: 0 bytes)