# Package for Rust-like borrow checker in Julia?

**URL:** <https://discourse.julialang.org/t/package-for-rust-like-borrow-checker-in-julia/124442>\
**Category:** General Usage\
**Tags:** question\
**Created:** [January 5, 2025, 11:17am UTC](https://discourse.julialang.org/t/package-for-rust-like-borrow-checker-in-julia/124442 "2025-01-05T11:17:12Z")\
**Posts on this page:** 1\
**Showing post:** 78

<div class="post-metadata">

**Author:** ![MilesCranmer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/milescranmer/32/21070_2.png) [@MilesCranmer](https://discourse.julialang.org/u/MilesCranmer)\
**Post date:** [January 7, 2025, 9:42pm UTC](https://discourse.julialang.org/t/package-for-rust-like-borrow-checker-in-julia/124442/78 "2025-01-07T21:42:01Z")

</div>

@mbauman nice recommendation – it works!! Check this out:

```julia
julia> using BorrowChecker

julia> @bind x = 1
Bound{Int64}(1)

julia> f(x::Int) = x^2
f (generic function with 1 method)

julia> result = BorrowChecker.managed() do
           # Enters Cassette.@overdub context
           f(x)
       end
1

julia> x # Successfully moved
[moved]

```

It succesfully called `@take(x)` on our bound variable `x` despite us not doing anything.

The code was surprisingly concise too:

```julia
maybe_take!(x) = x
function maybe_take!(arg::AllBound)
    is_moved(arg) && throw(MovedError(arg.symbol))
    value = unsafe_get_value(arg)
    mark_moved!(arg)
    return value
end
const SKIP_METHODS = (
    Base.getindex, Base.setindex!, Base.getproperty,
    Base.setproperty!, Base.getfield, Base.setfield!
)
function skip_method(f)
    parentmodule(parentmodule(f)) == parentmodule(@ __MODULE__ ) || f in SKIP_METHODS
end

Cassette.@context ManagedCtx

function Cassette.overdub(ctx::ManagedCtx, f, args...)
    if skip_method(f)
        Cassette.fallback(ctx, f, args...)
    else
        Cassette.recurse(ctx, f, map(maybe_take!, args)...)
    end
end
function managed(f)
    Cassette.@overdub(Cassette.disablehooks(ManagedCtx()), f())
end

```

That’s it!!

All it does is: goes through the arguments of EVERY SINGLE external function call, run `maybe_take!` on the arguments, and then call the function. This results in all `Bound` and `BoundMut` objects being taken, which voids their ownership in their original scope, thus

```julia
julia> x == 2
ERROR: Cannot use x: value has been moved

```

i.e., it is no longer owned, and can’t be used!

The only functions where `x` won’t be taken are ones we have defined explicitly. Otherwise we assume all external functions take ownership unless specified otherwise.

---

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