Issues DIP1000 can’t (yet) catch

D’s DIP1000 is its attempt to avoid memory safety issues in the language. It introduces the notion of scoped pointers and in principle (modulo bugs) prevents one from writing memory unsafe code.

I’ve been using it wherever I can since it was implemented, and it’s what makes it possible for me to have written a library allowing for fearless concurrency. I’ve also written a D version of C++’s std::vector that leverages it, which I thought was safe but turns out to have had a gaping hole.

I did wonder if Rust’s more complicated system would have advantages over DIP1000, and now it seems that the answer to that is yes. As the the linked github issue above shows,  there are ways of creating dangling pointers in the same stack frame:

void main() @safe {
    import automem.vector;

    auto vec1 = vector(1, 2, 3);
    int[] slice1 = vec1[];
    vec1.reserve(4096);  // invalidates slice1 here

Fortunately this is caught by ASAN if slice1 is used later. I’ve worked around the issue for now by returning a custom range object from the vector that has a pointer back to the container – that way it’s impossible to “invalidate iterators” in C++-speak. It probably costs more in performance though due to chasing an extra pointer. I haven’t measured to confirm the practical implications.

In Rust, the problem just can’t occur. This fails to compile (as it should):

use vec;

fn main() {
    let mut v = vec![1, 2, 3, 4, 5];
    let s = v.as_slice();
    println!("org v: {:?}\norg s: {:?}", v, s);
    v.resize(15, 42);
    println!("res: v: {:?}\norg s: {:?}", v, s);

I wonder if D can learn/borrow/steal an idea or two about only having one mutable borrow at a time. Food for thought.

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