For what it's worth there's reasonably active [1] work on implementing opt-in guaranteed tail calls - but it's not particularly fast going. LLVM (the backend rust uses) needs better support for musttail (e.g. some architectures just don't support it [2]).
By-default guaranteed tail calls really isn't rust's style, because it means subtle changes (introducing a destructor, re-ordering code, etc) can change semantics without you realizing it. If you want to guarantee that a call can't allocate a new stack frame you should have to say it.
Not so familiar with this area, but isn't the existing behavior of implicitly creating new stacks more of a problem than implicit tail-call elimination? Seems the latter is a kind of compiler-level optimization, of which there are already many (I think) that change the semantics internally but guarantee the outward behavior stays the same.
But I can understand the preference for an explicit opt-in, to make clear that it is enforced and not assumed.
I'd argue that it's explicit - that's what a function call does and you don't have implicit function calls in rust.
> Seems the latter is a kind of compiler-level optimization, of which there are already many (I think) that change the semantics internally but guarantee the outward behavior stays the same.
What you're asking for here already exists. Tail calls might be optimized into not allocating extra stack frames, the rust compiler just doesn't guarantee that it will perform that optimization (and almost certainly won't when code is compiled without optimizations... for instance).
What people want is the semantic guarantee that the stack frame won't be allocated. Not just a compiler that often performs the optimization. Otherwise you can't be sure that your code will keep working with new compiler flags/versions/architectures/... You could say "whenever the code is the right shape we'll guarantee the optimization" (C++ famously did this for things like copy elision)... but now the shape of code comes with non-obvious semantic guarantees and that's not rust's style. Hence the proposal for a keyword instead.
I see it, certain algorithms need guaranteed tail-call elimination, otherwise they are too inefficient and must be manually unrolled or rewritten to avoid blowing the stack. So a compiler optimization that is "nice to have" is not good enough.
I can see how it might require an explicit state machine (keep a mutable state number and switch over the inlined bodies of what could be functions), but I'm not seeing how it could require `goto`.
Are there more-complex relationships that might require it?
> because it means subtle changes (introducing a destructor, re-ordering code, etc) can change semantics without you realizing it.
No, it won't change semantics - if you say @musttail or similar, it will simply fail to compile if you, say, introduce a destructor - the semantics will not subtly change.
Uh, yes, if you guarantee the semantics only when the code explicitly opts in and not by default then semantics will not subtly change, that is the point of my comment
Guaranteeing an optimization that otherwise only might run is a change in semantics. The attribute doesn't allow (in any sensible language) the code to simply not compile because the optimizer doesn't feel like it today (or you compiled with -O0), it forces the compiler to not allocate a stack frame wherever the code fits the structure that makes that definitely possible and fails to compile wherever it doesn't (even if after other optimization passes it happens to fit a structure that makes it possible).
I don't think that's related? The bug alluded to looks something like
function rm(node) {
for (const child of ls(node))
rm(child);
unlink(node);
}
and no amount of tail call optimization will save you here, because this isn't tail recursion. Of course you could rewrite it using an explicit stack + tail recursion, but then you might as well be using a while loop.
So it's enumerating all the subdirectories first and unlinks the tree afterwards?
I get that this isn't transactional and inherently prone to race conditions, but if this is indeed the problem, it's rediculous. A single while loop could do the job correctly and use less RAM. It'd probably also be faster. But that wouldn't be a rusty thing to do?
I look at code from the heirloom project and, despite its warts, I think we've lost something in the past 45 years or so.
> A single while loop could do the job correctly and use less RAM. It'd
probably also be faster. But that wouldn't be a rusty thing to do?
No, the "single while loop" is just harder to implement than a naive
recursion, because recursion is a natural way to implement tree traversal.
With a while loop, you need an explicit stack, which is more complex.
(A stackless traversal seems unrealistic here, as getting the succeeding
node would be too expensive. Not that I've tried...)
> I look at code from the heirloom project and, despite its warts, I think
we've lost something in the past 45 years or so.
I've just tried and heirloom rm segfaults on the same test too. Which is
no wonder, seeing how that code also recurses.
(It's mentioned somewhere else in the thread, but this is exactly the
reason why GNU had to specify "no hard limits" as a policy. Unix used to
be full of such bugs.)
How interesting. I'd seen LISP(y) implementations like Scheme guarantee tail-call since recursion is a very common technique in that language family. But I didn't know Zig supported it.
They have an @call built-in that guarantees: always/never tail, as well as always/never inline. That's neat, I can see how that would be useful in various situations.
