alt Hacker NewsI like this change. I was wondering if I would've preferred to have something on the function signature (eg `tcc_fn foo() ...` as in Tail Call Constrained fn) and when encountering that the rust compiler would make checks about whether the body of the function is tail-callable.
My fear is that adding yet another keyword it might get lost in the sea of keywords that a Rust developer needs to remember. And if recursion is not something you do often you might not reach for it when actually needed. Having this signal in the function signature means that people would be exposed to it just by reading the documentation and eventually will learn it exists and (hopefully) how to wield it.
What do you folks think?
The property we care about isn't a property of functions but of callers, so marking a function doesn't help.
`become blah(foo, bar)` is the same thing as `blah(foo, bar)` except that we, the caller are promising that we have nothing further to do and so when blah returns it can return to our caller.
If somebody else calls blah they don't want that behaviour, they might have lots more to do and if they were skipped over that's a disaster.
In some languages it's very obvious when you're going to get TCO anyway, but Rust has what C++ calls RAII, when a function ends all the local variables get destroyed and this may be non-trivial work. Presumably destroying a local i32 is trivial & so is a [u8; 32] but destroying a local String isn't, let alone a HashMap and who knows how complicated it is to destroy a File or a DBQuery or a Foo...
So in a sense "all" become does is try to bring that destruction sooner a little, so it happens before the call, leaving nothing to do afterwards. We weren't using that String any more anyway, lets just destroy it first, and the HashMap? Sure, and oh... no, actually if we destroy that Foo before calling blah which needs the Foo that messes things up... Rust's borrowck comes in clutch here to help us avoid a terrible mistake, our code was nonsense, it doesn't build.
Edited: Improve explanation