This is sound- you aren't actually making more than one mutable reference here.
Stepping through the operations in your example: 1. i is a mutable pointer to an immutable memory location 2. Pass a pointer to i into `bar`, which can mutate `i`. 3. Deref that pointer so you have a copy of `i`, which still points to an immutable memory location. If you try to assign through `i` in `bar`, you'll quickly see that the compiler won't let you. Here's an example in the playpen [1] which fails to compile with "cannot assign to immutable dereference of `&`-pointer". If you were doing this in C, the difference between `let bar = &mut foo` and `let mut bar: &T = &mut foo` is equivalent to the difference between `const *` and `const *const`. The former can be changed to point somewhere else, but neither of them allows writing into the pointed-to memory. The real trick here is that when we specify the type of `bar` as `&T` we opt out of mutability- if you remove the type from that assignment, it's inferred to be `&mut T`. [1] http://is.gd/FqaGiL On Wed, Nov 26, 2014 at 10:26 AM, grayfox <gray...@outerhaven.de> wrote: > Hey guys, > > I'm really new to Rust (actually I've looked on Rust the last 5 hours the > first time) but I think I produced something that shouldn't be possible. From > the pointer guide I know that the following code will not compile because in > the end I would have two mutable pointers to the same address: > > let x = 5i; > let y = &x; > let z = &x; > > But in the following snippet I end up with two mutable pointer tmp and *i > which point both to the same address: > > fn bar<'a>(i: &mut &'a int) { > let mut tmp = *i; > println!("{} {}", *tmp, **i); > } > > fn foo<'a>() { > let mut i: &int = &mut 5; > bar(&mut i); > } > > fn main() { > foo(); > } > > Maybe I don't understand the concept of the Rust memory concept enough but if > I understand everything correct so far this shouldn't compile but it does > actually. > > Kind regards, > > grayfox > _______________________________________________ > Rust-dev mailing list > Rust-dev@mozilla.org > https://mail.mozilla.org/listinfo/rust-dev -- Peter Marheine Don't Panic _______________________________________________ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
