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Comment by enjoy-your-stay | Hacker News Reader

After reading this bit:

  // Vec is our growable heap-allocated array type.
  // This is just a handy way to make one with some fixed data.
  let mut v = vec![1i32, 2, 3, 4, 5];
  let x = v.get(2);
  v.push(6); // oops, sorry x!

I really hope that the Rust compiler has some really good error messages explaining what the problem is because I couldn't see what was wrong with this fragment until I read the explanation.

Saying that, after reading the explanation it became a bit more obvious, and I really like the way of describing the relationship to the pointer as a 'view' and 'loan', which as a c++ programmer (though clearly spending too much time with GC languages) I can understand.

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> I really hope that the Rust compiler has some really good error messages explaining what the problem is because I couldn't see what was wrong with this fragment until I read the explanation.

They're not always perfect, but in this case it's quite good.

    err.rs:4:9: 4:10 error: cannot borrow `v` as mutable because it is also borrowed as immutable
    err.rs:4         v.push(6); // oops, sorry x!
                     ^
    err.rs:3:17: 3:18 note: previous borrow of `v` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `v` until the borrow ends
    err.rs:3         let x = v.get(2);
                             ^
    err.rs:5:6: 5:6 note: previous borrow ends here
    err.rs:1     fn main() {
    err.rs:2         let mut v = vec![1i32, 2, 3, 4, 5];
    err.rs:3         let x = v.get(2);
    err.rs:4         v.push(6); // oops, sorry x!
    err.rs:5     }
                 ^

If you want to try out code examples, the Rust playpen is nice: http://play.rust-lang.org

I'm quite surprised by Rust behaviour here: if x isn't used after 'v.push(6);' (doesn't seem too difficult to know by the compiler) then I would have expected the compiler to allow this code and to reject it only if x is used after the 'v.push(6)'.. Sure as he explained you can do "let mut v = vec![1i32, 2, 3, 4, 5]; { let x = v.get(2); <work with x> } v.push(6);" but this must be quite annoying..

Currently, borrows last for block scope, which is why this doesn't work. There is a feature in the making called SEME (single entry, multiple exit) which will resolve this and other annoying borrow hazards. It at least at one point was considered a priority for 1.0.

I think this might be more difficult than it seems, given that Rust allows you to store references in structs. I'll ask the developers sometime.

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