Unclear clear error message for borrowed contexts

[ghost]

The error message for the following snippet could be improved, from IRC I was told to add .iter() onto line 7 (which worked). I believe a hint in the error message would be helpful.

struct MyStruct {
    member : Vec<u8>,
}

impl MyStruct {
    fn example (&self) {
        for foo in self.member { // line 7
            println!("{}", foo);
        }
    }
}

fn main() {
    let a = MyStruct { member : vec!(1, 2, 3) };
    a.example();
}

Error seen:

src/main.rs:7:20: 7:24 error: cannot move out of borrowed content
src/main.rs:7         for foo in self.member {
                                 ^~~~
note: in expansion of for loop expansion
src/main.rs:7:9: 9:10 note: expansion site
error: aborting due to previous error

rustc --version --verbose:

rustc 1.1.0-nightly (e5394240a 2015-05-14) (built 2015-05-13)
binary: rustc
commit-hash: e5394240a295650b567aa406b4a0e1e3a6749a5f
commit-date: 2015-05-14
build-date: 2015-05-13
host: x86_64-apple-darwin
release: 1.1.0-nightly

[Stebalien]

This could definitely be improved. Just as an FYI, for loops take IntoIterators by-value so the following also works:

for foo in &self.member {
    // ...
}

This is because IntoIterator is implemented on both &Vec<T> (producing an iterator that yields references to the items) and Vec<T> (producing an iterator that yields the items by value and consumes the vector.

We should probably add a detailed error message for this case; you're not the first to be confused.

[luke-gru]

I just stumbled into this, +1 for more helpful error message

[Eoin-ONeill-Yokai]

I also stumbled upon this myself. Adding iter() also worked. +1 for better error message handling...

[jleedev]

I just ran into this. Here's a slightly different example:

#[derive(Debug)]
struct X;  // not Copy

struct Y{field: Option<X>}

fn main() {
    let v: Vec<Y> = Vec::new();
    for y in v.iter() {
        println!("{:?}", y.field.unwrap_or(X));
    }
}

Error:

rustc 1.13.0 (2c6933acc 2016-11-07)
error[E0507]: cannot move out of borrowed content
 --> <anon>:9:26
  |
9 |         println!("{:?}", y.field.unwrap_or(X));
  |                          ^ cannot move out of borrowed content

Ultimately, the diagnostic here should explain why something is being moved, and what was borrowed in the first place. Namely, it's because the call to unwrap_or takes self.

The lifecycle of my example is worth nothing as well: The loop originally iterated over just v, which implicitly calls into_iter(), but I manually inserted iter() to make enumerate() work. No clue if it's generally useful, but a hint "did you mean into_iter" would have helped me here.

[jleedev]

This particular error pops up on me occasionally, so I have some more insight/complaints about my example. Let it: &Option<X>:

These three all give "cannot move out of borrowed content":

it.unwrap();
*it;
Option::unwrap(*it);

But this gives "expected type std::option::Option<_>, found type &std::option::Option<X>", which I think is more in line with my intent.

Option::unwrap(it);

When I type it.unwrap(), I am told that my error is an attempt to dereference it. But I think my error is in calling a method that takes ownership of a value which I have only borrowed. The compiler is inserting an implicit dereference and then chastising me about it.

[steveklabnik]

Triage: no changes, I do see people complain about this diagnostic fairly regularly...

[estebank]

Current output:

error[E0507]: cannot move out of `self.member` which is behind a shared reference
 --> src/main.rs:7:20
  |
7 |         for foo in self.member { // line 7
  |                    ^^^^^^^^^^^ move occurs because `self.member` has type `std::vec::Vec<u8>`, which does not implement the `Copy` trait

[estebank]

Current output:

error[E0507]: cannot move out of `self.member` which is behind a shared reference
 --> src/main.rs:7:20
  |
7 |         for foo in self.member { // line 7
  |                    ^^^^^^^^^^^
  |                    |
  |                    move occurs because `self.member` has type `std::vec::Vec<u8>`, which does not implement the `Copy` trait
  |                    help: consider iterating over a slice of the `Vec<_>`'s content: `&self.member`

error[E0507]: cannot move out of `y.field` which is behind a shared reference
 --> src/main.rs:9:26
  |
9 |         println!("{:?}", y.field.unwrap_or(X));
  |                          ^^^^^^^
  |                          |
  |                          move occurs because `y.field` has type `std::option::Option<X>`, which does not implement the `Copy` trait
  |                          help: consider borrowing the `Option`'s content: `y.field.as_ref()`