Improve error message when moving a captured value out of a closure

[gralpli]

I tried this code:

fn main() {
    let foo = String::new();
    
    bar(move || std::mem::forget(foo));
}

fn bar<F: Fn()>(a: F) {
    
}

I got this error:

error[E0507]: cannot move out of `foo`, a captured variable in an `Fn` closure
 --> src/main.rs:4:34
  |
2 |     let foo = String::new();
  |         --- captured outer variable
3 |     
4 |     bar(move || std::mem::forget(foo));
  |                                  ^^^ move occurs because `foo` has type `std::string::String`, which does not implement the `Copy` trait

The error message is because the closure is not FnOnce() – it could be called multiple times. So this is correct.

But I think the error message is confusing, because it says something about moving out of foo. What’s happening instead is that the captured foo is moved out of the closure that needs it to stay there for a second call.

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (a08c47310 2020-05-07)
binary: rustc
commit-hash: a08c47310c7d49cbdc5d7afb38408ba519967ecd
commit-date: 2020-05-07
host: x86_64-unknown-linux-gnu
release: 1.45.0-nightly
LLVM version: 9.0

[jonas-schievink]

That sounds correct – values are moved, not bindings like foo, so saying that a value is moved out of its binding foo seems right to me.

[edward-shen]

Was able to still repro this on 1.65-nightly (2022-08-11 20ffea6) with the following code (playground):

fn main() {
    let s = String::new();
    let _foo: Box<dyn Fn()> = Box::new(move || { s; });
}

For what it's worth, I think it's worth clarifying this diagnostic message. I ran into this today with the mindset of "Why doesn't this work? I have the move keyword, so it should have moved!". move felt like it wasn't actually doing it's job when the compiler then prints out something about the closed-over variable not being moved, rather than the issue being the constraint on the fn type.

I understand where I was wrong, but I would have loved to have a hint pointing to the fn constraint being the issue, not that I didn't specify move or something.