The "this function takes ownership of the receiver self" diagnostic hint not shown inside closures

[edmorley]

Given the following code:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bfca5a5df52348cba46546d72a36ccb6

fn testcase(items: &[u64]) -> Abc {
    let mut abc = Abc { bar: Vec::new() };
    items.iter().for_each(|item| abc.update(*item));
    abc
}

struct Abc {
    bar: Vec<u64>,
}

impl Abc {
    fn update(mut self, bar: u64) {
        //    ^^^^^^^^ the issue is here, this should be `&mut self`
        self.bar.push(bar);
    }
}

The current output is:

error[[E0507]](https://doc.rust-lang.org/nightly/error-index.html#E0507): cannot move out of `abc`, a captured variable in an `FnMut` closure
 [--> src/lib.rs:3:34
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut abc = Abc { bar: Vec::new() };
  |         ------- captured outer variable
3 |     items.iter().for_each(|item| abc.update(*item));
  |                           -------^^^--------------
  |                           |      |
  |                           |      move occurs because `abc` has type `Abc`, which does not implement the `Copy` trait
  |                           captured by this `FnMut` closure

warning: variable does not need to be mutable
 [--> src/lib.rs:2:9
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut abc = Abc { bar: Vec::new() };
  |         ----^^^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

error[[E0382]](https://doc.rust-lang.org/nightly/error-index.html#E0382): use of moved value: `abc`
 [--> src/lib.rs:4:5
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut abc = Abc { bar: Vec::new() };
  |         ------- move occurs because `abc` has type `Abc`, which does not implement the `Copy` trait
3 |     items.iter().for_each(|item| abc.update(*item));
  |                           ------ --- variable moved due to use in closure
  |                           |
  |                           value moved into closure here
4 |     abc
  |     ^^^ value used here after move

This diagnostic was confusing to me, since:

1. It doesn't mention the reason why update() takes ownership
2. It talks about the Copy trait, which whilst true, isn't really the correct way to fix this

Ideally the output should look like:

...
...
note: this function takes ownership of the receiver `self`, which moves `abc`
  [--> src/lib.rs:12:19
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)   |
12 |     fn update(mut self, bar: u64) {
   |                   ^^^^

ie: Have a note that explains the root issue - which is that the mut self takes ownership - and thus hopefully allow users to realise it should be &mut self in the definition of update(), rather than the problem being in testcase().

Notably, if I change the implementation to use a for loop (rather than a closure with .forEach()):

fn testcase(items: &[u64]) -> Abc {
    let mut abc = Abc { bar: Vec::new() };
    for item in items {
        abc.update(*item);
    }
    abc
}

(see https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=eb04a60d4d60c15c1911d14c64e2a692)

...then the output already includes the above suggestion:

warning: variable does not need to be mutable
 [--> src/lib.rs:2:9
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
2 |     let mut abc = Abc { bar: Vec::new() };
  |         ----^^^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

error[[E0382]](https://doc.rust-lang.org/nightly/error-index.html#E0382): use of moved value: `abc`
  [--> src/lib.rs:4:9
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)   |
2  |     let mut abc = Abc { bar: Vec::new() };
   |         ------- move occurs because `abc` has type `Abc`, which does not implement the `Copy` trait
3  |     for item in items {
4  |         abc.update(*item);
   |         ^^^ ------------- `abc` moved due to this method call, in previous iteration of loop
   |
note: this function takes ownership of the receiver `self`, which moves `abc`
  [--> src/lib.rs:14:19
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)   |
14 |     fn update(mut self, bar: u64) {
   |                   ^^^^