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add diagnostics for E0387 #27605

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55 changes: 54 additions & 1 deletion src/librustc_borrowck/diagnostics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -95,6 +95,60 @@ fn main(){
x = 5;
}
```
"##,

E0387: r##"
This error occurs when an attempt is made to mutate or mutably reference data
that a closure has captured immutably. Examples of this error are shown below:

```
// Accepts a function or a closure that captures its environment immutably.
// Closures passed to foo will not be able to mutate their closed-over state.
fn foo<F: Fn()>(f: F) { }

// Attempts to mutate closed-over data. Error message reads:
// `cannot assign to data in a captured outer variable...`
fn mutable() {
let mut x = 0u32;
foo(|| x = 2);
}

// Attempts to take a mutable reference to closed-over data. Error message
// reads: `cannot borrow data mutably in a captured outer variable...`
fn mut_addr() {
let mut x = 0u32;
foo(|| { let y = &mut x; });
}
```

The problem here is that foo is defined as accepting a parameter of type `Fn`.
Closures passed into foo will thus be inferred to be of type `Fn`, meaning that
they capture their context immutably.

If the definition of `foo` is under your control, the simplest solution is to
capture the data mutably. This can be done by defining `foo` to take FnMut
rather than Fn:

```
fn foo<F: FnMut()>(f: F) { }
```

Alternatively, we can consider using the `Cell` and `RefCell` types to achieve
interior mutability through a shared reference. Our example's `mutable` function
could be redefined as below:

```
use std::cell::Cell;

fn mutable() {
let x = Cell::new(0u32);
foo(|| x.set(2));
}
```

You can read more about cell types in the API documentation:

https://doc.rust-lang.org/std/cell/
"##

}
Expand All @@ -104,7 +158,6 @@ register_diagnostics! {
E0383, // partial reinitialization of uninitialized structure
E0385, // {} in an aliasable location
E0386, // {} in an immutable container
E0387, // {} in a captured outer variable in an `Fn` closure
E0388, // {} in a static location
E0389 // {} in a `&` reference
}