diagnostic_on_move

The tracking issue for this feature is: #154181

The diagnostic_on_move feature allows use of the #[diagnostic::on_move] attribute. It should be placed on struct, enum and union declarations, though it is not an error to be located in other positions. This attribute is a hint to the compiler to supplement the error message when the annotated type is involved in a borrowcheck error.

For example, File is annotated as such:

#![allow(unused)]
#![feature(diagnostic_on_move)]

fn main() {
#[diagnostic::on_move(note = "you can use `File::try_clone` \
                             to duplicate a `File` instance")]
pub struct File {
    // ...
}
}

When you try to use a File after it’s already been moved, it will helpfully tell you about try_clone.

The message and label can also be customized:

#![allow(unused)]
#![feature(diagnostic_on_move)]

fn main() {
use std::marker::PhantomData;

#[diagnostic::on_move(
    message = "`{Self}` cannot be used multiple times",
    label = "this token may only be used once",
    note = "you can create a new `Token` with `Token::conjure()`"
)]
pub struct Token<'brand> {
    spooky: PhantomData<&'brand ()>,
}

impl Token<'_> {
    pub fn conjure<'u>() -> Token<'u> {
        Token {
            spooky: PhantomData,
        }
    }
}
}

The user may try to use it like this:

#![feature(diagnostic_on_move)]

use std::marker::PhantomData;

#[diagnostic::on_move(
    message = "`{Self}` cannot be used multiple times",
    label = "this token may only be used once",
    note = "you can create a new `Token` with `Token::conjure()`"
)]
pub struct Token<'brand> {
    spooky: PhantomData<&'brand ()>,
}

impl Token<'_> {
    pub fn conjure<'u>() -> Token<'u> {
        Token {
            spooky: PhantomData,
        }
    }
}
fn main() {
let token = Token::conjure();
let _ = (token, token);
}

This will result in the following error:

error[E0382]: `Token` cannot be used multiple times
  --> src/main.rs:24:21
   |
 1 |     let token = Token::conjure();
   |         ----- this token may only be used once
 2 |     let _ = (token, token);
   |              -----  ^^^^^ value used here after move
   |              |
   |              value moved here
   |
   = note: you can create a new `Token` with `Token::conjure()`