|
| 1 | +# `diagnostic_on_move` |
| 2 | + |
| 3 | +The tracking issue for this feature is: [#154181] |
| 4 | + |
| 5 | +------------------------ |
| 6 | + |
| 7 | +The `diagnostic_on_move` feature allows use of the `#[diagnostic::on_move]` attribute. It should be |
| 8 | +placed on struct, enum and union declarations, though it is not an error to be located in other |
| 9 | +positions. This attribute is a hint to the compiler to supplement the error message when the |
| 10 | +annotated type is involved in a borrowcheck error. |
| 11 | + |
| 12 | +For example, [`File`] is annotated as such: |
| 13 | +```rust |
| 14 | +#![feature(diagnostic_on_move)] |
| 15 | + |
| 16 | +#[diagnostic::on_move(note = "you can use `File::try_clone` \ |
| 17 | + to duplicate a `File` instance")] |
| 18 | +pub struct File { |
| 19 | + // ... |
| 20 | +} |
| 21 | +``` |
| 22 | + |
| 23 | +When you try to use a `File` after it's already been moved, it will helpfully tell you about `try_clone`. |
| 24 | + |
| 25 | +The message and label can also be customized: |
| 26 | + |
| 27 | +```rust |
| 28 | +#![feature(diagnostic_on_move)] |
| 29 | + |
| 30 | +use std::marker::PhantomData; |
| 31 | + |
| 32 | +#[diagnostic::on_move( |
| 33 | + message = "`{Self}` cannot be used multiple times", |
| 34 | + label = "this token may only be used once", |
| 35 | + note = "you can create a new `Token` with `Token::conjure()`" |
| 36 | +)] |
| 37 | +pub struct Token<'brand> { |
| 38 | + spooky: PhantomData<&'brand ()>, |
| 39 | +} |
| 40 | + |
| 41 | +impl Token<'_> { |
| 42 | + pub fn conjure<'u>() -> Token<'u> { |
| 43 | + Token { |
| 44 | + spooky: PhantomData, |
| 45 | + } |
| 46 | + } |
| 47 | +} |
| 48 | +``` |
| 49 | +The user may try to use it like this: |
| 50 | +```rust,compile_fail,E0382 |
| 51 | +# #![feature(diagnostic_on_move)] |
| 52 | +# |
| 53 | +# use std::marker::PhantomData; |
| 54 | +# |
| 55 | +# #[diagnostic::on_move( |
| 56 | +# message = "`{Self}` cannot be used multiple times", |
| 57 | +# label = "this token may only be used once", |
| 58 | +# note = "you can create a new `Token` with `Token::conjure()`" |
| 59 | +# )] |
| 60 | +# pub struct Token<'brand> { |
| 61 | +# spooky: PhantomData<&'brand ()>, |
| 62 | +# } |
| 63 | +# |
| 64 | +# impl Token<'_> { |
| 65 | +# pub fn conjure<'u>() -> Token<'u> { |
| 66 | +# Token { |
| 67 | +# spooky: PhantomData, |
| 68 | +# } |
| 69 | +# } |
| 70 | +# } |
| 71 | +# fn main() { |
| 72 | +let token = Token::conjure(); |
| 73 | +let _ = (token, token); |
| 74 | +# } |
| 75 | +``` |
| 76 | +This will result in the following error: |
| 77 | +```text |
| 78 | +error[E0382]: `Token` cannot be used multiple times |
| 79 | + --> src/main.rs:24:21 |
| 80 | + | |
| 81 | + 1 | let token = Token::conjure(); |
| 82 | + | ----- this token may only be used once |
| 83 | + 2 | let _ = (token, token); |
| 84 | + | ----- ^^^^^ value used here after move |
| 85 | + | | |
| 86 | + | value moved here |
| 87 | + | |
| 88 | + = note: you can create a new `Token` with `Token::conjure()` |
| 89 | +``` |
| 90 | + |
| 91 | +[`File`]: https://doc.rust-lang.org/nightly/std/fs/struct.File.html "File in std::fs" |
| 92 | +[#154181]: https://github.com/rust-lang/rust/issues/154181 "Tracking Issue for #[diagnostic::on_move]" |
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