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Update `rustc_on_unimplemented` docs

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mejrs 2025-05-24 19:16:01 +02:00
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@ -866,19 +866,17 @@ struct](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/json/struct
(and sub-structs) for the JSON serialization. Don't confuse this with (and sub-structs) for the JSON serialization. Don't confuse this with
[`errors::Diag`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.Diag.html)! [`errors::Diag`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.Diag.html)!
## `#[rustc_on_unimplemented(...)]` ## `#[rustc_on_unimplemented]`
The `#[rustc_on_unimplemented]` attribute allows trait definitions to add specialized This attribute allows trait definitions to modify error messages when an implementation was
notes to error messages when an implementation was expected but not found. expected but not found. The string literals in the attribute are format strings and can be
You can refer to the trait's generic arguments by name and to the resolved type using `Self`. formatted with named parameters. See the Formatting
section below for what parameters are permitted.
For example:
```rust,ignore ```rust,ignore
#![feature(rustc_attrs)] #[rustc_on_unimplemented(message = "an iterator over \
elements of type `{A}` cannot be built from a \
#[rustc_on_unimplemented="an iterator over elements of type `{A}` \ collection of type `{Self}`")]
cannot be built from a collection of type `{Self}`"]
trait MyIterator<A> { trait MyIterator<A> {
fn next(&mut self) -> A; fn next(&mut self) -> A;
} }
@ -895,32 +893,26 @@ fn main() {
When the user compiles this, they will see the following; When the user compiles this, they will see the following;
```txt ```txt
error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied error[E0277]: an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]`
--> <anon>:14:5 --> src/main.rs:13:19
| |
14 | iterate_chars(&[1, 2, 3][..]); 13 | iterate_chars(&[1, 2, 3][..]);
| ^^^^^^^^^^^^^ an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]` | ------------- ^^^^^^^^^^^^^^ the trait `MyIterator<char>` is not implemented for `&[{integer}]`
| |
| required by a bound introduced by this call
| |
= help: the trait `MyIterator<char>` is not implemented for `&[{integer}]` note: required by a bound in `iterate_chars`
= note: required by `iterate_chars`
``` ```
`rustc_on_unimplemented` also supports advanced filtering for better targeting You can modify the contents of:
of messages, as well as modifying specific parts of the error message. You
target the text of:
- the main error message (`message`) - the main error message (`message`)
- the label (`label`) - the label (`label`)
- an extra note (`note`) - the note(s) (`note`)
For example, the following attribute For example, the following attribute
```rust,ignore ```rust,ignore
#[rustc_on_unimplemented( #[rustc_on_unimplemented(message = "message", label = "label", note = "note")]
message="message",
label="label",
note="note"
)]
trait MyIterator<A> { trait MyIterator<A> {
fn next(&mut self) -> A; fn next(&mut self) -> A;
} }
@ -930,42 +922,58 @@ Would generate the following output:
```text ```text
error[E0277]: message error[E0277]: message
--> <anon>:14:5 --> <file>:10:19
| |
14 | iterate_chars(&[1, 2, 3][..]); 10 | iterate_chars(&[1, 2, 3][..]);
| ^^^^^^^^^^^^^ label | ------------- ^^^^^^^^^^^^^^ label
| |
| required by a bound introduced by this call
| |
= note: note
= help: the trait `MyIterator<char>` is not implemented for `&[{integer}]` = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
= note: required by `iterate_chars` = note: note
note: required by a bound in `iterate_chars`
``` ```
To allow more targeted error messages, it is possible to filter the The functionality discussed so far is also available with
application of these fields based on a variety of attributes when using [`#[diagnostic::on_unimplemented]`](https://doc.rust-lang.org/nightly/reference/attributes/diagnostics.html#the-diagnosticon_unimplemented-attribute).
`on`: If you can, you should use that instead.
### Filtering
To allow more targeted error messages, it is possible to filter the
application of these fields with `on`.
You can filter on the following boolean flags:
- `crate_local`: whether the code causing the trait bound to not be - `crate_local`: whether the code causing the trait bound to not be
fulfilled is part of the user's crate. This is used to avoid suggesting fulfilled is part of the user's crate. This is used to avoid suggesting
code changes that would require modifying a dependency. code changes that would require modifying a dependency.
- Any of the generic arguments that can be substituted in the text can be - `direct`: whether this is an user-specified rather than derived obligation.
referred by name as well for filtering, like `Rhs="i32"`, except for - `from_desugaring`: whether we are in some kind of desugaring, like `?`
`Self`. or a `try` block for example. This flag can also be matched on, see below.
- `_Self`: to filter only on a particular calculated trait resolution, like
`Self="std::iter::Iterator<char>"`. This is needed because `Self` is a
keyword which cannot appear in attributes.
- `direct`: user-specified rather than derived obligation.
- `from_desugaring`: usable both as boolean (whether the flag is present)
or matching against a particular desugaring. The desugaring is identified
with its variant name in the `DesugaringKind` enum.
For example, the `Iterator` trait can be annotated in the following way: You can match on the following names and values, using `name = "value"`:
- `cause`: Match against one variant of the `ObligationCauseCode`
enum. Only `"MainFunctionType"` is supported.
- `from_desugaring`: Match against a particular variant of the `DesugaringKind`
enum. The desugaring is identified by its variant name, for example
`"QuestionMark"` for `?` desugaring or `"TryBlock"` for `try` blocks.
- `Self` and any generic arguments of the trait, like `Self = "alloc::string::String"`
or `Rhs="i32"`.
The compiler can provide several values to match on, for example:
- the self_ty, pretty printed with and without type arguments resolved.
- `"{integral}"`, if self_ty is an integral of which the type is known.
- `"[]"`, `"[{ty}]"`, `"[{ty}; _]"`, `"[{ty}; $N]"` when applicable.
- references to said slices and arrays.
- `"fn"`, `"unsafe fn"` or `"#[target_feature] fn"` when self is a function.
- `"{integer}"` and `"{float}"` if the type is a number but we haven't inferred it yet.
- combinations of the above, like `"[{integral}; _]"`.
For example, the `Iterator` trait can be filtered in the following way:
```rust,ignore ```rust,ignore
#[rustc_on_unimplemented( #[rustc_on_unimplemented(
on( on(Self = "&str", note = "call `.chars()` or `.as_bytes()` on `{Self}`"),
_Self="&str",
note="call `.chars()` or `.as_bytes()` on `{Self}`"
),
message = "`{Self}` is not an iterator", message = "`{Self}` is not an iterator",
label = "`{Self}` is not an iterator", label = "`{Self}` is not an iterator",
note = "maybe try calling `.iter()` or a similar method" note = "maybe try calling `.iter()` or a similar method"
@ -997,15 +1005,47 @@ error[E0277]: `&str` is not an iterator
= note: required by `std::iter::IntoIterator::into_iter` = note: required by `std::iter::IntoIterator::into_iter`
``` ```
If you need to filter on multiple attributes, you can use `all`, `any` or The `on` filter accepts `all`, `any` and `not` predicates similar to the `cfg` attribute:
`not` in the following way:
```rust,ignore ```rust,ignore
#[rustc_on_unimplemented( #[rustc_on_unimplemented(on(
on( all(Self = "&str", T = "alloc::string::String"),
all(_Self="&str", T="std::string::String"),
note = "you can coerce a `{T}` into a `{Self}` by writing `&*variable`" note = "you can coerce a `{T}` into a `{Self}` by writing `&*variable`"
) ))]
)] pub trait From<T>: Sized {
pub trait From<T>: Sized { /* ... */ } /* ... */
}
```
### Formatting
The string literals are format strings that accept parameters wrapped in braces
but positional and listed parameters and format specifiers are not accepted.
The following parameter names are valid:
- `Self` and all generic parameters of the trait.
- `This`: the name of the trait the attribute is on, without generics.
- `Trait`: the name of the "sugared" trait. See `TraitRefPrintSugared`.
- `ItemContext`: the kind of `hir::Node` we're in, things like `"an async block"`,
`"a function"`, `"an async function"`, etc.
Something like:
```rust,ignore
#![feature(rustc_attrs)]
#[rustc_on_unimplemented(message = "Self = `{Self}`, \
T = `{T}`, this = `{This}`, trait = `{Trait}`, \
context = `{ItemContext}`")]
pub trait From<T>: Sized {
fn from(x: T) -> Self;
}
fn main() {
let x: i8 = From::from(42_i32);
}
```
Will format the message into
```text
"Self = `i8`, T = `i32`, this = `From`, trait = `From<i32>`, context = `a function`"
``` ```