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Update must_use to use the attribute template #1892

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940f314
Unwrap must_use
ehuss Jun 16, 2025
05fea2f
Move examples into example blocks
ehuss Jun 16, 2025
89a82f7
Update diagnostic to use the attribute template
ehuss Jun 16, 2025
7ac731a
Minor update of must_use
ehuss Sep 22, 2025
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150 changes: 85 additions & 65 deletions src/attributes/diagnostics.md
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Expand Up @@ -345,90 +345,110 @@ The [RFC][1270-deprecation.md] contains motivations and more details.

[1270-deprecation.md]: https://github.com/rust-lang/rfcs/blob/master/text/1270-deprecation.md

<!-- template:attributes -->
r[attributes.diagnostics.must_use]
## The `must_use` attribute

r[attributes.diagnostics.must_use.intro]
The *`must_use` attribute* is used to issue a diagnostic warning when a value
is not "used".
The *`must_use` [attribute][attributes]* is used to issue a diagnostic warning when a value is not used.

r[attributes.diagnostics.must_use.allowed-positions]
The `must_use` attribute can be applied to user-defined composite types
([`struct`s][struct], [`enum`s][enum], and [`union`s][union]), [functions],
and [traits].
r[attributes.diagnostics.must_use.syntax]
The `must_use` attribute uses either the [MetaWord] syntax or the [MetaNameValueStr] syntax to be able to [specify a message][attributes.diagnostics.must_use.message].

r[attributes.diagnostics.must_use.message]
The `must_use` attribute may include a message by using the
[MetaNameValueStr] syntax such as `#[must_use = "example message"]`. The
message will be given alongside the warning.
> [!EXAMPLE]
> ```rust
> #[must_use]
> fn use_me() -> u8 { 0 }
>
> #[must_use = "explanation of why it must be used"]
> fn use_me2() -> u8 { 0 }
> ```

r[attributes.diagnostics.must_use.type]
When used on user-defined composite types, if the [expression] of an
[expression statement] has that type, then the `unused_must_use` lint is
violated.
r[attributes.diagnostics.must_use.allowed-positions]
The `must_use` attribute may be applied to a:

```rust
#[must_use]
struct MustUse {
// some fields
}
- [Struct]
- [Enumeration]
- [Union]
- [Function]
- [Trait]

# impl MustUse {
# fn new() -> MustUse { MustUse {} }
# }
#
// Violates the `unused_must_use` lint.
MustUse::new();
```
> [!NOTE]
> `rustc` ignores use in other positions but lints against it. This may become an error in the future.

r[attributes.diagnostics.must_use.fn]
When used on a function, if the [expression] of an [expression statement] is a
[call expression] to that function, then the `unused_must_use` lint is
violated.
r[attributes.diagnostics.must_use.duplicates]
The `must_use` attribute may be used only once on an item.

```rust
#[must_use]
fn five() -> i32 { 5i32 }
> [!NOTE]
> `rustc` lints against any use following the first. This may become an error in the future.

// Violates the unused_must_use lint.
five();
```
r[attributes.diagnostics.must_use.message]
The `must_use` attribute may include a message by using the [MetaNameValueStr] syntax such as `#[must_use = "example message"]`. The message will be given alongside the warning.

r[attributes.diagnostics.must_use.trait]
When used on a [trait declaration], a [call expression] of an [expression
statement] to a function that returns an [impl trait] or a [dyn trait] of that trait violates
the `unused_must_use` lint.
r[attributes.diagnostics.must_use.type]
When used on user-defined composite types, if the [expression] of an [expression statement] has that type, then the `unused_must_use` lint is violated.

```rust
#[must_use]
trait Critical {}
impl Critical for i32 {}
> [!EXAMPLE]
> ```rust
> #[must_use]
> struct MustUse {
> // some fields
> }
>
> # impl MustUse {
> # fn new() -> MustUse { MustUse {} }
> # }
> #
> // Violates the `unused_must_use` lint.
> MustUse::new();
> ```

fn get_critical() -> impl Critical {
4i32
}
r[attributes.diagnostics.must_use.fn]
When used on a function, if the [expression] of an [expression statement] is a [call expression] to that function, then the `unused_must_use` lint is violated.

// Violates the `unused_must_use` lint.
get_critical();
```
> [!EXAMPLE]
> ```rust
> #[must_use]
> fn five() -> i32 { 5i32 }
>
> // Violates the unused_must_use lint.
> five();
> ```

r[attributes.diagnostics.must_use.trait-function]
When used on a function in a trait declaration, then the behavior also applies
when the call expression is a function from an implementation of the trait.
r[attributes.diagnostics.must_use.trait]
When used on a [trait declaration], a [call expression] of an [expression statement] to a function that returns an [impl trait] or a [dyn trait] of that trait violates the `unused_must_use` lint.

```rust
trait Trait {
#[must_use]
fn use_me(&self) -> i32;
}
> [!EXAMPLE]
> ```rust
> #[must_use]
> trait Critical {}
> impl Critical for i32 {}
>
> fn get_critical() -> impl Critical {
> 4i32
> }
>
> // Violates the `unused_must_use` lint.
> get_critical();
> ```

impl Trait for i32 {
fn use_me(&self) -> i32 { 0i32 }
}
r[attributes.diagnostics.must_use.trait-function]
When used on a function in a trait declaration, then the behavior also applies when the call expression is a function from an implementation of the trait.

// Violates the `unused_must_use` lint.
5i32.use_me();
```
> [!EXAMPLE]
> ```rust
> trait Trait {
> #[must_use]
> fn use_me(&self) -> i32;
> }
>
> impl Trait for i32 {
> fn use_me(&self) -> i32 { 0i32 }
> }
>
> // Violates the `unused_must_use` lint.
> 5i32.use_me();
> ```

r[attributes.diagnostics.must_use.trait-impl-function]
When used on a function in a trait implementation, the attribute does nothing.
Expand Down Expand Up @@ -669,7 +689,7 @@ The first error message includes a somewhat confusing error message about the re
[call expression]: ../expressions/call-expr.md
[dyn trait]: ../types/trait-object.md
[enum variant]: ../items/enumerations.md
[enum]: ../items/enumerations.md
[enumeration]: ../items/enumerations.md
[expression statement]: ../statements.md#expression-statements
[expression]: ../expressions.md
[external block item]: ../items/external-blocks.md
Expand Down