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Update panic_handler to use the attribute template #1918

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Update panic_handler to use the attribute template #1918

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7aeb71d
Move the panic_handler example to the intro and an example block
ehuss Jun 23, 2025
731d62a
Update panic_handler to use the attribute template
ehuss Jun 23, 2025
d8e8071
Minor update of `panic_hanlder`
ehuss Sep 22, 2025
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82 changes: 46 additions & 36 deletions src/panic.md
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Expand Up @@ -16,54 +16,64 @@ There are also language features that provide a level of control over panic beha
> [!NOTE]
> The standard library provides the capability to explicitly panic via the [`panic!` macro][panic!].

<!-- template:attributes -->
r[panic.panic_handler]
## The `panic_handler` attribute

r[panic.panic_handler.intro]
The *`panic_handler` attribute* can be applied to a function to define the behavior of panics.
The *`panic_handler` [attribute][attributes]* can be applied to a function to define the behavior of panics.

r[panic.panic_handler.allowed-positions]
The `panic_handler` attribute can only be applied to a function with signature `fn(&PanicInfo) -> !`.
> [!EXAMPLE]
> Below is shown a `panic_handler` function that logs the panic message and then halts the thread.
> <!-- ignore: test infrastructure can't handle no_std -->
> ```rust,ignore
> #![no_std]
>
> use core::fmt::{self, Write};
> use core::panic::PanicInfo;
>
> struct Sink {
> // ..
> # _0: (),
> }
> #
> # impl Sink {
> # fn new() -> Sink { Sink { _0: () }}
> # }
> #
> # impl fmt::Write for Sink {
> # fn write_str(&mut self, _: &str) -> fmt::Result { Ok(()) }
> # }
>
> #[panic_handler]
> fn panic(info: &PanicInfo) -> ! {
> let mut sink = Sink::new();
>
> // logs "panicked at '$reason', src/main.rs:27:4" to some `sink`
> let _ = writeln!(sink, "{}", info);
>
> loop {}
> }
> ```

> [!NOTE]
> The [`PanicInfo`] struct contains information about the location of the panic.

r[panic.panic_handler.syntax]
The `panic_handler` attribute uses the [MetaWord] syntax.

r[panic.panic_handler.allowed-positions]
The `panic_handler` attribute may only be applied to a function with signature `fn(&PanicInfo) -> !`.

r[panic.panic_handler.duplicates]
The `panic_handler` attribute may be used any number of times on a function.

> [!NOTE]
> `rustc` lints against any use following the first.

r[panic.panic_handler.unique]
There must be a single `panic_handler` function in the dependency graph.

Below is shown a `panic_handler` function that logs the panic message and then halts the thread.

<!-- ignore: test infrastructure can't handle no_std -->
```rust,ignore
#![no_std]

use core::fmt::{self, Write};
use core::panic::PanicInfo;

struct Sink {
// ..
# _0: (),
}
#
# impl Sink {
# fn new() -> Sink { Sink { _0: () }}
# }
#
# impl fmt::Write for Sink {
# fn write_str(&mut self, _: &str) -> fmt::Result { Ok(()) }
# }

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
let mut sink = Sink::new();

// logs "panicked at '$reason', src/main.rs:27:4" to some `sink`
let _ = writeln!(sink, "{}", info);

loop {}
}
```

r[panic.panic_handler.std]
### Standard behavior

Expand Down