\#\[no\_panic\]
===============

[github](https://github.com/dtolnay/no-panic)
[crates.io](https://crates.io/crates/no-panic)
[docs.rs](https://docs.rs/no-panic)
[build status](https://github.com/dtolnay/no-panic/actions?query=branch%3Amaster)

A Rust attribute macro to require that the compiler prove a function can't ever
panic.

```toml
[dependencies]
no-panic = "0.1"
```

```rust
use no_panic::no_panic;

#[no_panic]
fn demo(s: &str) -> &str {
    &s[1..]
}

fn main() {
    println!("{}", demo("input string"));
}
```

If the function does panic (or the compiler fails to prove that the function
cannot panic), the program fails to compile with a linker error that identifies
the function name. Let's trigger that by passing a string that cannot be sliced
at the first byte:

```rust
fn main() {
    println!("{}", demo("\u{1f980}input string"));
}
```

```console
   Compiling no-panic-demo v0.0.1
error: linking with `cc` failed: exit code: 1
  |
  = note: /no-panic-demo/target/release/deps/no_panic_demo-7170785b672ae322.no_p
anic_demo1-cba7f4b666ccdbcbbf02b7348e5df1b2.rs.rcgu.o: In function `_$LT$no_pani
c_demo..demo..__NoPanic$u20$as$u20$core..ops..drop..Drop$GT$::drop::h72f8f423002
b8d9f':
          no_panic_demo1-cba7f4b666ccdbcbbf02b7348e5df1b2.rs:(.text._ZN72_$LT$no
_panic_demo..demo..__NoPanic$u20$as$u20$core..ops..drop..Drop$GT$4drop17h72f8f42
3002b8d9fE+0x2): undefined reference to `

          ERROR[no-panic]: detected panic in function `demo`
          '
          collect2: error: ld returned 1 exit status
```

The error is not stellar but notice the ERROR\[no-panic\] part at the end that
provides the name of the offending function.

### Caveats

- Functions that require some amount of optimization to prove that they do not
  panic may no longer compile in debug mode after being marked `#[no_panic]`.

- Panic detection happens at link time across the entire dependency graph, so
  any Cargo commands that do not invoke a linker will not trigger panic
  detection. This includes `cargo build` of library crates and `cargo check` of
  binary and library crates.

- The attribute is useless in code built with `panic = "abort"`. Code must be
  built with `panic = "unwind"` (the default) in order for any panics to be
  detected. After confirming absence of panics, you can of course still ship
  your software as a `panic = "abort"` build.

- Const functions are not supported. The attribute will fail to compile if
  placed on a `const fn`.

If you find that code requires optimization to pass `#[no_panic]`, either make
no-panic an optional dependency that you only enable in release builds, or add a
section like the following to your Cargo.toml or .cargo/config.toml to enable
very basic optimization in debug builds.

```toml
[profile.dev]
opt-level = 1
```

If the code that you need to prove isn't panicking makes function calls to
non-generic non-inline functions from a different crate, you may need thin LTO
enabled for the linker to deduce those do not panic.

```toml
[profile.release]
lto = "thin"
```

If thin LTO isn't cutting it, the next thing to try would be fat LTO with a
single codegen unit:

```toml
[profile.release]
lto = "fat"
codegen-units = 1
```

If you want no\_panic to just assume that some function you call doesn't panic,
and get Undefined Behavior if it does at runtime, see [dtolnay/no-panic#16]; try
wrapping that call in an `unsafe extern "C"` wrapper.

[dtolnay/no-panic#16]: https://github.com/dtolnay/no-panic/issues/16

### Acknowledgments

The linker error technique is based on [Kixunil]'s crate [`dont_panic`]. Check
out that crate for other convenient ways to require absence of panics.

[Kixunil]: https://github.com/Kixunil
[`dont_panic`]: https://github.com/Kixunil/dont_panic

#### License

Licensed under either of Apache License, Version
2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.

