no-panic

#[no_panic]

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

[dependencies]
no-panic = "0.1"

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:

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

…

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.

[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.

[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:

[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.

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.

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.