#[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"));
}

   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.

[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.}