1\#\[no\_panic\]
2===============
3
4[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/no--panic-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/no-panic)
5[<img alt="crates.io" src="https://img.shields.io/crates/v/no-panic.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/no-panic)
6[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-no--panic-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/no-panic)
7[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/no-panic/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/no-panic/actions?query=branch%3Amaster)
8
9A Rust attribute macro to require that the compiler prove a function can't ever
10panic.
11
12```toml
13[dependencies]
14no-panic = "0.1"
15```
16
17```rust
18use no_panic::no_panic;
19
20#[no_panic]
21fn demo(s: &str) -> &str {
22    &s[1..]
23}
24
25fn main() {
26    println!("{}", demo("input string"));
27}
28```
29
30If the function does panic (or the compiler fails to prove that the function
31cannot panic), the program fails to compile with a linker error that identifies
32the function name. Let's trigger that by passing a string that cannot be sliced
33at the first byte:
34
35```rust
36fn main() {
37    println!("{}", demo("\u{1f980}input string"));
38}
39```
40
41```console
42   Compiling no-panic-demo v0.0.1
43error: linking with `cc` failed: exit code: 1
44  |
45  = note: /no-panic-demo/target/release/deps/no_panic_demo-7170785b672ae322.no_p
46anic_demo1-cba7f4b666ccdbcbbf02b7348e5df1b2.rs.rcgu.o: In function `_$LT$no_pani
47c_demo..demo..__NoPanic$u20$as$u20$core..ops..drop..Drop$GT$::drop::h72f8f423002
48b8d9f':
49          no_panic_demo1-cba7f4b666ccdbcbbf02b7348e5df1b2.rs:(.text._ZN72_$LT$no
50_panic_demo..demo..__NoPanic$u20$as$u20$core..ops..drop..Drop$GT$4drop17h72f8f42
513002b8d9fE+0x2): undefined reference to `
52
53          ERROR[no-panic]: detected panic in function `demo`
54          '
55          collect2: error: ld returned 1 exit status
56```
57
58The error is not stellar but notice the ERROR\[no-panic\] part at the end that
59provides the name of the offending function.
60
61<br>
62
63### Caveats
64
65- Functions that require some amount of optimization to prove that they do not
66  panic may no longer compile in debug mode after being marked `#[no_panic]`.
67
68- Panic detection happens at link time across the entire dependency graph, so
69  any Cargo commands that do not invoke a linker will not trigger panic
70  detection. This includes `cargo build` of library crates and `cargo check` of
71  binary and library crates.
72
73- The attribute is useless in code built with `panic = "abort"`.
74
75If you find that code requires optimization to pass `#[no_panic]`, either make
76no-panic an optional dependency that you only enable in release builds, or add a
77section like the following to Cargo.toml to enable very basic optimization in
78debug builds.
79
80```toml
81[profile.dev]
82opt-level = 1
83```
84
85If the code that you need to prove isn't panicking makes function calls to
86non-generic non-inline functions from a different crate, you may need thin LTO
87enabled for the linker to deduce those do not panic.
88
89```toml
90[profile.release]
91lto = "thin"
92```
93
94If you want no\_panic to just assume that some function you call doesn't panic,
95and get Undefined Behavior if it does at runtime, see [dtolnay/no-panic#16]; try
96wrapping that call in an `unsafe extern "C"` wrapper.
97
98[dtolnay/no-panic#16]: https://github.com/dtolnay/no-panic/issues/16
99
100<br>
101
102### Acknowledgments
103
104The linker error technique is based on [Kixunil]'s crate [`dont_panic`]. Check
105out that crate for other convenient ways to require absence of panics.
106
107[Kixunil]: https://github.com/Kixunil
108[`dont_panic`]: https://github.com/Kixunil/dont_panic
109
110<br>
111
112#### License
113
114<sup>
115Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
1162.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
117</sup>
118
119<br>
120
121<sub>
122Unless you explicitly state otherwise, any contribution intentionally submitted
123for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
124be dual licensed as above, without any additional terms or conditions.
125</sub>
126