For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

The Rust runtime

This section documents features that define some aspects of the Rust runtime.

The panic_handler attribute

The panic_handler attribute can only be applied to a function with signature fn(&PanicInfo) -> !. The function marked with this attribute defines the behavior of panics. The PanicInfo struct contains information about the location of the panic. There must be a single panic_handler function in the dependency graph of a binary, dylib or cdylib crate.

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


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(()) }
# }

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

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

    loop {}

Standard behavior

The standard library provides an implementation of panic_handler that defaults to unwinding the stack but that can be changed to abort the process. The standard library's panic behavior can be modified at runtime with the set_hook function.

The global_allocator attribute

The global_allocator attribute is used on a static item implementing the GlobalAlloc trait to set the global allocator.

The windows_subsystem attribute

The windows_subsystem attribute may be applied at the crate level to set the subsystem when linking on a Windows target. It uses the MetaNameValueStr syntax to specify the subsystem with a value of either console or windows. This attribute is ignored on non-Windows targets, and for non-bin crate types.

#![windows_subsystem = "windows"]