Macro core::panic

macro_rules! panic {
    ($($arg:tt)*) => { ... };
}

Panics the current thread.

This allows a program to terminate immediately and provide feedback to the caller of the program.

This macro is the perfect way to assert conditions in example code and in tests. panic! is closely tied with the unwrap method of both Option and Result enums. Both implementations call panic! when they are set to None or Err variants.

When using panic!() you can specify a string payload that is built using formatting syntax. That payload is used when injecting the panic into the calling Rust thread, causing the thread to panic entirely.

The behavior of the default std hook, i.e. the code that runs directly after the panic is invoked, is to print the message payload to stderr along with the file/line/column information of the panic!() call. You can override the panic hook using std::panic::set_hook(). Inside the hook a panic can be accessed as a &dyn Any + Send, which contains either a &str or String for regular panic!() invocations. (Whether a particular invocation contains the payload at type &str or String is unspecified and can change.) To panic with a value of another other type, panic_any can be used.

See also the macro compile_error!, for raising errors during compilation.

When to use panic! vs Result

The Rust language provides two complementary systems for constructing / representing, reporting, propagating, reacting to, and discarding errors. These responsibilities are collectively known as “error handling.” panic! and Result are similar in that they are each the primary interface of their respective error handling systems; however, the meaning these interfaces attach to their errors and the responsibilities they fulfill within their respective error handling systems differ.

The panic! macro is used to construct errors that represent a bug that has been detected in your program. With panic! you provide a message that describes the bug and the language then constructs an error with that message, reports it, and propagates it for you.

Result on the other hand is used to wrap other types that represent either the successful result of some computation, Ok(T), or error types that represent an anticipated runtime failure mode of that computation, Err(E). Result is used alongside user defined types which represent the various anticipated runtime failure modes that the associated computation could encounter. Result must be propagated manually, often with the help of the ? operator and Try trait, and they must be reported manually, often with the help of the Error trait.

For more detailed information about error handling check out the book or the std::result module docs.

Current implementation

If the main thread panics it will terminate all your threads and end your program with code 101.

Editions

Behavior of the panic macros changed over editions.

2021 and later

In Rust 2021 and later, panic! always requires a format string and the applicable format arguments, and is the same in core and std. Use std::panic::panic_any(x) to panic with an arbitrary payload.

2018 and 2015

In Rust Editions prior to 2021, std::panic!(x) with a single argument directly uses that argument as a payload. This is true even if the argument is a string literal. For example, panic!("problem: {reason}") panics with a payload of literally "problem: {reason}" (a &'static str).

core::panic!(x) with a single argument requires that x be &str, but otherwise behaves like std::panic!. In particular, the string need not be a literal, and is not interpreted as a format string.

Examples

panic!();
panic!("this is a terrible mistake!");
panic!("this is a {} {message}", "fancy", message = "message");
std::panic::panic_any(4); // panic with the value of 4 to be collected elsewhere