std::unreachable!
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macro_rules! unreachable { ( ) => { ... }; ( $ msg : expr ) => { ... }; ( $ fmt : expr , $ ( $ arg : tt ) * ) => { ... }; }
A utility macro for indicating unreachable code.
This is useful any time that the compiler can't determine that some code is unreachable. For example:
- Match arms with guard conditions.
- Loops that dynamically terminate.
- Iterators that dynamically terminate.
Panics
This will always panic.
Examples
Match arms:
fn foo(x: Option<i32>) { match x { Some(n) if n >= 0 => println!("Some(Non-negative)"), Some(n) if n < 0 => println!("Some(Negative)"), Some(_) => unreachable!(), // compile error if commented out None => println!("None") } }
Iterators:
fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3 for i in 0.. { if 3*i < i { panic!("u32 overflow"); } if x < 3*i { return i-1; } } unreachable!(); }