Macro std::unreachable
1.0.0 · source · macro_rules! unreachable { ($($arg:tt)*) => { ... }; }
Expand description
Indicates 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.
If the determination that the code is unreachable proves incorrect, the
program immediately terminates with a panic!
.
The unsafe counterpart of this macro is the unreachable_unchecked
function, which
will cause undefined behavior if the code is reached.
§Panics
This will always panic!
because unreachable!
is just a shorthand for panic!
with a
fixed, specific message.
Like panic!
, this macro has a second form for displaying custom values.
§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")
}
}
RunIterators:
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!("The loop should always return");
}
Run