Error code E0158
An associated const
, const
parameter or static
has been referenced
in a pattern.
Erroneous code example:
#![allow(unused)] fn main() { enum Foo { One, Two } trait Bar { const X: Foo; } fn test<A: Bar>(arg: Foo) { match arg { A::X => println!("A::X"), // error: E0158: associated consts cannot be // referenced in patterns Foo::Two => println!("Two") } } }
Associated const
s cannot be referenced in patterns because it is impossible
for the compiler to prove exhaustiveness (that some pattern will always match).
Take the above example, because Rust does type checking in the generic
method, not the monomorphized specific instance. So because Bar
could have
theoretically infinite implementations, there's no way to always be sure that
A::X
is Foo::One
. So this code must be rejected. Even if code can be
proven exhaustive by a programmer, the compiler cannot currently prove this.
The same holds true of const
parameters and static
s.
If you want to match against an associated const
, const
parameter or
static
consider using a guard instead:
#![allow(unused)] fn main() { trait Trait { const X: char; } static FOO: char = 'j'; fn test<A: Trait, const Y: char>(arg: char) { match arg { c if c == A::X => println!("A::X"), c if c == Y => println!("Y"), c if c == FOO => println!("FOO"), _ => () } } }