I tried this code:

#![feature(effects)] enum FlipFlop { Flip, Flop, } impl FlipFlop { const fn negating(self) -> FlipFlop { !self } } impl core::ops::Not for FlipFlop { type Output = FlipFlop; fn not(self) -> FlipFlop { match self { FlipFlop::Flip => FlipFlop::Flop, FlipFlop::Flop => FlipFlop::Flip, } } }

I expected this code to fail, since impl Not for FlipFlop is not a constant implementation, but const fn negating requires a constant implementation.

Instead, this code succeeds with no error, failing if you actually use the method like so:

const FLOP: FlipFlop = !FlipFlop::Flip;

Giving the following result:

error[E0080]: evaluation of constant value failed --> src/lib.rs:21:24 | 21 | const FLOP: FlipFlop = !FlipFlop::Flip; | ^^^^^^^^^^^^^^^ calling non-const function `<FlipFlop as Not>::not` For more information about this error, try `rustc --explain E0080`. error: could not compile `playground` (lib) due to 1 previous error 

I believe that this code should fail to compile regardless of whether it's used at constant time.

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rustc --version --verbose:

1.80.0-nightly (2024-05-30 6f3df08aadf71e8d4bf7)