E0807.md

A keyword was used to move control flow out of a const or static initializer.

This can occur in the following scenario.

A ? was used to return from inside a const initializer:

const fn foo() -> Result<(), ()> { Ok(()) }

fn main() -> Result<(), ()> {
    // error: the `?` operator cannot be used to return from inside a `const` initializer
    const A: () = foo()?;
    Ok(A)
}

To fix this error, either use the keyword or operator outside the const or static initializer:

const fn foo() -> Result<(), ()> { Ok(()) }

fn main() -> Result<(), ()> {
    // store the `Result` in the const and apply `?` later
    const A: Result<(), ()> = foo();
    Ok(A?)
}

or use let instead of const:

const fn foo() -> Result<(), ()> { Ok(()) }

fn main() -> Result<(), ()> {
    let a = foo()?;
    Ok(a)
}