c43a9ea146
make unstable Result::flatten a const fn This method is still unstable (tracked at https://github.com/rust-lang/rust/issues/70142), but there's no reason I can see for it not to be const -- after all, `Option::flatten` is const. So let's make the `Result` one `const` as well, under the same feature gate. Cc https://github.com/rust-lang/rust/issues/70142