https://play.rust-lang.org/?gist=188e52dc2ba423a8f78c6e381f278cf1&version=nightly&backtrace=1

source (copied by @pnkfelix from gist linked above):

fn main() {}

fn t() -> Result<(), String> {
    return Err("".into());
    Ok(())
}

Maybe it's because Ok(()) is unreachable?

See rust-lang-deprecated/error-chain#132 for the original report.

The breakage happens between nightlies 668864d 2017-02-16 and 536a900 2017-02-17.