`cargo fmt -- --help` displays no output

[carols10cents]

Problem

I ran cargo fmt -- --help and didn't get any output. I expected to get the same output that I do get when I run rustfmt --help, especially because the last line in cargo fmt --help says Arguments after `--` are passed to rustfmt.

The weirdest part is if I run cargo fmt -- --help=config, I do get the same output as I get if I run rustfmt --help=config, the configuration specific help. So cargo is passing through --help when it has an argument? And not passing it through when there isn't? I haven't investigated the cause at all, I'm just guessing based on the symptoms.

Steps

1. Be on stable Rust with the rustfmt component installed.
2. Run cargo fmt -- --help.
3. Observe that there is no output.
4. Observe the expected output by running rustfmt --help.

Notes

Output of cargo version: cargo 1.35.0 (6f3e9c3 2019-04-04)
Output of rustfmt --version: rustfmt 1.2.0-stable (09940a70 2019-03-27)
OS: macOS Mojave

[ehuss]

This is a bug in rustfmt. I posted a fix at rust-lang/rustfmt#3620.

[ehuss]

This is now fixed on nightly via rust-lang/rustfmt#3620 and rust-lang/rust#62688.