std: make ExitCode Eq

[djc]

rustup currently has its own utility ExitCode type, which is sometimes converted from std::process::ExitStatus and it would be nice to be able to check whether it represents success.

Not sure if this kind of thing needs to be unstable for a while?

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[ehuss]

There was some discussion about this in various places, such as #93840 (comment), where there was some intent to keep ExitCode somewhat opaque. In particular, exit codes can be integers, but that is one-way (to_i32 was intentionally not stabilized, but supporting Eq would provide a roundabout way of determining the internal code).

There were questions about adding is_success, but I don't see those ever being answered (tracking #48711 and stabilization #93840). However, I could imagine that being a thorny problem.

[jhpratt]

Not sure if this kind of thing needs to be unstable for a while?

Trait impls cannot currently be unstable, unfortunately.