Be able to use fleet without using a nightly toolchain

[Milo123459]

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Nightly is unstable and I don't want to use it for my application.

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An option to disable whatever options require nightly rust to be used.

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[suptejas]

We'll work on this.

[lucasfernog]

An interesting approach for now would be to allow by default Cargo to be the stable one so I can easily use fleet only in the projects that I want or can. So you'd need to change your process::Command usage to include the +nightly argument.

[omac777]

We wish fleet would be a drop-in replacement for cargo accepting all cargo conformant cli args.
These currently are not valid with fleet, but I'm certain others would hope it would be:
RUSTFLAGS=-Awarnings fleet +stable build --release --verbose
RUSTFLAGS=-Awarnings fleet +nightly build --release --verbose
RUSTFLAGS=-Awarnings fleet +nightly-2022-04-19 build --release --verbose

Currently it's unclear which toolchain is used when building. I believe it should be stated as part of the output at the beginning.
I also believe it should state whether it is building a release or a debug binary as well.

Cheers and thank you for listening.

[Milo123459]

No real point reopening. Fleets optimisations in 9/10 cases don't help anymore due to new cargo optimisations.