cargo install errors if same version already installed

[gnzlbg]

Problem

cargo install is not idempotent on nightly (probably has never been)

Steps

cargo install xargo # OK if xargo not installed
cargo install xargo # Errors

errors with:

Updating crates.io index
  Installing xargo v0.3.13
error: binary `xargo` already exists in destination as part of `xargo v0.3.13`

but the version to install is identical with the version currently installed. This should be a warning instead of a hard error.

[dwijnand]

Will be fixed if we go ahead with the proposal in #6667.

[gnzlbg]

@dwijnand we didn't go with that, @ehuss is there a plan for solving this ?

For CI systems it is often useful to use the cargo: cache on, e.g., Travis. I want cargo install foo --XYZ to only install foo if it is not already installed, or if there is a newer version, or similar. But if the exact same version is installed, I don't want cargo to just "fail" here.

[ehuss]

@gnzlbg This has been implemented on nightly. cargo +nightly install foo -Z install-upgrade. If you can try it out and leave feedback on the tracking issue (#6797), I'd appreciate it.

[gnzlbg]

Nice! Thank you!