Install skip existing crates

[liamdawson]

Describe the problem you are trying to solve
I would like to install some cargo utilities via a bootstrap script, and would like it to skip utilities that have already been installed.

Describe the solution you'd like

$ cargo install --skip-existing cargo-edit
warning: crate `cargo-add v0.3.1` already exists in destination, skipping install

Prior art from rbenv:

$ rbenv install --help
...
-s/--skip-existing Skip if the version appears to be installed already
...

This flag would output a message to stderr indicating the crate is already installed, and return a status code of 0 (instead of the current 101).

Notes
I was working around this with command -v cargo-tree | cargo install cargo-tree, but that approach fails with cargo-edit, because it adds multiple binaries, none of which are called cargo-edit. I also feel this is a simple, partial workaround to the multiple requests for using install to upgrade crates.

[ehuss]

Thanks for the report! I've recently implemented a feature that will automatically skip the install if it is already installed. It hasn't been merged yet (#6798), but could you take a look at the documentation and let me know if it covers your needs?

[liamdawson]

It would definitely cover my needs in this scenario! I thought I'd propose this as a (hopefully) uncontroversial/uncomplicated way to achieve my goals in the short term, but do you think there's any value in it vs. your PR there? (I'm happy to open a PR for this suggestion).

[ehuss]

Probably not at this time. New features tend to go through the unstable process, and I don't think we need multiple ones doing similar functionality.

[ehuss]

Closing in favor of #6797.