feat(custom-toolchains): rustup show now reporting installed targets

[FranciscoTGouveia]

Closes #4251.

Hi everyone! This is my first PR on rustup, part of my community bonding period of GSoC (on the "Making Rustup Concurrent" project).

I'd appreciate your feedback on a few decisions I made, such as:

• Assuming the existence of a rustlib/components file, even for custom toolchains.
• Placing the function for listing custom targets in toolchain.rs.

Additionally, before I add a test, I’d like to ask where it would be most appropriate to place it: should it go in tests/cli_v2.rs or tests/cli_rustup.rs?

Thank you in advance for your time and input!

[rami3l]

Nice changes overall!

Pre-existing issue, but it'd probably be better if we make this new behavior more consistent across the board if rustup component list and rustup target list come into play as well. You can give the same outputs with or without the flag --installed if it's a custom toolchain, I believe.

As for the use of the rustlib/components file, I believe it's the best thing to do now, but do make sure the user experience is not heavily damaged when the file is missing (meaning you need to add a test case for the failing case as well).

cli_v1 and cli_v2 are for tests specific to a certain manifest version. Since this machinery is a fallback in the manifest-free case, it's better to put it in another file.

Many thanks in advance 🙏

[FranciscoTGouveia]

Pre-existing issue, but it'd probably be better if we make this new behavior more consistent across the board if rustup component list and rustup target list come into play as well.

Should I open a new issue/PR for this in order to keep this PR as small/atomic as possible, or should I do it here?

As for the use of the rustlib/components file, I believe it's the best thing to do now, but do make sure the user experience is not heavily damaged when the file is missing (meaning you need to add a test case for the failing case as well).

In the failing case should I display any kind of error or should I keep the previous behavior of simply not reporting any installed target?