feat(cli)!: remove implicit toolchain installation

[rami3l]

Closes #3943.

As a follow-up of #3983, this PR makes rustup toolchain install (or rustup install) the only way to ensure the installation of the active toolchain (apart from installing it manually).

When the active toolchain needs to be installed, the existing error messages now come to our rescue:

rustup/tests/suite/cli_v2.rs

Lines 320 to 336 in e857897

	cx.config
	.expect_ok(&["rustup", "override", "add", "beta"])
	.await;
	cx.config
	.expect_ok(&["rustup", "toolchain", "remove", "beta"])
	.await;
	cx.config
	.expect_err_ex(
	&["rustc", "--version"],
	"",
	for_host!(
	r"error: toolchain 'beta-{0}' is not installed
	help: run `rustup toolchain install beta-{0}` to install it
	"
	),
	)
	.await;

This PR also makes it possible to merge find_or_install_active_toolchain() and find_active_toolchain() by reducing the number of call sites of the former.

Given the current state of this PR, the CI will certainly fail, since even the test suite also relies on implicit installation. Fixup commits has been added accordingly, which will of course be squashed onto the original commit respectively before actually merging this PR.

[djc]

This is looking great!

[djc]

I think after this lands we should probably do a release!

[rami3l]

I think after this lands we should probably do a release!

@djc There are 20-ish items remaining in the milestone: https://github.com/rust-lang/rustup/milestone/8. I invite you to check them out and let's decide which ones are here to stay and which ones will be left for v1.28.1, or even v1.29.0.

I'm even for having a very long beta cycle for this one: there are just so many changes and I'm afraid of breaking anything by accident: I just don't have that much bandwidth, plus I have to do the legwork around cutting a new release.

---

Update: I've created a new milestone for v1.28.1 and have moved some not-so-urgent items there: https://github.com/rust-lang/rustup/milestone/13

Also, please remember we can add new stuff even in the beta stage, as long as they are minor enough (doc changes, new warnings, etc). We've done this with v1.27.0 as well.

[rukai]

Hi, does this PR mean that if I have a rust-toolchain.toml set to a specific release, rustup will no longer automatically install that release, instead a manual command must be run before that toolchain is available?

If so this is going to break CI for many projects who rely on a pinned nightly or pinned stable + fail on warnings.

[rami3l]

Hi, does this PR mean that if I have a rust-toolchain.toml set to a specific release, rustup will no longer automatically install that release, instead a manual command must be run before that toolchain is available?

If so this is going to break CI for many projects who rely on a pinned nightly or pinned stable + fail on warnings.

Yes, you've understood it correctly, and this breakage is intentional: #3635 (comment)

Before the actual release, we'll definitely clearly communicate this change with the community, and we'll use the beta phase to collect user feedback.

[ickk]

I am looking forward to this change, so thanks for working on it

re: breakage

Might I suggest adding the new behaviour (#3983), and then waiting a release or two before completely removing the implicit installs? My concern is having a path with some continuity; rustup toolchain install on <= 1.27.1 is an error, but after >= 1.28 it will be an error not to do so, leaving no grace period.

If there is need - perhaps some tool that is blocked waiting on the new behaviour - during the transitional period an extra flag could be added to opt-out of implicit installs, at the risk of some complexity.

[rami3l]

We currently propose #3635 (comment) as a migration path. If you have any concerns/suggestions/comments, let's continue the discussion over there.