Improve error message for incomplete toolchains

[Inconnu08]

Improve the error message shown when rustup encounters a toolchain that is missing its manifest.

Interrupted installations can leave a partially installed toolchain behind. Rustup already improves the recovery experience when reinstalling such toolchains (#4725), but users can still encounter a confusing "Missing manifest" error when interacting with the incomplete toolchain before retrying installation.

• Added a test that removes the manifest file from an installed toolchain and verifies the improved error message

Related to #4724
Builds on the interrupted-install UX improvements from #4725

[rami3l]

Thanks for working on this!

View changes since this review

[rami3l]

Suggest adding an extra step to see if the suggestion really works.

[Inconnu08]

Edit: I tried verifying the suggested reinstall step.

In the test environment, rustup toolchain install nightly reports the toolchain as unchanged and does not repair the missing-manifest state.

However, when testing manually with a real dist server, I noticed that rustup component list --toolchain nightly actually triggers a re-sync and repairs the toolchain automatically.

So recovery currently depends on the command used, and toolchain install does not explicitly detect and repair incomplete states.

Given this, I think the current suggestion may be misleading, so I’m considering adjusting the help text to be less prescriptive and soften it.

[Inconnu08]

So after further testing, I re-ran the reproduction using the exact same dated toolchain (nightly-2026-01-01) to avoid channel updates.

In that case, both:

• rustup component list --toolchain nightly-2026-01-01
• rustup toolchain install nightly-2026-01-01

fail to recover the toolchain, and toolchain install still reports it as unchanged.

So it seems the issue is not just command inconsistency, but that toolchain install does not detect or repair an incomplete toolchain in this state.

I’ll open a follow-up issue to track this behavior, and keep this PR focused on just improving the diagnostic and softening the help text.

[rami3l]

@Inconnu08 Ah, I see. In that case it's worth going for #4825. Many thanks for the investigation :)

PS: Rethinking about it, we can indeed have a vague wording to begin with, it's better than having nothing 🙏

[rami3l]

Let's merge this first and see how it goes, I guess #4825 can wait until later :)