Show that a toolchain download is due to auto installation

[nc7s]

Problem you are trying to solve

I'm in directory atuin/, trying to run the tool bacon, and wanted to check if bacon has a newer version because it's cargo install-ed:

atuin  [!]
❯ cargo info bacon
info: syncing channel updates for '1.88-x86_64-unknown-linux-gnu'
info: latest update on 2025-06-26, rust version 1.88.0 (6b00bc388 2025-06-23)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-docs'
info: installing component 'rust-std'
info: installing component 'rustc'
info: installing component 'rustfmt'
error: the 'cargo' binary, normally provided by the 'cargo' component, is not applicable to the '1.88-x86_64-unknown-linux-gnu' toolchain

Why is cargo suddenly updating the toolchain?

Solution you'd like

Show an obvious notice that it's downloading a toolchain due to rust-toolchain.toml.

Notes

No response

[rami3l]

@nc7s Thanks for your report! I totally understand your confusion (see also https://users.rust-lang.org/t/is-it-normal-behavior-that-the-rustup-toolchain-list-command-will-download-and-install-toolchains/130168), and you can simply put RUSTUP_AUTO_INSTALL=0 to avoid this one.

A little bit of background here: we have removed this behavior back in v1.28.0, but this turned out to be a premature move and has broken many people's workflow, so we reverted it in v1.28.1 (https://github.com/rust-lang/rustup/issues/4211#issuecomment-2700966450).

While now the team is still inclined towards that direction, we need to be much more cautious than before. Your suggestion has pointed out a possible move forward: instead of showing an obvious notice that auto-installation is in effect, would it be better if we show a deprecation warning so that the users can detect where auto-installation is used and at least try to move away from it?

cc @djc @ChrisDenton

[nc7s]

Thanks for linking to the issue, I wasn't aware it's called implicit installation and missed it.

I'm not against auto-installation here, to be clear, just confused by a sudden invocation without notable trace. A deprecation warning is enough to tell me what's going on in place of a "normal" notice, so if that seems better to you, sure!

[djc]

I agree that even if we don't deprecate implicit installation, it would be useful to log a message when implicitly downloading a new toolchain.