Include toolchain specification in error message on missing component

[GreenPenguino]

Describe the problem you are trying to solve

I try to run an example from https://github.com/Rahix/avr-hal.

$ cargo +nightly-2021-01-07 run --example leonardo-blink
Output:

error: "/home/<redacted>/.rustup/toolchains/nightly-2021-01-07-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/Cargo.lock" does not exist, unable to build with the standard library, try:
        rustup component add rust-src

So I try the suggested fix:
$ rustup component add rust-src

Output:
info: component 'rust-src' is up to date

After a while I realised that the last command tries to install the component for the default toolchain instead of the nightly toolchain. This is the first time I use a toolchain other than the default, and therefore the first time I use rustup.

Entering the following command solved the issue:
$ rustup component add --toolchain nightly-2021-01-07 rust-src

Describe the solution you'd like

It would be much clearer to include the toolchain specification in the suggested command.

$ cargo +nightly-2021-01-07 run --example leonardo-blink

Desired output:

error: "/home/<redacted>/.rustup/toolchains/nightly-2021-01-07-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/Cargo.lock" does not exist, unable to build with the standard library, try:
        rustup component add --toolchain nightly-2021-01-07 rust-src

[kinnison]

I'm not certain but I think that error is not coming from Rustup but from Cargo. Certainly we'd never have a reason to mention Cargo.lock from the component.

[kinnison]

@ehuss is this Cargo?

[GreenPenguino]

I think it is indeed Cargo. Could this issue be moved to the Cargo repository?

[kinnison]

We need to wait for a Cargo dev to agree, and then we can look to do so.

[ehuss]

Yea, that is from cargo. It is probably pretty easy to include the toolchain in the suggestion.

[kinnison]

If RUSTUP_TOOLCHAIN is set in the environment you can use that to hint it @ehuss :D Are you able to migrate the issue to the cargo repo?

[kinnison]

Frankly I'd also soften it to "You can try running rustup ...." because if it's a custom toolchain it might fail.

[ehuss]

Unfortunately I don't have write access here to transfer.

[kinnison]

Aah I think it needs someone from infra. @pietroalbini perhaps?

[Rustin170506]

A PR is created to add this information.

[pietroalbini]

Transferred the issue to cargo.

[Rustin170506]

@ehuss I think we can close it now. 😄

[ehuss]

Yea, this was fixed by #9625, thanks!