Improve "invoke \`cargo\` through \`rustup\` to handle \`+toolchain\`" comment to be more helpful

[sliedes]

Problem

I today for the first time tried to run MIRI and discovered I needed to use an unstable toolchain. I figured out how that can be installed using rustup, but then got this:

$ cargo +nightly miri test
error: no such command: `+nightly`

help: invoke `cargo` through `rustup` to handle `+toolchain` directives

Based on this, I tried all kinds of things (including reading through documentation), yet it took me probably 20 minutes to figure out the correct syntax. I tried:

rustup +toolchain run cargo miri test
rustup run +toolchain cargo miri test

... etc. I found many mentions of cargo +toolchain on the net, but it was really quite hard to find the syntax to actually get this to run. One reason is that as a beginner I really had no idea if I was looking for an option to run, not rustup or cargo..

Proposed Solution

Now, having found that the correct invocation is rustup run nightly cargo miri test, I think it would have been most helpful if the "help:" message would have given a bigger hint, maybe something like

help: Use `rustup run <toolchain> cargo ...` instead of `cargo +toolchain`.

, or even pointing me, the new user, to rustup run --help.

Notes

Place in code:

cargo/src/bin/cargo/main.rs

Line 294 in 4f15cc8

help: invoke `cargo` through `rustup` to handle `+toolchain` directives{script_suggestion}",

A related issue which lead to some changes to make this supposedly easier to discover: #8058

[epage]

I opened rust-lang/rustup#4564 for improving the error message in rustup.

There are two routes for a user getting Cargo's error message:

1. Switch from an externally-managed Cargo to a rustup-managed Cargo so the existing command will work
2. Use rustup run nightly cargo

A concern with how we word the diagnostic is us cutting users off of going down route (1). My main concern is that users think cargo +nightly is never valid

[epage]

A third case is someone has resolved to the actual copy of cargo behind the rustup proxy.

I feel like at least one step is to have a note saying that +toolchain overrides require using rustup's copy of cargo. How to get the nuance right with that is difficult.

[sliedes]

Right. I in fact suspected also something funny in NixOS since I run this in a nix direnv—not the most typical setting. It took me quite a while to figure out where this string is printed, but Debian code search helped :P.

But I agree, I think any knowledge like that would likely have helped. I was left suspecting that "cargo +toolchain" is some obsolete thing. And the other thing is that it suggested using rustup but didn't really give an example command. I tried inserting "+toolchain" in all kinds of places before I figured out I need to drop the plus!

[epage]

I realized I forgot to create an issue for rustup run +toolchain cargo miri test, see #4571