Improve error message for no such subcommand

[CAD97]

Problem

With the rustup changes to set $RUSTC et al, it's become more likely that people will accidentally use +toolchain on cargo/rustc/etc directly rather than through the rustup shims. While we shouldn't try to handle the toolchain flag ourselves, it would be beneficial to recognize it in the "no such subcommand" path and print a help note notifying the user that cargo received the +toolchain argument but can't handle it.

〉main:pegcel〉rustup which cargo
D:\.rust\rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe
〉main:pegcel〉D:\.rust\rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe +nightly check
error: no such subcommand: `+nightly`

Proposed Solution

〉main:pegcel〉rustup which cargo
D:\.rust\rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe
〉main:pegcel〉D:\.rust\rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe +nightly check
error: no such subcommand: `+nightly`

        Did you mean to run cargo through rustup?
        Cargo does not handle `+toolchain` directives itself.

Notes

Some rustup context:

• rustup breaks with cargo-make due to environment variables rustup#3029
• rustup 1.25 sets RUSTC, which overrides custom toolchain specified in sub-Cargo invocation rustup#3031
• un-revert "Set RUSTC and RUSTDOC env for child processes run through the proxy" rustup#3035
• rustup 1.25: On Windows, nested cargo invocation with a toolchain specified fails rustup#3036

[ehuss]

Thanks for the suggestion! This is essentially a duplicate of #9652. However, since this has more detail I'll close the other issue. However, #9652 contains a few different experience reports.

[CAD97]

The change should IIUC slot in right here:

cargo/src/bin/cargo/main.rs

Lines 164 to 174 in 85b500c

	fn execute_external_subcommand(config: &Config, cmd: &str, args: &[&str]) -> CliResult {
	let path = find_external_subcommand(config, cmd);
	let command = match path {
	Some(command) => command,
	None => {
	let suggestions = list_commands(config);
	let did_you_mean = closest_msg(cmd, suggestions.keys(), |c| c);
	let err = anyhow::format_err!("no such subcommand: `{}`{}", cmd, did_you_mean);
	return Err(CliError::new(err, 101));
	}
	};

[ehuss]

Another thought I had that would be useful (unrelated to +toolchain) is to also print a note, recommending the user run cargo --list to get a complete list of subcommands. Otherwise the error can be a bit terse:

> cargo cargo
error: no such subcommand: `cargo`

[bindsdev]

@rustbot claim