fix(config): warn if host is incompatible with the toolchain in rustup default

[rami3l]

Closes #3972:

FWIW we do emit this warning for rustup toolchain install:

rustup/src/cli/rustup_mode.rs

Lines 820 to 825 in 4207838

	error!("DEPRECATED: future versions of rustup will require --force-non-host to install a non-host toolchain.");
	warn!("toolchain '{name}' may not be able to run on this system.");
	warn!(
	"If you meant to build software to target that platform, perhaps try `rustup target add {}` instead?",
	target_triple.to_string()
	);

> uname -mrs
Darwin 23.5.0 arm64

> rustup toolchain install stable-unknown-linux-gnu --profile=minimal
error: DEPRECATED: future versions of rustup will require --force-non-host to install a non-host toolchain.
warn: toolchain 'stable-unknown-linux-gnu' may not be able to run on this system.
warn: If you meant to build software to target that platform, perhaps try `rustup target add aarch64-unknown-linux-gnu` instead?
info: syncing channel updates for 'stable-aarch64-unknown-linux-gnu'
info: latest update on 2024-07-25, rust version 1.80.0 (051478957 2024-07-21)
info: downloading component 'cargo'
info: downloading component 'rust-std'
^C⏎                           

... it could be that this is not covered by rustup default somehow. So adding the same warning to that path should address this issue.

#3972 (comment)

A unified ensure_installed() function also paves the way to addressing #2686.

[djc]

I like the direction here, would mainly like a cleaner commit history here to make it easier to review the details.

[rami3l]

@djc Thanks for your guidance! I'm a bit occupied with other stuff right now so I'll take another look later.

Just to be sure, I actually prefer removing --force-non-host and the related deprecation notice after searching for the existing usages I think we should keep the flag itself, but remove the deprecation notice and leave the warnings as-is. Is that okay to you?

[rbtcollins]

Just to be sure, I actually prefer removing --force-non-host and the related deprecation notice after searching for the existing usages I think we should keep the flag itself, but remove the deprecation notice and leave the warnings as-is. Is that okay to you?

@rami3l So the point of this was to support cross-rs properly (they need non-host toolchains), while stopping people that are just following their nose from getting stuck and confused. E.g. installing an arm64 toolchain on an amd64 machine and wondering why that breaks.

cross-rs should be updated by now I believe, so the appropriate action is to remove the warning and reject non-host toolchains unless the flag is provided.