Allow changing the default toolchain for each channel

[CryZe]

I set my default toolchain to the stable-x86_64-pc-windows-msvc toolchain from the gnu one, but if I type cargo +nightly it still defaults to the nightly-x86_64-pc-windows-gnu toolchain. Setting the default should either affect all channels or there should be a way to select the specific channel you want to change.

This may already be a thing, but it at least doesn't seem to be documented in the CLI.

[CryZe]

Actually, looking at it further, there seems to be a default host, but it's not clear how to change that. (Apparently you can manually change it in the settings.toml)

[rukai]

Just got tripped up by this.
In my case I have a rust-toolchain file which specifies which nightly release to use.
The correct nightly release was being used but it was for the wrong toolchain (gnu instead of msvc) It took a long time to realize this was my problem.
I was running rustup default stable-pc-windows-msvc expecting this to change the toolchain to use msvc but it wasn't.

After finding this issue I was able to follow CryZe's instructions and inspect settings.toml to find the setting default_host_triple = "x86_64-pc-windows-gnu" that was causing the gnu toolchain to still be used. Manually changing it to msvc resolved the issue. The file also contained the setting default_toolchain = "nightly-x86_64-pc-windows-msvc" appears to be correct.

I'm not sure what command does change this setting, but I think it should be in sync with the current toolchain. i.e. running rustup default stable-pc-windows-msvc should change it to msvc

[rbtcollins]

The issue here is a flexability vs ease of use one.

Ease of use: if you use a particular host, we switch you over to that everywhere, and things just happen.

Flexability: everything is explicit, and you can have stable-x86_64-pc-windows-gnu, stable-x86_64-pc-windows-mvsc, nightly-x86_64-pc-linux-gnu, beta-i686-pc-windows-msvc all installed at once, and rustup will support you in doing that.

The underlying confusion you're facing here is the idea that a channel is unique: it is not - you can have multiple instances of stable installed on your machine at once as my example shows.

Should we permit that? Perhaps not. Most folk don't need it. But folk on linux machines can use OS personalities to do this and then run binaries from vastly different platforms (like ARM on x86 via qemu) , which can make building for other devices without cross-compiling very convenient. (Avoiding cross compiling can be useful to allow test suites to run).

If you have suggestions for making this more useful friendly or more clear, we'd certainly love to hear them, but for now I think the tradeoff is probably about right.