Warn nightly users if rls is missing in the active toolchain

[m-rutter]

So I've been fighting for the past couple hours with rls-preview on the latest nightly build after doing an unwise rustup update. Turns out rls is missing in the latest nightly and has been missing on and off for the past few weeks due to breaking changes in nightly (as is to be expected).

rust-lang/rls#611

While trying to work out what was the last working nightly/rls release I saw it mentioned in the issue linked above that rls has a release on stable now (yay!), and sure enough setting rust-client.channel to stable got rls working again with this extension. Might it make sense to default to stable rather than asking the user if they want to install the latest nightly, which has a small chance of missing rls-preview?

[Xanewok]

Current design, as in #179, is to default to user's current default toolchain.

I do recognize that ergonomics regarding rustup update with possibly missing rls component on nightly (hopefully fixed later by rust-lang/rustup#1277) are far from perfect and need a lot of work. Also thanks to rust-lang/rust#46554 tool management will be easier and so hopefully rls breakage should occur a lot less frequently from now on.

One idea how to improve the ergonomics is to warn nightly users if active toolchain has no rls component and to detect and suggest newest nightly that has the component available. I'll change the title to better reflect work left to do.

[m-rutter]

Yes, it makes much better sense to have it default to the active toolchain over stable.

[m-rutter]

I think this has now been addressed. rls-vscode now defaults to the default toolchain set by rustup, and rustup no longer updates nightly if there is no upgrade path for a something like RLS.