Splice overrideToolchain

[szlend]

Motivation

overrideToolchain can now accept a function used to splice the rust toolchain:

craneLib = (crane.mkLib pkgs).overrideToolchain (p: p.rust-bin.stable.latest.default);

I don't fully understand why the toolchain needs be offset from pkgsBuildHost even when spliced. But this has always been the case.

Fixes #648

Fixes #551

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md

[szlend]

Actually, on second testing, it doesn't seem there's any difference to just doing:

craneLib = (crane.mkLib pkgs).overrideToolchain pkgsBuildHost.rust-bin.stable.latest.default;

So it kind of seems like splicing isn't working correctly. Though I did verify that cargo passed into nativeBuildInputs in various crane derivations has the __spliced attrset. So I don't understand what's going on here.

Edit: Fixed now.

[szlend]

I don't fully understand why the toolchain needs be offset from pkgsBuildHost even when spliced. But this has always been the case.

I should have seen this as a red flag. This works fine after working out the issues:

craneLib = (crane.mkLib pkgs).overrideToolchain (p: p.rust-bin.stable.latest.default);

[ipetkov]

Thanks again for the PR, and for making it backwards compatible! Overall I think it looks good. The hoops we have to jump through to make everything work are no where near as bad as I feared so I'm okay with moving in that direction!

Going to do a little bit of my own testing and move forward from there if it looks fine