Toolchain types should be aliasable

[brandjon]

Suppose you define a toolchain_type with label "//pkg:tt", and an alias that points to it named "//tt2". Then you should be able to use either //pkg:tt and //pkg:tt2interchangeably. This is important for smooth migration of toolchain_type definitions from one name/package/repository to another.

Currently this works for a toolchain target's toolchain_type attribute: When you specify //pkg:tt2, it retrieves the original ToolchainTypeInfo provider from the alias target and sees the original //pkg:tt label.

It does not work for a rule that declares a required toolchain type of //pkg:tt2. The problem can be traced back to the fact that ToolchainResolutionFunction directly compares the label of the requested toolchain type, when it should instead resolve this label to a configured target and retrieve the label in the ToolchainTypeInfo provider.

[brandjon]

Note that once this is fixed, it would still not be expected that you can require a toolchain type under one name but retrieve it from ctx.toolchains under another.

[katre]

This seems related to the concerns @aragos has about constraint values proliferating.

[brandjon]

How so? I did a cursory check, and aliasing appears to be fully transparent to constraint values and constraint settings.

Or are you referring to the problem of explicitly merging two distinct constraint values that are not declared as aliases?

[katre]

I meant the latter, which won't directly involve alias rules, but which is conceptually related.