support generic target tables and env variables

[jameshilliard]

Problem

When building for targets from a meta build system like buildroot it is preferable to be able to unconditionally set target config/env variables without having to care about the target triple as we use target specific toolchains that will only support a single target architecture typically.

Proposed Solution

#9603

[epage]

The linked PR doesn't provide much details on the proposed solution besides a code dump. Could you provide more details on the problem, what the user-facing solution would be, and how that helps?

[jameshilliard]

Could you provide more details on the problem, what the user-facing solution would be, and how that helps?

Currently to say set a target linker we have to generate an env variable like CARGO_TARGET_$(call UPPERCASE,$(RUSTC_TARGET_NAME))_LINKER, it would be nice to be able to set the target linker for all target artifact builds using a non-target-triple specific variable like CARGO_TARGET_LINKER.

[epage]

In TOML terms,

[target]
linker = ...
[target.<triple>]
linker = ...

translates to

{
    "target": {
        "linker": "...",
        "<triple>": {
            "linker": "...",
        }
    }
}

Personally, I would worry about putting target-config fields at the same level as <triple> / <cfg>

Some options

• built-in value that means "all"
• extend which fields can be used with <cfg> (I'm assuming there are technical limitations for this)

Huh, we don't specify what happens if multiple targets apply. In moving this forward, we should probably verify what is done in that case and be able to talk whether a design for solving this problem works with that solution or not.

[jameshilliard]

Huh, we don't specify what happens if multiple targets apply. In moving this forward, we should probably verify what is done in that case and be able to talk whether a design for solving this problem works with that solution or not.

So in my pull request I actually wrote a test to cover this specific scenario. The way I implemented the logic was to have any target triple specific configs take precedence over generic target configs if they exist.