Cannot extend build directives within cargo

[epage]

Because of cargo:KEY=VALUE, it isn't safe to add cargo:error= (or any other directive) as it can be interpreted as a KEY/VALUE, blocking #10159.

[epage]

tl;dr we decided to extend the build directive syntax with cargo:: (maintaining compatibility on cargo: support). cargo:KEY=VALUE metadata syntax would move to something like cargo::metadata=KEY=VALUE. The name "metadata" was not fully settled on.

Ideally, cargo:KEY=VALUE would be changed to cargo:metadata=KEY=VALUE (name to be bikesheded) and we can extend as needed.

How to do it

• Change it on an edition boundary
  • A directive could opt-in to the style
    • We'd need "begin" and "end"s to allow composing with third-party dependencies
  • Provide an env variable with the package.edition for what directive syntax is accepted by default
    • There are target-specific editions but the build script is tied to the package and not the target
• Hack in a new namespace, say cargo:: instead of cargo:
  • For cargo:KEY=VALUE, we'd get :KEY=VALUE which seems a corner case enough that we'd consider this a minor incompatibility instead of a major incompatibility
  • Update all documentation to use :: with a note that : exists for compatiblity purposes with support for some directives

In the cargo team meeting we feel like cargo:: is a relatively simple solution with little documentation impact (new syntax, not concepts or interactions). We can always add directive versioning in the future within the syntax we have. We do not see any other compatibility traps within this syntax.

[XaurDesu]

Could i take a look into this? Currently learning rust and it is marked as 'help wanted'

[epage]

Sure. Feel free to message me on Zulip if you have questions

[XaurDesu]

@rustbot claim

[0xPoe]

@rustbot claim

Since there was no progress for a long time, I wanted to try to implement it.