Platform transition added to Rust binary rules.

[rickvanprim]

Without much insight into core Bazel's plans for supporting "flagless" builds across platforms, this seems like the most direct way to select the platform for a given Rust binary target (all the terminal rules) that doesn't need additional work to forward providers.

Currently I'm using platform_transition_binary (https://github.com/aspect-build/bazel-lib/blob/main/docs/transitions.md), but it doesn't work with Rust Analyzer/Clippy/etc. presumably due to not forwarding the providers. In the case of this particular rule, it also doesn't work for something like rust_shared_library() which is explicitly not an executable = True target.

@illicitonion brought to my attention with_cfg (https://github.com/fmeum/with_cfg.bzl) which could be used as an alternative by consumers of rules_rust instead of this PR. That feels reasonable, though certainly more work for individual users and appears to rely on a lot of Bazel black magic.

Along the same lines, this could wait on rule inheritance as presumably a more formally supported with_cfg approach, which would also be up to individual users and not rules_rust.

Tagging @aiuto, @katre, and @comius per @illicitonion's suggestion of relevant inputs 🙂

[illicitonion]

Tagged folks - @aiuto @katre @fmeum - I know there's been long-standing discussion around flagless builds and arbitrary targets being transitionable - this PR seems like a reasonable step in that direction, but feels like a very specific instance/implementation of a very general pattern... Any thoughts on whether we should do this as-is, and whether there's anything we should change to be forwards-compatible with any larger more general efforts folks may be considering?

[katre]

This change is compatible with our goals, as outlined in Standard Platforms Transitions.

Once we have the common change_platform transition available, it'd be nice if you migrated to use it, but the intent is the same.

[illicitonion]

LGTM!

I'll leave this open for a day or two in case any other maintainers have thoughts.

[rickvanprim]

@katre thanks for the link. I'd be happy to switch this over to change_platform whenever that's available 🙂