feat: add extra_rustc_flags_triples (#2999)

Currently, this attribute can be either a dict or a list:

https://github.com/bazelbuild/rules_rust/blob/d383a1aef9865e59172af6c63f29c96fed19b04a/rust/repositories.bzl#L152

Resolved here:

https://github.com/bazelbuild/rules_rust/blob/d383a1aef9865e59172af6c63f29c96fed19b04a/rust/repositories.bzl#L1047

This adds `bzlmod` parity to that feature.

Author: finn-ball

rust/extensions.bzl

@@ -51,17 +51,21 @@ def _rust_impl(module_ctx):
     toolchains = root.tags.toolchain or rules_rust.tags.toolchain
 
     for toolchain in toolchains:
+        if toolchain.extra_rustc_flags and toolchain.extra_rustc_flags_triples:
+            fail("Cannot define both extra_rustc_flags and extra_rustc_flags_triples")
         if len(toolchain.versions) == 0:
             # If the root module has asked for rules_rust to not register default
             # toolchains, an empty repository named `rust_toolchains` is created
             # so that the `register_toolchains()` in MODULES.bazel is still
             # valid.
             _empty_repository(name = "rust_toolchains")
         else:
+            extra_rustc_flags = toolchain.extra_rustc_flags if toolchain.extra_rustc_flags else toolchain.extra_rustc_flags_triples
+
             rust_register_toolchains(
                 dev_components = toolchain.dev_components,
                 edition = toolchain.edition,
-                extra_rustc_flags = toolchain.extra_rustc_flags,
+                extra_rustc_flags = extra_rustc_flags,
                 extra_exec_rustc_flags = toolchain.extra_exec_rustc_flags,
                 allocator_library = toolchain.allocator_library,
                 rustfmt_version = toolchain.rustfmt_version,
@@ -117,6 +121,9 @@ _RUST_TOOLCHAIN_TAG = tag_class(
         extra_rustc_flags = attr.string_list(
             doc = "Extra flags to pass to rustc in non-exec configuration",
         ),
+        extra_rustc_flags_triples = attr.string_list_dict(
+            doc = "Extra flags to pass to rustc in non-exec configuration. Key is the triple, value is the flag.",
+        ),
         rust_analyzer_version = attr.string(
             doc = "The version of Rustc to pair with rust-analyzer.",
         ),