Fix toolchain repo aliases and add a test.

[sputt]

No description provided.

[sputt]

@UebelAndre a fix and test for the repo aliasing

[UebelAndre]

Thanks! Just one question

[UebelAndre]

What exactly is this fixing?

[sputt]

When I originally implemented this feature, I added a check that when you aliased a repository that repository existed. I did it by removing entries from this dict as they were consumed.

It worked for the check, but it emptied the dict for when we actually make the repos below. Unfortunately I thought bringing the repo into scope with use_repo in the test was sufficient, but actually we needed to use the repo in the build for it to be fetched.

[sputt]

Functional test

Alias a junk repo name:

(deck@steamdeck aliased_toolchains)$ bazel build ...
INFO: Streaming build results to: https://req-compile.buildbuddy.io/invocation/74dd1f1d-4be0-4ef7-8d90-b5df6695ccad
ERROR: /home/deck/.cache/bazel/_bazel_deck/596758dc673d4f9d583c4ae1c5ffbc6e/external/rules_rust~/rust/repositories.bzl:317:13: Traceback (most recent call last):
        File "/home/deck/.cache/bazel/_bazel_deck/596758dc673d4f9d583c4ae1c5ffbc6e/external/rules_rust~/rust/extensions.bzl", line 106, column 37, in _rust_impl
                rust_register_toolchains(
        File "/home/deck/.cache/bazel/_bazel_deck/596758dc673d4f9d583c4ae1c5ffbc6e/external/rules_rust~/rust/repositories.bzl", line 317, column 13, in rust_register_toolchains
                fail("No repositories were created matching the requested names to alias:\n{}".format("\n".join(sorted(aliases))))
Error in fail: No repositories were created matching the requested names to alias:
Junk repo

Add tests in tests/aliased_toolchains to actually use an aliased repository.

Example failed test before the dict(aliases) call:

(1)(deck@steamdeck aliased_toolchains)$ bazel build ...
INFO: Streaming build results to: https://req-compile.buildbuddy.io/invocation/fbeb544b-8a41-4df6-b89c-ed16ff8e8716
ERROR: /home/deck/rules_rust/test/aliased_toolchains/BUILD.bazel:3:13: While resolving toolchains for target //:hello (5c86453): invalid registered toolchain '@rust_toolchains//:rust_linux': no such target '@@rules_rust~~rust~rust_toolchains//:rust_linux': target 'rust_linux' not declared in package '' defined by /home/deck/.cache/bazel/_bazel_deck/596758dc673d4f9d583c4ae1c5ffbc6e/external/rules_rust~~rust~rust_toolchains/BUILD.bazel

[sputt]

@UebelAndre questions resolved, should be good to go.