commit c6ad23abadf2c26f354db515ad2378f1bf21d414
author scentini <rosica@google.com> Thu Apr 20 14:25:20 2023 +0200
committer GitHub <noreply@github.com> Thu Apr 20 12:25:20 2023 +0000
tree 5cf7770793bdcc911b17838dce1c3773fc239945
parent d787525049e4cbaadabb2f531fe86c871a6cf505

Respect `#[global_allocator]` in `cc_common.link` builds (#1926)

`rustc` generates allocator symbols at link time. When we link via `cc_common.link` those symbols are missing because `rustc` is not driving the linking anymore. We compensate by providing the symbols via the `rust_toolchain.allocator_library`.
Rust also allows a global allocator to be specified (via `#[global_allocator]`). The symbols `rustc` generates at link time are different based on whether the system allocator is used, or a global allocator was specified.

Currently, when linking via `cc_common.link`, `#[global_allocator]` is completely ignored. This PR makes using a global allocator actually work:
1. Adding a `rust_toolchain.global_allocator_library` attribute that can point to a set of symbols needed when `#[global_allocator]` is used.
2. Adding a build flag `--@rules_rust//rust/settings:experimental_use_global_allocator` flag that is only observed when `--@rules_rust//rust/settings:experimental_use_cc_common_link==True`. At this point this flag only applies to the target configuration.

Unrelated to the goal of this PR, I also modified visibility of a `rust_toolchain` in `test/toolchain/toolchain_test.bzl` to address failures of Bazel rolling releases tasks (https://buildkite.com/bazel/rules-rust-rustlang/builds/8457#018799ff-5908-4b57-918b-a582ed12a248)