Add support for optional cc_toolchain (#3665)

Changes:
- Moved `rust_allocator_libraries` rule implementation into it's own
file
- Updated all rules to set `mandatory = False` for
`@bazel_tools//tools/cpp:toolchain_type` toolchains.
- Refactored rules_rust internals to work in cases where
`find_cc_toolchain` returns `None`
- Added `linker` as an optional attribute to `rust_toolchain`. This is
expected to be `rust-lld` for now.
- Added a new setting
`@rules_rust//rust/settings:toolchain_linker_preference` which in the
event `rust_toolchain.linker` and a `cc_toolchain` are present, chooses
the linker to use. (Defaults to `cc`).
- Introduce `--@rules_rust//settings:default_allocator_library` as a
flag to globally control this value.

---------

Co-authored-by: Daniel Wagner-Hall <[email protected]>

Author: UebelAndre

.bazelci/presubmit.yml

@@ -723,19 +723,43 @@ tasks:
     working_directory: examples/compile_opt
     build_targets:
       - "//..."
-  # TODO: https://github.com/bazelbuild/rules_rust/issues/2075
-  # cross_compile_zig:
-  #   name: Cross compile example with Zig
+  # TODO: This test requires musl or for rustc to provide necessary gnu libraries for
+  # linking gnu libraries.
+  # https://github.com/bazelbuild/rules_rust/issues/390
+  # cross_compile_linux:
+  #   name: Cross compile example on Linux without cc_toolchain
   #   platform: ubuntu2204
-  #   working_directory: examples/cross_compile_zig
-  #   build_targets:
+  #   working_directory: examples/cross_compile
+  #   shell_commands:
+  #     - sed -i 's/_FORCE_DISABLE_CC_TOOLCHAIN = False/_FORCE_DISABLE_CC_TOOLCHAIN = True/' ../../rust/private/utils.bzl
+  #   test_flags:
+  #     - "--keep_going"
+  #   test_targets:
   #     - "//..."
+  cross_compile_macos:
+    name: Cross compile example on MacOS without cc_toolchain
+    platform: macos_arm64
+    working_directory: examples/cross_compile
+    shell_commands:
+      - sed -i '' 's/_FORCE_DISABLE_CC_TOOLCHAIN = False/_FORCE_DISABLE_CC_TOOLCHAIN = True/' ../../rust/private/utils.bzl
+    test_flags:
+      - "--keep_going"
+    test_targets:
+      - "//..."
+  cross_compile_windows:
+    name: Cross compile example on Windows without cc_toolchain
+    platform: windows
+    working_directory: examples/cross_compile
+    batch_commands:
+      - powershell -Command "(Get-Content rust/private/utils.bzl) -replace '_FORCE_DISABLE_CC_TOOLCHAIN = False', '_FORCE_DISABLE_CC_TOOLCHAIN = True' | Set-Content ../../rust/private/utils.bzl"
+    test_flags:
+      - "--keep_going"
+    test_targets:
+      - "//..."
   cross_compile_musl_macos_to_linux:
     name: Cross compile example Musl from macOS to Linux
     platform: macos_arm64
     working_directory: examples/cross_compile_musl
-    build_targets:
-      - "//..."
     test_targets:
       - "//..."
   cross_compile_musl_linux_to_linux:
@@ -775,6 +799,13 @@ tasks:
       - "//:all"
     test_targets:
       - "//..."
+  # TODO: https://github.com/bazelbuild/rules_rust/issues/2075
+  # cross_compile_zig:
+  #   name: Cross compile example with Zig
+  #   platform: ubuntu2204
+  #   working_directory: examples/cross_compile_zig
+  #   build_targets:
+  #     - "//..."
   example_ffi_linux:
     platform: ubuntu2204
     working_directory: examples/ffi

.bazelrc

@@ -49,6 +49,10 @@ build:unpretty --output_groups=+rust_unpretty
 # https://github.com/rust-lang/rust/issues/43364
 build:unpretty --config=nightly
 
+# Disable cc toolchains to test rust targets can be built without one.
+build:no_cc_toolchain --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
+build:no_cc_toolchain --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
+
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 ## Incompatibility flags
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