[feat] Add a build setting to announce crate scope

This build setting is not used internally, but provides a way for
dependencies to be aware that they are being compiled for a rust crate.

Examples of complex scenarios allowed by this change:

1. Build with a mac-x86_64 toolchain on mac-aarch64 platforms (rosetta)
   - everything (including host tools) is compiled for aarch64 while
   proc_macros must be compiled for x86_64.
2. A `cc_library` linked to both a `cc_binary` and a `rust_binary`, but
   needs slightly different build flags when used for rust crates.

Author: yuzhy8701

rust/private/rust.bzl

@@ -750,13 +750,13 @@ _common_attrs = {
         doc = "A version to inject in the cargo environment variable.",
         default = "0.0.0",
     ),
+    "_allowlist_function_transition": attr.label(
+        default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+    ),
     "_stamp_flag": attr.label(
         doc = "A setting used to determine whether or not the `--stamp` flag is enabled",
         default = Label("//rust/private:stamp"),
     ),
-    "_allowlist_function_transition": attr.label(
-        default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
-    ),
 } | RUSTC_ATTRS
 
 _coverage_attrs = {

test/crate_scope/BUILD.bazel

@@ -46,7 +46,7 @@ rust_library(
     srcs = ["proc_macro_helper.rs"],
     data = select({
         "//rust/settings:proc_macro_scope": [":helper_data"],
-        # Analysis fails if rust_library doesn't propagate the condition above.
+        "//rust/settings:regular_crate_scope": [],
     }),
     edition = "2021",
     rustc_env = {"CARGO_MANIFEST_DIR": package_name()},