Make toolchain vars available but don't automatically set them

MODULE.bazel

@@ -184,3 +184,8 @@ use_repo(rust_host_tools, "rust_host_tools")
 
 cargo_bazel_bootstrap = use_extension("//crate_universe/private/module_extensions:cargo_bazel_bootstrap.bzl", "cargo_bazel_bootstrap")
 use_repo(cargo_bazel_bootstrap, "cargo_bazel_bootstrap")
+
+register_toolchains(
+    "//test/foreign_toolchain_make_variables:toolchain_for_test",
+    dev_dependency = True,
+)

WORKSPACE.bazel

@@ -91,3 +91,5 @@ http_archive(
 load("//test/3rdparty/crates:crates.bzl", test_crate_repositories = "crate_repositories")
 
 test_crate_repositories()
+
+register_toolchains("//test/foreign_toolchain_make_variables:toolchain_for_test")

cargo/private/cargo_build_script.bzl

@@ -435,6 +435,8 @@ def _cargo_build_script_impl(ctx):
     # Add environment variables from the Rust toolchain.
     env.update(toolchain.env)
 
+    known_variables = {}
+
     # Gather data from the `toolchains` attribute.
     for target in ctx.attr.toolchains:
         if DefaultInfo in target:
@@ -449,7 +451,7 @@ def _cargo_build_script_impl(ctx):
             toolchain_tools.append(all_files)
         if platform_common.TemplateVariableInfo in target:
             variables = getattr(target[platform_common.TemplateVariableInfo], "variables", depset([]))
-            env.update(variables)
+            known_variables.update(variables)
 
     _merge_env_dict(env, expand_dict_value_locations(
         ctx,
@@ -459,6 +461,7 @@ def _cargo_build_script_impl(ctx):
         getattr(ctx.attr, "tools", []) +
         script_info.data +
         script_info.tools,
+        known_variables,
     ))
 
     tools = depset(

rust/private/rust.bzl

@@ -261,6 +261,7 @@ def _rust_binary_impl(ctx):
             ctx,
             ctx.attr.env,
             ctx.attr.data,
+            {},
         ),
     ))
 
@@ -345,6 +346,7 @@ def _rust_test_impl(ctx):
             ctx,
             ctx.attr.rustc_env,
             data_paths,
+            {},
         ))
         aliases = dict(crate.aliases)
         aliases.update(ctx.attr.aliases)
@@ -396,6 +398,7 @@ def _rust_test_impl(ctx):
             ctx,
             ctx.attr.rustc_env,
             data_paths,
+            {},
         )
 
         # Target is a standalone crate. Build the test binary as its own crate.
@@ -432,6 +435,7 @@ def _rust_test_impl(ctx):
         ctx,
         getattr(ctx.attr, "env", {}),
         data,
+        {},
     )
     if toolchain.llvm_cov and ctx.configuration.coverage_enabled:
         if not toolchain.llvm_profdata:

rust/private/rustc.bzl

@@ -991,6 +991,14 @@ def construct_arguments(
     # Deduplicate data paths due to https://github.com/bazelbuild/bazel/issues/14681
     data_paths = depset(direct = getattr(attr, "data", []), transitive = [crate_info.compile_data_targets]).to_list()
 
+    rustc_flags.add_all(
+        expand_list_element_locations(
+            ctx,
+            getattr(attr, "rustc_flags", []),
+            data_paths,
+            {},
+        ),
+    )
     add_edition_flags(rustc_flags, crate_info)
 
     # Link!
@@ -1053,6 +1061,7 @@ def construct_arguments(
             ctx,
             crate_info.rustc_env,
             data_paths,
+            {},
         ))
 
     # Ensure the sysroot is set for the target platform
@@ -1096,6 +1105,7 @@ def construct_arguments(
             ctx,
             getattr(attr, "rustc_flags", []),
             data_paths,
+            {},
         ),
     )
 

rust/private/utils.bzl

@@ -242,13 +242,14 @@ def _deduplicate(xs):
 def concat(xss):
     return [x for xs in xss for x in xs]
 
-def _expand_location_for_build_script_runner(ctx, env, data):
+def _expand_location_for_build_script_runner(ctx, env, data, known_variables):
     """A trivial helper for `expand_dict_value_locations` and `expand_list_element_locations`
 
     Args:
         ctx (ctx): The rule's context object
         env (str): The value possibly containing location macros to expand.
         data (sequence of Targets): See one of the parent functions.
+        known_variables (dict): Make variables (probably from toolchains) to substitute in when doing make variable expansion.
 
     Returns:
         string: The location-macro expanded version of the string.
@@ -260,10 +261,10 @@ def _expand_location_for_build_script_runner(ctx, env, data):
     return ctx.expand_make_variables(
         env,
         dedup_expand_location(ctx, env, data),
-        {},
+        known_variables,
     )
 
-def expand_dict_value_locations(ctx, env, data):
+def expand_dict_value_locations(ctx, env, data, known_variables):
     """Performs location-macro expansion on string values.
 
     $(execroot ...) and $(location ...) are prefixed with ${pwd},
@@ -288,13 +289,14 @@ def expand_dict_value_locations(ctx, env, data):
         data (sequence of Targets): The targets which may be referenced by
             location macros. This is expected to be the `data` attribute of
             the target, though may have other targets or attributes mixed in.
+        known_variables (dict): Make variables (probably from toolchains) to substitute in when doing make variable expansion.
 
     Returns:
         dict: A dict of environment variables with expanded location macros
     """
-    return dict([(k, _expand_location_for_build_script_runner(ctx, v, data)) for (k, v) in env.items()])
+    return dict([(k, _expand_location_for_build_script_runner(ctx, v, data, known_variables)) for (k, v) in env.items()])
 
-def expand_list_element_locations(ctx, args, data):
+def expand_list_element_locations(ctx, args, data, known_variables):
     """Performs location-macro expansion on a list of string values.
 
     $(execroot ...) and $(location ...) are prefixed with ${pwd},
@@ -311,11 +313,12 @@ def expand_list_element_locations(ctx, args, data):
         data (sequence of Targets): The targets which may be referenced by
             location macros. This is expected to be the `data` attribute of
             the target, though may have other targets or attributes mixed in.
+        known_variables (dict): Make variables (probably from toolchains) to substitute in when doing make variable expansion.
 
     Returns:
         list: A list of arguments with expanded location macros
     """
-    return [_expand_location_for_build_script_runner(ctx, arg, data) for arg in args]
+    return [_expand_location_for_build_script_runner(ctx, arg, data, known_variables) for arg in args]
 
 def name_to_crate_name(name):
     """Converts a build target's name into the name of its associated crate.

test/cargo_build_script/tools_exec/BUILD.bazel

@@ -17,7 +17,7 @@ cargo_build_script(
     edition = "2018",
     links = "beep",
     # Add a flag to test that they're exposed to the build script
-    rustc_flags = ["--verbose"],
+    rustc_flags = ["--cfg=foo=\"bar\""],
     toolchains = ["//rust/toolchain:current_rust_toolchain"],
     tools = [":tool"],
 )

test/cargo_build_script/tools_exec/build.rs

@@ -15,7 +15,7 @@ fn test_encoded_rustflags() {
     // Ensure the `pwd` template has been resolved
     assert!(!flags[0].contains("${pwd}"));
 
-    assert_eq!(flags[1], "--verbose");
+    assert_eq!(flags[1], "--cfg=foo=\"bar\"");
 }
 
 /// Ensure Make variables provided by the `toolchains` attribute are expandable.

test/foreign_toolchain_make_variables/BUILD.bazel

@@ -0,0 +1,25 @@
+load("//cargo:defs.bzl", "cargo_build_script")
+load("//test/foreign_toolchain_make_variables:toolchain.bzl", "current_dummy_env_var_toolchain_toolchain", "dummy_env_var_toolchain")
+
+cargo_build_script(
+    name = "bs",
+    srcs = ["build.rs"],
+    build_script_env = {
+        "FROM_TOOLCHAIN": "$(FROM_TOOLCHAIN)",
+        "MODIFIED_FROM_TOOLCHAIN": "modified$(FROM_TOOLCHAIN)",
+    },
+    edition = "2021",
+    toolchains = [":current_dummy_env_var_toolchain_toolchain"],
+)
+
+toolchain_type(name = "toolchain_type_for_test")
+
+toolchain(
+    name = "toolchain_for_test",
+    toolchain = ":dummy_env_var_toolchain",
+    toolchain_type = ":toolchain_type_for_test",
+)
+
+dummy_env_var_toolchain(name = "dummy_env_var_toolchain")
+
+current_dummy_env_var_toolchain_toolchain(name = "current_dummy_env_var_toolchain_toolchain")

test/foreign_toolchain_make_variables/build.rs

@@ -0,0 +1,9 @@
+fn main() {
+    assert_eq!(std::env::var("FROM_TOOLCHAIN").unwrap(), "present");
+    assert_eq!(
+        std::env::var("MODIFIED_FROM_TOOLCHAIN").unwrap(),
+        "modifiedpresent"
+    );
+    // This was not explicitly forwarded by the cargo_build_script target, so should not be present.
+    assert!(std::env::var_os("ALSO_FROM_TOOLCHAIN").is_none());
+}

test/foreign_toolchain_make_variables/toolchain.bzl

@@ -0,0 +1,34 @@
+"""Utilties for testing forwarding Make variables from toolchains."""
+
+def _dummy_env_var_toolchain_impl(_ctx):
+    make_variables = platform_common.TemplateVariableInfo({
+        "ALSO_FROM_TOOLCHAIN": "absent",
+        "FROM_TOOLCHAIN": "present",
+    })
+
+    return [
+        platform_common.ToolchainInfo(
+            make_variables = make_variables,
+        ),
+        make_variables,
+    ]
+
+dummy_env_var_toolchain = rule(
+    implementation = _dummy_env_var_toolchain_impl,
+)
+
+def _current_dummy_env_var_toolchain_impl(ctx):
+    toolchain = ctx.toolchains[str(Label("@rules_rust//test/foreign_toolchain_make_variables:toolchain_type_for_test"))]
+
+    return [
+        toolchain,
+        toolchain.make_variables,
+    ]
+
+current_dummy_env_var_toolchain_toolchain = rule(
+    doc = "A rule for exposing the current registered `dummy_env_var_toolchain`.",
+    implementation = _current_dummy_env_var_toolchain_impl,
+    toolchains = [
+        str(Label("@rules_rust//test/foreign_toolchain_make_variables:toolchain_type_for_test")),
+    ],
+)