Further isolate need for cc_toolchain

Author: UebelAndre

examples/cross_compile/.bazelrc

@@ -20,6 +20,10 @@ coverage --experimental_fetch_all_coverage_outputs
 # https://bazel.build/reference/command-line-reference#flag--experimental_cc_shared_library
 common --experimental_cc_shared_library
 
+# Disable the allocator library by pointing to a unique target which satisfies the `CcInfo`
+# contract but does not require a `cc_toolchain` to produce it.
+common --@rules_rust//rust/settings:default_allocator_library=@rules_rust//ffi/cc/empty
+
 ###############################################################################
 ## Unique configuration groups
 ###############################################################################

ffi/cc/allocator_library/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@rules_cc//cc:defs.bzl", "cc_library")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
 
 cc_library(
     name = "allocator_library",

ffi/cc/empty/BUILD.bazel

@@ -0,0 +1,5 @@
+alias(
+    name = "empty",
+    actual = "//rust/private/cc:empty",
+    visibility = ["//visibility:public"],
+)

ffi/cc/global_allocator_library/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@rules_cc//cc:defs.bzl", "cc_library")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
 
 cc_library(
     name = "global_allocator_library",

rust/extensions.bzl

@@ -132,7 +132,7 @@ def _rust_impl(module_ctx):
     return module_ctx.extension_metadata(**metadata_kwargs)
 
 _COMMON_TAG_DEFAULTS = {
-    "allocator_library": "@rules_rust//ffi/cc/allocator_library",
+    "allocator_library": "",
     "rustfmt_version": DEFAULT_NIGHTLY_VERSION,
     "urls": DEFAULT_STATIC_RUST_URL_TEMPLATES,
 }

rust/private/cc/BUILD.bazel

@@ -0,0 +1,24 @@
+load(":cc_utils.bzl", "cc_empty_library")
+
+cc_empty_library(
+    name = "empty",
+    visibility = ["//visibility:public"],
+)
+
+alias(
+    name = "malloc",
+    actual = select({
+        "//rust/settings:experimental_use_cc_common_link_on": "@bazel_tools//tools/cpp:malloc",
+        "//conditions:default": ":empty",
+    }),
+    visibility = ["//visibility:public"],
+)
+
+alias(
+    name = "global_allocator_library",
+    actual = select({
+        "//rust/settings:experimental_use_global_allocator_on": "//ffi/cc/global_allocator_library",
+        "//conditions:default": ":empty",
+    }),
+    visibility = ["//visibility:public"],
+)

rust/private/cc/cc_utils.bzl

@@ -0,0 +1,12 @@
+"""Rust CC utilities"""
+
+load("@rules_cc//cc/common:cc_info.bzl", "CcInfo")
+
+def _cc_empty_library_impl(_ctx):
+    return [CcInfo()]
+
+cc_empty_library = rule(
+    doc = "A rule that provides an empty `CcInfo`.",
+    implementation = _cc_empty_library_impl,
+    provides = [CcInfo],
+)

rust/private/rust.bzl

@@ -868,11 +868,11 @@ _experimental_use_cc_common_link_attrs = {
         default = -1,
     ),
     "malloc": attr.label(
-        default = Label("@bazel_tools//tools/cpp:malloc"),
+        default = Label("//rust/private/cc:malloc"),
         doc = """Override the default dependency on `malloc`.
 
 By default, Rust binaries linked with cc_common.link are linked against
-`@bazel_tools//tools/cpp:malloc"`, which is an empty library and the resulting binary will use
+`//rust/private/cc:malloc"`, which is an empty library and the resulting binary will use
 libc's `malloc`. This label must refer to a `cc_library` rule.
 """,
         mandatory = False,

rust/private/rust_allocator_libraries.bzl

@@ -89,21 +89,22 @@ def make_libstd_and_allocator_ccinfo(
 
         # Include C++ toolchain files as additional inputs for cross-compilation scenarios
         additional_inputs = []
-        if cc_toolchain and cc_toolchain.all_files:
-            additional_inputs = cc_toolchain.all_files.to_list()
+        if cc_toolchain:
+            if cc_toolchain.all_files:
+                additional_inputs = cc_toolchain.all_files.to_list()
 
-        linking_context, _linking_outputs = cc_common.create_linking_context_from_compilation_outputs(
-            name = label.name,
-            actions = actions,
-            feature_configuration = feature_configuration,
-            cc_toolchain = cc_toolchain,
-            compilation_outputs = compilation_outputs,
-            additional_inputs = additional_inputs,
-        )
+            linking_context, _linking_outputs = cc_common.create_linking_context_from_compilation_outputs(
+                name = label.name,
+                actions = actions,
+                feature_configuration = feature_configuration,
+                cc_toolchain = cc_toolchain,
+                compilation_outputs = compilation_outputs,
+                additional_inputs = additional_inputs,
+            )
 
-        cc_infos.append(CcInfo(
-            linking_context = linking_context,
-        ))
+            cc_infos.append(CcInfo(
+                linking_context = linking_context,
+            ))
 
     if rust_stdlib_info.std_rlibs:
         allocator_library_inputs = []

rust/private/rustc.bzl

@@ -791,7 +791,9 @@ def collect_inputs(
     if linker_script:
         nolinkstamp_compile_direct_inputs.append(linker_script)
 
-    if crate_info.type in ["dylib", "cdylib"]:
+    if not cc_toolchain:
+        runtime_libs = depset()
+    elif crate_info.type in ["dylib", "cdylib"]:
         # For shared libraries we want to link C++ runtime library dynamically
         # (for example libstdc++.so or libc++.so).
         runtime_libs = cc_toolchain.dynamic_runtime_lib(feature_configuration = feature_configuration)
@@ -1342,10 +1344,7 @@ def rustc_compile_action(
         # One or more of the transitive deps is a cc_library / cc_import
         extra_disabled_features = []
     cc_toolchain, feature_configuration = find_cc_toolchain(ctx, extra_disabled_features)
-    if not cc_toolchain or not _are_linkstamps_supported(
-        feature_configuration = feature_configuration,
-        has_grep_includes = hasattr(ctx.attr, "_use_grep_includes"),
-    ):
+    if not cc_toolchain or not _are_linkstamps_supported(feature_configuration = feature_configuration):
         linkstamps = depset([])
 
     # Determine if the build is currently running with --stamp