Support optional toolchains with find_cpp_toolchain

Rules that want to use optional toolchains but still support builds that don't use C++ toolchain resolution should point their `_cc_toolchain` attribute to the new
`@bazel_tools//tools/cpp:optional_current_cc_toolchain` target.

Fixes #16966

Closes #16968.

PiperOrigin-RevId: 505707967
Change-Id: I251749348f982ae063b51f7d9cc0078a1b61a948

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainAliasRule.java

@@ -33,6 +33,7 @@
 import com.google.devtools.build.lib.analysis.TemplateVariableInfo;
 import com.google.devtools.build.lib.analysis.platform.ToolchainInfo;
 import com.google.devtools.build.lib.packages.RuleClass;
+import com.google.devtools.build.lib.packages.Type;
 import javax.annotation.Nullable;
 
 /** Implementation of the {@code cc_toolchain_alias} rule. */
@@ -50,6 +51,7 @@ public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env)
  .add(
  attr(CcToolchain.CC_TOOLCHAIN_TYPE_ATTRIBUTE_NAME, NODEP_LABEL)
  .value(CppRuleClasses.ccToolchainTypeAttribute(env)))
+ .add(attr("mandatory", Type.BOOLEAN).value(true))
  .requiresConfigurationFragments(PlatformConfiguration.class)
  .addToolchainTypes(CppRuleClasses.ccToolchainTypeRequirement(env))
  .build();

src/main/starlark/builtins_bzl/common/cc/cc_helper.bzl

@@ -201,18 +201,22 @@ def _get_dynamic_library_for_runtime_or_none(library, linking_statically):
 
 _CPP_TOOLCHAIN_TYPE = "@" + objc_semantics.get_repo() + "//tools/cpp:toolchain_type"
 
-def _find_cpp_toolchain(ctx):
+def _find_cpp_toolchain(ctx, *, mandatory = True):
  """
  Finds the c++ toolchain.
 
  If the c++ toolchain is in use, returns it. Otherwise, returns a c++
- toolchain derived from legacy toolchain selection.
+ toolchain derived from legacy toolchain selection, constructed from
+ the CppConfiguration.
 
  Args:
  ctx: The rule context for which to find a toolchain.
+ mandatory: If this is set to False, this function will return None rather
+ than fail if no toolchain is found.
 
  Returns:
- A CcToolchainProvider.
+ A CcToolchainProvider, or None if the c++ toolchain is declared as
+ optional, mandatory is False and no toolchain has been found.
  """
 
  # Check the incompatible flag for toolchain resolution.
@@ -221,6 +225,9 @@ def _find_cpp_toolchain(ctx):
  fail("In order to use find_cpp_toolchain, you must include the '//tools/cpp:toolchain_type' in the toolchains argument to your rule.")
  toolchain_info = ctx.toolchains[_CPP_TOOLCHAIN_TYPE]
  if toolchain_info == None:
+ if not mandatory:
+ return None
+
  # No cpp toolchain was found, so report an error.
  fail("Unable to find a CC toolchain using toolchain resolution. Target: %s, Platform: %s, Exec platform: %s" %
  (ctx.label, ctx.fragments.platform.platform, ctx.fragments.platform.host_platform))

src/main/starlark/builtins_bzl/common/cc/cc_toolchain_alias.bzl

@@ -22,7 +22,9 @@ TemplateVariableInfo = _builtins.toplevel.platform_common.TemplateVariableInfo
 ToolchainInfo = _builtins.toplevel.platform_common.ToolchainInfo
 
 def _impl(ctx):
- cc_toolchain = cc_helper.find_cpp_toolchain(ctx)
+ cc_toolchain = cc_helper.find_cpp_toolchain(ctx, mandatory = ctx.attr.mandatory)
+ if not cc_toolchain:
+ return []
  make_variables = cc_toolchain.get_additional_make_variables()
  cc_provider_make_variables = cc_helper.get_toolchain_global_make_variables(cc_toolchain)
  template_variable_info = TemplateVariableInfo(make_variables | cc_provider_make_variables)
@@ -43,6 +45,7 @@ cc_toolchain_alias = rule(
  implementation = _impl,
  fragments = ["cpp", "platform"],
  attrs = {
+ "mandatory": attr.bool(default = True),
  "_cc_toolchain": attr.label(default = configuration_field(fragment = "cpp", name = "cc_toolchain"), providers = [CcToolchainInfo]),
  "_cc_toolchain_type": attr.label(default = "@" + semantics.get_repo() + "//tools/cpp:toolchain_type"),
  },

src/test/shell/bazel/cc_integration_test.sh

@@ -1910,4 +1910,71 @@ EOF
  expect_log "runtime error: index 10 out of bounds"
 }
 
+function setup_find_optional_cpp_toolchain() {
+ mkdir -p pkg
+
+ cat > pkg/BUILD <<'EOF'
+load(":rules.bzl", "my_rule")
+
+my_rule(
+ name = "my_rule",
+)
+
+platform(
+ name = "exotic_platform",
+ constraint_values = [
+ "@platforms//cpu:wasm64",
+ "@platforms//os:windows",
+ ],
+)
+EOF
+
+ cat > pkg/rules.bzl <<'EOF'
+load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain", "use_cpp_toolchain")
+
+def _my_rule_impl(ctx):
+ out = ctx.actions.declare_file(ctx.attr.name)
+ toolchain = find_cpp_toolchain(ctx, mandatory = False)
+ if toolchain:
+ ctx.actions.write(out, "Toolchain found")
+ else:
+ ctx.actions.write(out, "Toolchain not found")
+ return [DefaultInfo(files = depset([out]))]
+
+my_rule = rule(
+ implementation = _my_rule_impl,
+ attrs = {
+ "_cc_toolchain": attr.label(
+ default = "@bazel_tools//tools/cpp:optional_current_cc_toolchain",
+ ),
+ },
+ toolchains = use_cpp_toolchain(mandatory = False),
+)
+EOF
+}
+
+function test_find_optional_cpp_toolchain_present_without_toolchain_resolution() {
+ setup_find_optional_cpp_toolchain
+
+ bazel build //pkg:my_rule --noincompatible_enable_cc_toolchain_resolution \
+ &> "$TEST_log" || fail "Build failed"
+ assert_contains "Toolchain found" bazel-bin/pkg/my_rule
+}
+
+function test_find_optional_cpp_toolchain_present_with_toolchain_resolution() {
+ setup_find_optional_cpp_toolchain
+
+ bazel build //pkg:my_rule --incompatible_enable_cc_toolchain_resolution \
+ &> "$TEST_log" || fail "Build failed"
+ assert_contains "Toolchain found" bazel-bin/pkg/my_rule
+}
+
+function test_find_optional_cpp_toolchain_not_present_with_toolchain_resolution() {
+ setup_find_optional_cpp_toolchain
+
+ bazel build //pkg:my_rule --incompatible_enable_cc_toolchain_resolution \
+ --platforms=//pkg:exotic_platform &> "$TEST_log" || fail "Build failed"
+ assert_contains "Toolchain not found" bazel-bin/pkg/my_rule
+}
+
 run_suite "cc_integration_test"

tools/cpp/BUILD.tools

@@ -57,12 +57,9 @@ constraint_value(
 
 cc_toolchain_alias(name = "current_cc_toolchain")
 
-# In future versions of Bazel, this target will not fail if no C++ toolchain is
-# available. Instead, it will not advertise the cc_common.CcToolchainInfo
-# provider.
-alias(
+cc_toolchain_alias(
  name = "optional_current_cc_toolchain",
- actual = ":current_cc_toolchain",
+ mandatory = False,
 )
 
 cc_host_toolchain_alias(name = "current_cc_host_toolchain")

tools/cpp/toolchain_utils.bzl

@@ -29,12 +29,14 @@ def find_cpp_toolchain(ctx, *, mandatory = True):
 
  Args:
  ctx: The rule context for which to find a toolchain.
- mandatory: This is currently a no-op. In future releases of Bazel, if this
- is set to False, this function will return None rather than fail if no
- toolchain is found.
+ mandatory: If this is set to False, this function will return None rather
+ than fail if no toolchain is found. To use this parameter, the calling
+ rule should have a `_cc_toolchain` label attribute with default
+ `@bazel_tools//tools/cpp:optional_current_cc_toolchain`.
 
  Returns:
- A CcToolchainProvider.
+ A CcToolchainProvider, or None if the c++ toolchain is declared as
+ optional, mandatory is False and no toolchain has been found.
  """
 
  # Check the incompatible flag for toolchain resolution.
@@ -43,6 +45,9 @@ def find_cpp_toolchain(ctx, *, mandatory = True):
  fail("In order to use find_cpp_toolchain, you must include the '%s' in the toolchains argument to your rule." % CPP_TOOLCHAIN_TYPE)
  toolchain_info = ctx.toolchains[CPP_TOOLCHAIN_TYPE]
  if toolchain_info == None:
+ if not mandatory:
+ return None
+
  # No cpp toolchain was found, so report an error.
  fail("Unable to find a CC toolchain using toolchain resolution. Target: %s, Platform: %s, Exec platform: %s" %
  (ctx.label, ctx.fragments.platform.platform, ctx.fragments.platform.host_platform))