Use incompatible_flag rule

Author: Vinh Tran

rust/private/rustc.bzl

@@ -38,6 +38,7 @@ load(
     "make_static_lib_symlink",
     "relativize",
 )
+load("//rust/settings:incompatible.bzl", "IncompatibleFlagInfo")
 
 BuildInfo = _BuildInfo
 
@@ -966,7 +967,7 @@ def construct_arguments(
         rustc_flags.add(linker_script, format = "--codegen=link-arg=-T%s")
 
     if hasattr(attr, "_experimental_toolchain_generated_sysroot"):
-        if attr._experimental_toolchain_generated_sysroot[BuildSettingInfo].value == True:
+        if attr._experimental_toolchain_generated_sysroot[IncompatibleFlagInfo].enabled == True:
             rustc_flags.add("--sysroot", toolchain.sysroot)
 
     # Tell Rustc where to find the standard library (or libcore)

rust/settings/BUILD.bazel

@@ -1,5 +1,6 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_flag")
+load(":incompatible.bzl", "incompatible_flag")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -65,7 +66,8 @@ bzl_library(
 )
 
 # A flag to set rustc --sysroot flag to the sysroot generated by rust_toolchain
-bool_flag(
+incompatible_flag(
     name = "experimental_toolchain_generated_sysroot",
-    build_setting_default = True,
+    build_setting_default = False,
+    issue = "https://github.com/bazelbuild/rules_rust/issues/2039",
 )

test/toolchain/toolchain_test.bzl

@@ -36,12 +36,25 @@ def _toolchain_adds_rustc_flags_impl(ctx):
         "Found exec toolchain flag ({}) in rustc flags: {}".format(EXEC_TOOLCHAIN_FLAG, action.argv),
     )
 
+    asserts.true(
+        env,
+        "--sysroot" in action.argv,
+        "Missing --sysroot flag",
+    )
+
+    asserts.true(
+        env,
+        action.argv[action.argv.index("--sysroot") + 1].endswith("test/toolchain/rust_extra_flags_toolchain"),
+        "--sysroot does not set to directory generated by rust_toolchain",
+    )
+
     return analysistest.end(env)
 
 toolchain_adds_rustc_flags_test = analysistest.make(
     _toolchain_adds_rustc_flags_impl,
     config_settings = {
         str(Label("//:extra_rustc_flags")): [CONFIG_FLAG],
+        str(Label("//rust/settings:experimental_toolchain_generated_sysroot")): True,
     },
 )