Make variable substituion for `javacopts`

[protocol7]

The documentation for java_library and java_binary states that the javacopts attribute supports make variable substitution, e.g.

rules_java/java/common/rules/java_library.bzl

Line 202 in 624b208

"javacopts": attr.string_list(

. However, this does not seem to be the case. Here's a small test case:

  MODULE.bazel
  module(name = "make_var_bug_repro")

  bazel_dep(name = "rules_java", version = "8.6.3")

  java = use_extension("@rules_java//java:extensions.bzl", "java")
  java.toolchain(version = "21")

  make_var_provider.bzl
  "A simple rule that provides a make variable via TemplateVariableInfo."

  def _make_var_provider_impl(ctx):
      return [
          platform_common.TemplateVariableInfo({
              ctx.attr.var_name: ctx.attr.var_value,
          }),
      ]

  make_var_provider = rule(
      implementation = _make_var_provider_impl,
      attrs = {
          "var_name": attr.string(mandatory = True),
          "var_value": attr.string(mandatory = True),
      },
  )

  BUILD.bazel
  load("@rules_java//java:defs.bzl", "java_library")
  load(":make_var_provider.bzl", "make_var_provider")

  make_var_provider(
      name = "my_make_var",
      var_name = "MY_CUSTOM_OPT",
      var_value = "-Xlint:none",
  )

  # According to docs, javacopts supports "Make variable" substitution
  # but this fails with: error: invalid flag: $(MY_CUSTOM_OPT)
  java_library(
      name = "hello",
      srcs = ["Hello.java"],
      javacopts = ["$(MY_CUSTOM_OPT)"],
      toolchains = [":my_make_var"],
  )

  Hello.java
  public class Hello {
      public static void main(String[] args) {
          System.out.println("Hello, World!");
      }
  }

  Run with: bazel build //:hello --java_runtime_version=remotejdk_21

This fails with:

$ bazel build //:hello --java_runtime_version=remotejdk_21
INFO: Analyzed target //:hello (0 packages loaded, 0 targets configured).
ERROR: /private/tmp/claude/rules_java_make_var_bug/BUILD.bazel:14:13: Building libhello.jar (1 source file) failed: (Exit 1): java failed: error executing Javac command (from java_library rule target //:hello) external/rules_java++toolchains+remotejdk25_macos_aarch64/bin/java '--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED' '--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED' ... (remaining 19 arguments skipped)
java.lang.IllegalArgumentException: error: invalid flag: $(MY_CUSTOM_OPT)
	at jdk.compiler/com.sun.tools.javac.main.Arguments.reportDiag(Arguments.java:914)
	at jdk.compiler/com.sun.tools.javac.main.Arguments.doProcessArgs(Arguments.java:409)
	at jdk.compiler/com.sun.tools.javac.main.Arguments.processArgs(Arguments.java:361)
	at jdk.compiler/com.sun.tools.javac.main.Arguments.init(Arguments.java:260)
	at jdk.compiler/com.sun.tools.javac.api.JavacTool.getTask(JavacTool.java:191)
	at com.google.devtools.build.buildjar.javac.BlazeJavacMain.compile(BlazeJavacMain.java:137)
	at com.google.devtools.build.buildjar.ReducedClasspathJavaLibraryBuilder.compileSources(ReducedClasspathJavaLibraryBuilder.java:57)
	at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder.compileJavaLibrary(SimpleJavaLibraryBuilder.java:110)
	at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder.run(SimpleJavaLibraryBuilder.java:118)
	at com.google.devtools.build.buildjar.BazelJavaBuilder.build(BazelJavaBuilder.java:111)
	at com.google.devtools.build.buildjar.BazelJavaBuilder.parseAndBuild(BazelJavaBuilder.java:91)
	at com.google.devtools.build.buildjar.BazelJavaBuilder.lambda$main$0(BazelJavaBuilder.java:52)
	at com.google.devtools.build.lib.worker.WorkRequestHandler$WorkRequestCallback.apply(WorkRequestHandler.java:252)
	at com.google.devtools.build.lib.worker.WorkRequestHandler.respondToRequest(WorkRequestHandler.java:480)
	at com.google.devtools.build.lib.worker.WorkRequestHandler.lambda$startResponseThread$0(WorkRequestHandler.java:433)
	at java.base/java.lang.Thread.run(Thread.java:1474)

This shows that the make variable has not been substitued before javac is invoked. Is this expected, and the docs should be updated, or should support for substitution be added?