Clean up jmh rule a bit (#811)

Author: johnynek

jmh/jmh.bzl

@@ -75,18 +75,22 @@ def jmh_repositories(maven_servers = ["http://central.maven.org/maven2"]):
         actual = "@io_bazel_rules_scala_org_apache_commons_commons_math3//jar",
     )
 
-def _scala_construct_runtime_classpath(deps):
-    files = []
-    [files.append(target[JavaInfo].transitive_runtime_deps) for target in deps if JavaInfo in target]
-
-    return depset(transitive = files)
-
 def _scala_generate_benchmark(ctx):
-    class_jar = ctx.attr.src.scala.outputs.class_jar
-    classpath = _scala_construct_runtime_classpath([ctx.attr.src])
+    # we use required providers to ensure JavaInfo exists
+    info = ctx.attr.src[JavaInfo]
+    # TODO, if we emit more than one jar, which scala_library does not,
+    # this might fail. We could possibly extend the BenchmarkGenerator
+    # to accept more than one jar to scan, and then allow multiple labels
+    # in ctx.attr.src
+    outs = info.outputs.jars
+    if len(outs) != 1:
+        print("expected exactly 1 output jar in: " + ctx.label)
+    # just try to take the first one and see if that works
+    class_jar = outs[0].class_jar
+    classpath = info.transitive_runtime_deps
     ctx.actions.run(
         outputs = [ctx.outputs.src_jar, ctx.outputs.resource_jar],
-        inputs = depset([class_jar], transitive = [classpath]),
+        inputs = classpath,
         executable = ctx.executable._generator,
         arguments = [ctx.attr.generator_type] + [
             f.path
@@ -99,7 +103,7 @@ def _scala_generate_benchmark(ctx):
 scala_generate_benchmark = rule(
     implementation = _scala_generate_benchmark,
     attrs = {
-        "src": attr.label(allow_single_file = True, mandatory = True),
+        "src": attr.label(mandatory = True, providers = [[JavaInfo]]),
         "generator_type": attr.string(
             default = "reflection",
             mandatory = False,