Add prefix_root option

go/def.bzl

@@ -48,6 +48,13 @@ def _go_prefix(ctx):
     prefix = prefix + "/"
   return prefix
 
+def _prefix_root(ctx):
+  """slash terminated go-prefix"""
+  root = ctx.attr.go_prefix.prefix_root
+  if root != "" and not root.endswith("/"):
+    root = root + "/"
+  return root
+
 # TODO(bazel-team): it would be nice if Bazel had this built-in.
 def symlink_tree_commands(dest_dir, artifact_dict):
   """Symlink_tree_commands returns a list of commands to create the
@@ -165,8 +172,12 @@ def _go_importpath(ctx):
     Go importpath of the library
   """
   path = _go_prefix(ctx)[:-1]
+  prefix_root = _prefix_root(ctx)
   if ctx.label.package:
-    path += "/" + ctx.label.package
+    label = ctx.label.package
+    if ctx.label.package.startswith(prefix_root):
+      label = label[len(prefix_root):]
+    path += "/" + label
   if ctx.label.name != _DEFAULT_LIB:
     path += "/" + ctx.label.name
   if path.rfind(_VENDOR_PREFIX) != -1:

go/private/go_prefix.bzl

@@ -22,18 +22,23 @@
 
 def _go_prefix_impl(ctx):
   """go_prefix_impl provides the go prefix to use as a transitive info provider."""
-  return struct(go_prefix = ctx.attr.prefix)
+  return struct(go_prefix = ctx.attr.prefix, prefix_root = ctx.attr.prefix_root)
 
 _go_prefix_rule = rule(
     _go_prefix_impl,
     attrs = {
         "prefix": attr.string(mandatory = True),
+        "prefix_root": attr.string(default = ""),
     },
 )
 
-def go_prefix(prefix):
-  """go_prefix sets the Go import name to be used for this workspace."""
+def go_prefix(prefix, prefix_root=""):
+  """
+    go_prefix sets the Go import name to be used for this workspace.
+    prefix_root (optional) - subdirectory within the workspace in which go code is located
+  """
   _go_prefix_rule(name = "go_prefix",
     prefix = prefix,
+    prefix_root = prefix_root,
     visibility = ["//visibility:public" ]
   )