From 5150e8d5b9bce6ef7095d54aa26b64292dc81262 Mon Sep 17 00:00:00 2001
From: Robert Sayre <sayrer@gmail.com>
Date: Wed, 17 Apr 2019 12:47:02 -0700
Subject: [PATCH] Add proc-macro support from
 https://github.com/bazelbuild/rules_rust/issues/205

---
 rust/private/rust.bzl | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/rust/private/rust.bzl b/rust/private/rust.bzl
index 2cdcf98ff6..9e29466469 100644
--- a/rust/private/rust.bzl
+++ b/rust/private/rust.bzl
@@ -328,10 +328,27 @@ _rust_test_attrs = {
     ),
 }
 
+def _platform_to_host_transition(settings, attr):
+    if attr.crate_type == "proc-macro":
+        return {
+            "//command_line_option:platforms": "@bazel_tools//platforms:host_platform",
+        }
+    else:
+        return {
+            "//command_line_option:platforms": "@bazel_tools//platforms:target_platform",
+        }
+
+platform_to_host_transition = transition(
+    implementation = _platform_to_host_transition,
+    inputs = [],
+    outputs = ["//command_line_option:platforms"],
+)
+
 rust_library = rule(
     _rust_library_impl,
     attrs = dict(_rust_common_attrs.items() +
                  _rust_library_attrs.items()),
+    cfg = platform_to_host_transition,
     fragments = ["cpp"],
     host_fragments = ["cpp"],
     toolchains = ["@io_bazel_rules_rust//rust:toolchain"],