Add toolchains argument to unittests.make (#483)

* Add toolchains argument to unittests.make

Make unittests.make bypass toolchains arguments to target rule's constructor.

* update doc

---------

Co-authored-by: JiaYan Lin <jiayanl@google.com>

docs/unittest_doc.md

diff --git a/docs/unittest_doc.md b/docs/unittest_doc.md
index 353d05b..99000e4 100755
--- a/docs/unittest_doc.md
+++ b/docs/unittest_doc.md
@@ -426,7 +426,7 @@
 ## unittest.make
 
 <pre>
-unittest.make(<a href="#unittest.make-impl">impl</a>, <a href="#unittest.make-attrs">attrs</a>, <a href="#unittest.make-doc">doc</a>)
+unittest.make(<a href="#unittest.make-impl">impl</a>, <a href="#unittest.make-attrs">attrs</a>, <a href="#unittest.make-doc">doc</a>, <a href="#unittest.make-toolchains">toolchains</a>)
 </pre>
 
 Creates a unit test rule from its implementation function.
@@ -439,6 +439,9 @@
 The optional `attrs` argument can be used to define dependencies for this
 test, in order to form unit tests of rules.
 
+The optional `toolchains` argument can be used to define toolchain
+dependencies for this test.
+
 An example of a unit test:
 
 ```
@@ -463,6 +466,7 @@
 | <a id="unittest.make-impl"></a>impl |  The implementation function of the unit test.   |  none |
 | <a id="unittest.make-attrs"></a>attrs |  An optional dictionary to supplement the attrs passed to the unit test's <code>rule()</code> constructor.   |  <code>{}</code> |
 | <a id="unittest.make-doc"></a>doc |  A description of the rule that can be extracted by documentation generating tools.   |  <code>""</code> |
+| <a id="unittest.make-toolchains"></a>toolchains |  An optional list to supplement the toolchains passed to the unit test's <code>rule()</code> constructor.   |  <code>[]</code> |
 
 **RETURNS**
 

lib/unittest.bzl

diff --git a/lib/unittest.bzl b/lib/unittest.bzl
index 02e374c..3860ad1 100644
--- a/lib/unittest.bzl
+++ b/lib/unittest.bzl
@@ -148,7 +148,7 @@
     impl_name = impl_name.partition("<function ")[-1]
     return impl_name.rpartition(">")[0]
 
-def _make(impl, attrs = {}, doc = ""):
+def _make(impl, attrs = {}, doc = "", toolchains = []):
     """Creates a unit test rule from its implementation function.
 
     Each unit test is defined in an implementation function that must then be
@@ -159,6 +159,9 @@
     The optional `attrs` argument can be used to define dependencies for this
     test, in order to form unit tests of rules.
 
+    The optional `toolchains` argument can be used to define toolchain
+    dependencies for this test.
+
     An example of a unit test:
 
     ```
@@ -179,6 +182,8 @@
       attrs: An optional dictionary to supplement the attrs passed to the
           unit test's `rule()` constructor.
       doc: A description of the rule that can be extracted by documentation generating tools.
+      toolchains: An optional list to supplement the toolchains passed to
+          the unit test's `rule()` constructor.
 
     Returns:
       A rule definition that should be stored in a global whose name ends in
@@ -193,7 +198,7 @@
         attrs = attrs,
         _skylark_testable = True,
         test = True,
-        toolchains = [TOOLCHAIN_TYPE],
+        toolchains = toolchains + [TOOLCHAIN_TYPE],
     )
 
 _ActionInfo = provider(