diff --git a/.bazelversion b/.bazelversion
index 5e8f65f..9c47c19 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1,4 +1,4 @@
-4.2.0
+5.0.0-pre.20211011.2
 # The first line of this file is used by Bazelisk and Bazel to be sure
 # the right version of Bazel is used to build and test this repo.
 # This also defines which version is used on CI.
diff --git a/BUILD.bazel b/BUILD.bazel
index dc1cc22..4faf351 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -17,7 +17,6 @@ gazelle(
 pkg_tar(
     name = "bazel_lib-" + VERSION,
     srcs = [
-        "LICENSE",
         "README.md",
         "version.bzl",
         "//lib:package_content",
diff --git a/WORKSPACE b/WORKSPACE
index 199482f..ef688a5 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -10,6 +10,11 @@ load(":internal_deps.bzl", "bazel_lib_internal_deps")
 # Fetch deps needed only locally for development
 bazel_lib_internal_deps()
 
+# For running our own unit tests
+load("@bazel_skylib//lib:unittest.bzl", "register_unittest_toolchains")
+
+register_unittest_toolchains()
+
 ############################################
 # Gazelle, for generating bzl_library targets
 load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
diff --git a/docs/expand_make_vars.md b/docs/expand_make_vars.md
index de8fb22..658fb15 100644
--- a/docs/expand_make_vars.md
+++ b/docs/expand_make_vars.md
@@ -2,6 +2,33 @@
 
 Public API for expanding variables
 
+<a id="#expand_template"></a>
+
+## expand_template
+
+<pre>
+expand_template(<a href="#expand_template-name">name</a>, <a href="#expand_template-data">data</a>, <a href="#expand_template-is_executable">is_executable</a>, <a href="#expand_template-out">out</a>, <a href="#expand_template-substitutions">substitutions</a>, <a href="#expand_template-template">template</a>)
+</pre>
+
+Template expansion
+    
+This performs a simple search over the template file for the keys in substitutions,
+and replaces them with the corresponding values.
+
+
+**ATTRIBUTES**
+
+
+| Name  | Description | Type | Mandatory | Default |
+| :------------- | :------------- | :------------- | :------------- | :------------- |
+| <a id="expand_template-name"></a>name |  A unique name for this target.   | <a href="https://bazel.build/docs/build-ref.html#name">Name</a> | required |  |
+| <a id="expand_template-data"></a>data |  List of targets for additional lookup information.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
+| <a id="expand_template-is_executable"></a>is_executable |  Whether to mark the output file as executable.   | Boolean | optional | False |
+| <a id="expand_template-out"></a>out |  Where to write the expanded file.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |
+| <a id="expand_template-substitutions"></a>substitutions |  Mapping of strings to substitutions.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | required |  |
+| <a id="expand_template-template"></a>template |  The template file to expand.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |
+
+
 <a id="#expand_locations"></a>
 
 ## expand_locations
diff --git a/lib/expand_make_vars.bzl b/lib/expand_make_vars.bzl
index 134749b..527a2c3 100644
--- a/lib/expand_make_vars.bzl
+++ b/lib/expand_make_vars.bzl
@@ -3,8 +3,10 @@
 load(
     "//lib/private:expand_make_vars.bzl",
     _expand_locations = "expand_locations",
+    _expand_template = "expand_template",
     _expand_variables = "expand_variables",
 )
 
 expand_locations = _expand_locations
 expand_variables = _expand_variables
+expand_template = _expand_template
diff --git a/lib/private/expand_make_vars.bzl b/lib/private/expand_make_vars.bzl
index b1f6371..ab4c21e 100644
--- a/lib/private/expand_make_vars.bzl
+++ b/lib/private/expand_make_vars.bzl
@@ -162,3 +162,47 @@ def expand_variables(ctx, s, outs = [], output_dir = False, attribute_name = "ar
     additional_substitutions["RULEDIR"] = "/".join([o for o in rule_dir if o])
 
     return ctx.expand_make_variables(attribute_name, s, additional_substitutions)
+
+def _expand_template_impl(ctx):
+    ctx.actions.expand_template(
+        template = ctx.file.template,
+        output = ctx.outputs.out,
+        substitutions = {
+            k: expand_locations(v, ctx.attr.data)
+            for k, v in ctx.attr.substitutions.items()
+        },
+        is_executable = ctx.attr.is_executable,
+    )
+
+expand_template = rule(
+    doc = """Template expansion
+    
+This performs a simple search over the template file for the keys in substitutions,
+and replaces them with the corresponding values.
+""",
+    implementation = _expand_template_impl,
+    attrs = {
+        "template": attr.label(
+            doc = "The template file to expand.",
+            mandatory = True,
+            allow_single_file = True,
+        ),
+        "substitutions": attr.string_dict(
+            doc = "Mapping of strings to substitutions.",
+            mandatory = True,
+        ),
+        "out": attr.output(
+            doc = "Where to write the expanded file.",
+            mandatory = True,
+        ),
+        "is_executable": attr.bool(
+            doc = "Whether to mark the output file as executable.",
+            default = False,
+            mandatory = False,
+        ),
+        "data": attr.label_list(
+            doc = "List of targets for additional lookup information.",
+            allow_files = True,
+        ),
+    },
+)
diff --git a/lib/private/params_file.bzl b/lib/private/params_file.bzl
index 12c5a2a..db61435 100644
--- a/lib/private/params_file.bzl
+++ b/lib/private/params_file.bzl
@@ -2,9 +2,6 @@
 
 load("//lib/private:expand_make_vars.bzl", "expand_locations")
 
-_DOC = """Generates a params file from a list of arguments."""
-
-# See params_file macro below for docstrings
 _ATTRS = {
     "args": attr.string_list(),
     "data": attr.label_list(allow_files = True),
@@ -53,5 +50,4 @@ params_file = rule(
     implementation = _impl,
     provides = [DefaultInfo],
     attrs = _ATTRS,
-    doc = _DOC,
 )
diff --git a/lib/tests/BUILD.bazel b/lib/tests/BUILD.bazel
new file mode 100644
index 0000000..580c74e
--- /dev/null
+++ b/lib/tests/BUILD.bazel
@@ -0,0 +1,3 @@
+load(":expand_make_vars_test.bzl", "expand_make_vars_test_suite")
+
+expand_make_vars_test_suite(name = "expand_make_vars_test")
diff --git a/lib/tests/expand_make_vars_test.bzl b/lib/tests/expand_make_vars_test.bzl
new file mode 100644
index 0000000..e63c6f4
--- /dev/null
+++ b/lib/tests/expand_make_vars_test.bzl
@@ -0,0 +1,29 @@
+"""Unit tests for starlark helpers
+See https://docs.bazel.build/versions/main/skylark/testing.html#for-testing-starlark-utilities
+"""
+
+load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
+load("//lib/private:expand_make_vars.bzl", "expand_variables")
+
+def _variables_test_impl(ctx):
+    env = unittest.begin(ctx)
+    capture_subs = {}
+    fake_ctx = struct(
+        bin_dir = struct(path = "bazel-bin"),
+        label = struct(
+            workspace_root = "my-wksp",
+            package = "path/to",
+        ),
+        expand_make_variables = lambda attr, expr, subs: capture_subs.update(subs),
+    )
+    expand_variables(fake_ctx, "output=$(@D)")
+    expected = {"@D": "bazel-bin/my-wksp/path/to", "RULEDIR": "bazel-bin/my-wksp/path/to"}
+    asserts.equals(env, expected, capture_subs)
+    return unittest.end(env)
+
+# The unittest library requires that we export the test cases as named test rules,
+# but their names are arbitrary and don't appear anywhere.
+t0_test = unittest.make(_variables_test_impl)
+
+def expand_make_vars_test_suite(name):
+    unittest.suite(name, t0_test)