Define unit tests for location expansion

Addressing review comment
#132 (comment)

WORKSPACE

@@ -16,6 +16,10 @@ load(
 
 # For tests
 
+load("@bazel_skylib//lib:unittest.bzl", "register_unittest_toolchains")
+
+register_unittest_toolchains()
+
 nixpkgs_git_repository(
     name = "remote_nixpkgs",
     remote = "https://github.com/NixOS/nixpkgs",

tests/BUILD.bazel

@@ -1,6 +1,9 @@
 package(default_testonly = 1)
 
 load("@io_bazel_rules_go//go:def.bzl", "go_binary")
+load(":location_expansion_unit_test.bzl", "expand_location_unit_test_suite")
+
+expand_location_unit_test_suite()
 
 [
     # All of these tests use the "hello" binary to see

tests/location_expansion_unit_test.bzl

@@ -0,0 +1,145 @@
+load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
+load(
+    "//nixpkgs:private/location_expansion.bzl",
+    "parse_expand_location",
+    "resolve_label",
+)
+
+def _parse_expand_location_test(ctx):
+    env = unittest.begin(ctx)
+
+    asserts.equals(
+        env,
+        expected = ([], None),
+        actual = parse_expand_location(""),
+        msg = "Parses the empty string",
+    )
+
+    asserts.equals(
+        env,
+        expected = ([("string", "plain string")], None),
+        actual = parse_expand_location("plain string"),
+        msg = "Parses a plain string",
+    )
+
+    asserts.equals(
+        env,
+        expected = ([("string", "$")], None),
+        actual = parse_expand_location("$$"),
+        msg = "Parses an escaped dollar sign",
+    )
+
+    asserts.equals(
+        env,
+        expected = ([("location", "@workspace//package:target")], None),
+        actual = parse_expand_location("$(location @workspace//package:target)"),
+        msg = "Parses a location command",
+    )
+
+    asserts.equals(
+        env,
+        expected = ([
+            ("string", "before "),
+            ("location", "//label:1"),
+            ("string", " "),
+            ("string", "$"),
+            ("string", " "),
+            ("location", "//label:2"),
+            ("string", " after"),
+        ], None),
+        actual = parse_expand_location(
+            "before $(location //label:1) $$ $(location //label:2) after",
+        ),
+        msg = "Parses a complex location expansion string",
+    )
+
+    asserts.equals(
+        env,
+        expected = (None, "Unescaped '$' in location expansion at position 0 of input."),
+        actual = parse_expand_location("$"),
+        msg = "Fails on unescaped dollar sign",
+    )
+
+    asserts.equals(
+        env,
+        expected = (None, "Unbalanced parentheses in location expansion for '$(location //label:1'."),
+        actual = parse_expand_location("$(location //label:1"),
+        msg = "Fails on unbalanced parentheses",
+    )
+
+    asserts.equals(
+        env,
+        expected = (None, "Unrecognized location expansion '$(misspelled)'."),
+        actual = parse_expand_location("$(misspelled)"),
+        msg = "Fails on unknown location expansion command",
+    )
+
+    return unittest.end(env)
+
+parse_expand_location_test = unittest.make(_parse_expand_location_test)
+
+def _resolve_label_test(ctx):
+    env = unittest.begin(ctx)
+
+    asserts.equals(
+        env,
+        expected = ("correct/path", None),
+        actual = resolve_label(
+            "@workspace//package:target",
+            {
+                Label("@workspace//package:target"): "correct/path",
+                Label("@another//package:target"): "wrong/path",
+            },
+        ),
+        msg = "Finds an absolute label",
+    )
+
+    asserts.equals(
+        env,
+        expected = ("correct/path", None),
+        actual = resolve_label(
+            "//package:target",
+            {
+                Label("@workspace//package:target"): "correct/path",
+                Label("@another//different:target"): "wrong/path",
+            },
+        ),
+        msg = "Finds an unambiguous relative label",
+    )
+
+    asserts.equals(
+        env,
+        expected = (None, "Unknown label '@unknown//package:target' in location expansion."),
+        actual = resolve_label(
+            "@unknown//package:target",
+            {
+                Label("@workspace//package:target"): "wrong/path",
+                Label("@another//package:target"): "another/wrong/path",
+            },
+        ),
+        msg = "Fails on an unknown label",
+    )
+
+    asserts.equals(
+        env,
+        expected = (None, "Ambiguous label '//package:target' in location expansion. Candidates: @workspace//package:target, @another//package:target"),
+        actual = resolve_label(
+            "//package:target",
+            {
+                Label("@workspace//package:target"): "wrong/path",
+                Label("@another//package:target"): "another/wrong/path",
+            },
+        ),
+        msg = "Fails on an ambiguous relative label",
+    )
+
+    return unittest.end(env)
+
+resolve_label_test = unittest.make(_resolve_label_test)
+
+def expand_location_unit_test_suite():
+    unittest.suite(
+        "expand_location_unit_test_suite",
+        parse_expand_location_test,
+        resolve_label_test,
+    )