Factor out location expansion

Addressing review comment
#132 (comment)

nixpkgs/BUILD.bazel

@@ -37,6 +37,7 @@ bzl_library(
     name = "nixpkgs",
     srcs = [
         "nixpkgs.bzl",
+        "private/location_expansion.bzl",
     ],
     visibility = ["//visibility:public"],
     deps = [

nixpkgs/nixpkgs.bzl

@@ -1,6 +1,5 @@
 """Rules for importing Nixpkgs packages."""
 
-load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_skylib//lib:sets.bzl", "sets")
 load("@bazel_skylib//lib:versions.bzl", "versions")
 load("@bazel_tools//tools/cpp:cc_configure.bzl", "cc_autoconf_impl")
@@ -11,6 +10,7 @@ load(
     "write_builtin_include_directory_paths",
 )
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
+load(":private/location_expansion.bzl", "expand_location")
 
 def _nixpkgs_git_repository_impl(repository_ctx):
     repository_ctx.file(
@@ -82,74 +82,6 @@ nixpkgs_local_repository = repository_rule(
 def _is_supported_platform(repository_ctx):
     return repository_ctx.which("nix-build") != None
 
-def _expand_location(repository_ctx, string, labels, attr = None):
-    """Expand `$(location label)` to a path.
-
-    Raises an error on unexpected occurrences of `$`.
-    Use `$$` to insert a verbatim `$`.
-
-    Attrs:
-      repository_ctx: The repository rule context.
-      string: string, Replace instances of `$(location )` in this string.
-      labels: dict from label to path: Known label to path mappings.
-      attr: string, The rule attribute to use for error reporting.
-
-    Returns:
-      The string with all instances of `$(location )` replaced by paths.
-    """
-    result = ""
-    offset = 0
-
-    # Step through occurrences of `$`. This is bounded by the length of the string.
-    for _ in range(len(string)):
-        start = string.find("$", offset)
-        if start == -1:
-            result += string[offset:]
-            break
-        else:
-            result += string[offset:start]
-        if start + 1 == len(string):
-            fail("Unescaped '$' in location expansion at end of input", attr)
-        elif string[start + 1] == "$":
-            # Insert verbatim '$'.
-            result += "$"
-            offset = start + 2
-        elif string[start + 1] == "(":
-            group_start = start + 2
-            group_end = string.find(")", group_start)
-            if group_end == -1:
-                fail("Unbalanced parentheses in location expansion for '{}'.".format(string[start:]), attr)
-            group = string[group_start:group_end]
-            if group.startswith("location "):
-                label_str = group[len("location "):]
-                label_candidates = [
-                    (lbl, path)
-                    for (lbl, path) in labels.items()
-                    if lbl.relative(label_str) == lbl
-                ]
-                if len(label_candidates) == 0:
-                    fail("Unknown label '{}' in location expansion for '{}'.".format(label_str, string), attr)
-                elif len(label_candidates) > 1:
-                    fail(
-                        "Ambiguous label '{}' in location expansion for '{}'. Candidates: {}".format(
-                            label_str,
-                            string,
-                            ", ".join([str(lbl) for lbl in label_candidates]),
-                        ),
-                        attr,
-                    )
-                location = paths.join(".", paths.relativize(
-                    str(repository_ctx.path(label_candidates[0][1])),
-                    str(repository_ctx.path(".")),
-                ))
-                result += location
-            else:
-                fail("Unrecognized location expansion '$({})'.".format(group), attr)
-            offset = group_end + 1
-        else:
-            fail("Unescaped '$' in location expansion at position {} of input.".format(start), attr)
-    return result
-
 def _nixpkgs_package_impl(repository_ctx):
     repository = repository_ctx.attr.repository
     repositories = repository_ctx.attr.repositories
@@ -215,7 +147,7 @@ def _nixpkgs_package_impl(repository_ctx):
     ])
 
     expr_args.extend([
-        _expand_location(repository_ctx, opt, nix_file_deps, "nixopts")
+        expand_location(repository_ctx, opt, nix_file_deps, "nixopts")
         for opt in repository_ctx.attr.nixopts
     ])
 

nixpkgs/private/location_expansion.bzl

@@ -0,0 +1,69 @@
+load("@bazel_skylib//lib:paths.bzl", "paths")
+
+def expand_location(repository_ctx, string, labels, attr = None):
+    """Expand `$(location label)` to a path.
+
+    Raises an error on unexpected occurrences of `$`.
+    Use `$$` to insert a verbatim `$`.
+
+    Attrs:
+      repository_ctx: The repository rule context.
+      string: string, Replace instances of `$(location )` in this string.
+      labels: dict from label to path: Known label to path mappings.
+      attr: string, The rule attribute to use for error reporting.
+
+    Returns:
+      The string with all instances of `$(location )` replaced by paths.
+    """
+    result = ""
+    offset = 0
+
+    # Step through occurrences of `$`. This is bounded by the length of the string.
+    for _ in range(len(string)):
+        start = string.find("$", offset)
+        if start == -1:
+            result += string[offset:]
+            break
+        else:
+            result += string[offset:start]
+        if start + 1 == len(string):
+            fail("Unescaped '$' in location expansion at end of input", attr)
+        elif string[start + 1] == "$":
+            # Insert verbatim '$'.
+            result += "$"
+            offset = start + 2
+        elif string[start + 1] == "(":
+            group_start = start + 2
+            group_end = string.find(")", group_start)
+            if group_end == -1:
+                fail("Unbalanced parentheses in location expansion for '{}'.".format(string[start:]), attr)
+            group = string[group_start:group_end]
+            if group.startswith("location "):
+                label_str = group[len("location "):]
+                label_candidates = [
+                    (lbl, path)
+                    for (lbl, path) in labels.items()
+                    if lbl.relative(label_str) == lbl
+                ]
+                if len(label_candidates) == 0:
+                    fail("Unknown label '{}' in location expansion for '{}'.".format(label_str, string), attr)
+                elif len(label_candidates) > 1:
+                    fail(
+                        "Ambiguous label '{}' in location expansion for '{}'. Candidates: {}".format(
+                            label_str,
+                            string,
+                            ", ".join([str(lbl) for lbl in label_candidates]),
+                        ),
+                        attr,
+                    )
+                location = paths.join(".", paths.relativize(
+                    str(repository_ctx.path(label_candidates[0][1])),
+                    str(repository_ctx.path(".")),
+                ))
+                result += location
+            else:
+                fail("Unrecognized location expansion '$({})'.".format(group), attr)
+            offset = group_end + 1
+        else:
+            fail("Unescaped '$' in location expansion at position {} of input.".format(start), attr)
+    return result