Add lib/labels.bzl

Author: EricCousineau-TRI

lib/BUILD

@@ -20,6 +20,11 @@ bzl_library(
     srcs = ["dicts.bzl"],
 )
 
+bzl_library(
+    name = "labels",
+    srcs = ["labels.bzl"],
+)
+
 bzl_library(
     name = "partial",
     srcs = ["partial.bzl"],

lib/labels.bzl

@@ -0,0 +1,77 @@
+"""
+Skylib module containing additional sugar for resolving or analyzing labels
+across workspaces / repositories and packages.
+"""
+
+# Possibly helpful pointers / complaints:
+# https://github.com/bazelbuild/bazel/issues/16210
+
+def strip_prefix(prefix, value):
+    if not value.startswith(prefix):
+        fail("bad")
+    return value[len(prefix):]
+
+def qualify_package(label):
+    """
+    Returns a "qualified package" name, that is "@repository//package".
+    """
+    if in_root_workspace(label):
+        return "//" + label.package
+    elif label.package == "":
+        return "@" + label.workspace_name
+    else:
+        return "@" + label.workspace_name + "//" + label.package
+
+def current_qualified_package():
+    """
+    Gets the current package as "qualified package".
+    """
+    label_str = "//" + native.package_name()
+    repository = native.repository_name()
+    if repository == "@":
+        return "//" + native.package_name()
+    else:
+        return repository + "//" + native.package_name()
+
+def to_label(label_str):
+    """
+    Converts any label string to a Label instance.
+    Normally, Label() requires absolute paths, so this wraps the constructor to
+    allow automatic scoping.
+    This can be used to parse / canonicalize labels.
+    """
+    if label_str.startswith(":"):
+        label_str = current_qualified_package() + label_str
+    return Label(label_str)
+
+def in_workspace(label, workspace):
+    """True if given Label is in the specified workspace / repository."""
+    if workspace.startswith("@"):
+        fail("Please supply name without @")
+    return label.workspace_name == workspace
+
+def in_root_workspace(label):
+    """True if given Label is in source workspace."""
+    return in_workspace(label, "")
+
+def workspace_name():
+    """Repository name, but without leading @."""
+    return strip_prefix("@", native.repository_name())
+
+def in_current_package(label):
+    """True if given label is in the current package."""
+    return (
+        label.workspace_name == workspace_name() and
+        label.package == native.package_name()
+    )
+
+labels = struct(
+    strip_prefix = strip_prefix,
+    qualify_package = qualify_package,
+    current_qualified_package = current_qualified_package,
+    to_label = to_label,
+    in_workspace = in_workspace,
+    in_root_workspace = in_root_workspace,
+    workspace_name = workspace_name,
+    in_current_package = in_current_package,
+)