Allow arbitrary expressions in nixpkgs_package

This allows us to be much more flexible when we want to be while
maintaining convenience for simple attribute cases.

Author: Fuuzetsu

WORKSPACE

@@ -10,3 +10,15 @@ nixpkgs_git_repository(
 )
 
 nixpkgs_package(name = "hello", repository = "@nixpkgs")
+
+nixpkgs_package(
+  name = "expr-test",
+  expression = "let pkgs = import <nixpkgs> {}; in pkgs.hello",
+  repository = "@nixpkgs"
+)
+
+nixpkgs_package(
+  name = "attribute-test",
+  attribute = "hello",
+  repository = "@nixpkgs"
+)

nixpkgs/nixpkgs.bzl

@@ -15,6 +15,16 @@ nixpkgs_git_repository = repository_rule(
   local = False,
 )
 
+def _mk_build_expression(ctx):
+  """Generate a nix expression that picks a package from nixpkgs.
+  """
+  if ctx.attr.attribute and ctx.attr.expression:
+    fail("'attribute' and 'expression' are mutually exclusive.")
+  elif ctx.attr.expression:
+    return ["-E", ctx.attr.expression]
+  else:
+    return ["-E", "(import <nixpkgs> {{}}).{0}".format(ctx.attr.attribute or ctx.attr.name)]
+
 def _nixpkgs_package_impl(ctx):
   if ctx.attr.build_file and ctx.attr.build_file_content:
     fail("Specify one of 'build_file' or 'build_file_content', but not both.")
@@ -25,19 +35,22 @@ def _nixpkgs_package_impl(ctx):
   else:
     ctx.template("BUILD", Label("@io_tweag_rules_nixpkgs//nixpkgs:BUILD.pkg"))
 
-  path = '<nixpkgs>'
+  # If neither repository or path are set, leave empty which will use
+  # default value from NIX_PATH
+  path = ''
   if ctx.attr.repository and ctx.attr.path:
     fail("'repository' and 'path' fields are mutually exclusive.")
   if ctx.attr.repository:
     # XXX Another hack: the repository label typically resolves to
     # some top-level package in the external workspace. So we use
     # dirname to get the actual workspace path.
-    path = ctx.path(ctx.attr.repository).dirname
+    path = "-I nixpkgs={0}".format(ctx.path(ctx.attr.repository).dirname)
   if ctx.attr.path:
-    path = ctx.attr.path
+    path = "-I nixpkgs={0}".format(ctx.attr.path)
+
+  buildExpr = _mk_build_expression(ctx)
+  buildCmd = ["nix-build", path, "--no-out-link"] + buildExpr
 
-  attr_path = ctx.attr.attribute_path or ctx.name
-  buildCmd = ["nix-build", path, "-A", attr_path, "--no-out-link"]
   res = ctx.execute(buildCmd, quiet = False)
   if res.return_code == 0:
     output_path = res.stdout.splitlines()[-1]
@@ -48,7 +61,12 @@ def _nixpkgs_package_impl(ctx):
 nixpkgs_package = repository_rule(
   implementation = _nixpkgs_package_impl,
   attrs = {
-    "attribute_path": attr.string(),
+    "attribute": attr.string(
+      doc="Nix attribute to build. Exclusive to expression."
+    ),
+    "expression": attr.string(
+      doc="Nix expression to build. Rule name used as attribute if not present.",
+    ),
     "path": attr.string(),
     "repository": attr.label(),
     "build_file": attr.string(),

tests/BUILD

@@ -1,9 +1,13 @@
 package(default_testonly = 1)
 
-sh_test(
-  name = "run-hello",
+[sh_test(
+  name= "run-{0}".format(test),
   srcs = ["test_bin.sh"],
-  args = ["$(location @hello//:bin)"],
-  data = ["@hello//:bin"],
+  args=["$(location @{0}//:bin)".format(test)],
+  data = ["@{0}//:bin".format(test)],
   timeout = "short",
-)
+) for test in [
+    "hello",
+    "expr-test",
+    "attribute-test",
+]]