Expand arg location in rustc_flags attribute (#809)

Expand locations in rustc_flags.
Fixes #801.

Author: sayrer

cargo/cargo_build_script.bzl

@@ -8,7 +8,7 @@ load("//rust:rust.bzl", "rust_binary")
 load("//rust/private:rustc.bzl", "BuildInfo", "get_compilation_mode_opts", "get_linker_and_args")
 
 # buildifier: disable=bzl-visibility
-load("//rust/private:utils.bzl", "expand_locations", "find_cc_toolchain", "find_toolchain", "name_to_crate_name")
+load("//rust/private:utils.bzl", "expand_dict_value_locations", "find_cc_toolchain", "find_toolchain", "name_to_crate_name")
 
 def get_cc_compile_env(cc_toolchain, feature_configuration):
     """Gather cc environment variables from the given `cc_toolchain`
@@ -117,7 +117,7 @@ def _build_script_impl(ctx):
     for f in ctx.attr.crate_features:
         env["CARGO_FEATURE_" + f.upper().replace("-", "_")] = "1"
 
-    env.update(expand_locations(
+    env.update(expand_dict_value_locations(
         ctx,
         ctx.attr.build_script_env,
         getattr(ctx.attr, "data", []) +

docs/defs.md

@@ -0,0 +1,23 @@
+load(
+    "@rules_rust//rust:rust.bzl",
+    "rust_test",
+)
+
+# generate a file containing cfg flags
+genrule(
+    name = "flag_generator",
+    outs = ["generated_flag.data"],
+    cmd = "echo --cfg=test_flag > $@",
+)
+
+rust_test(
+    name = "test",
+    srcs = [
+        "main.rs",
+    ],
+    data = [":flag_generator"],
+    edition = "2018",
+    rustc_flags = [
+        "@$(location :flag_generator)",
+    ],
+)

docs/flatten.md

@@ -0,0 +1,9 @@
+#[test]
+fn test() {
+    // we should be able to read rustc args from a generated file
+    if cfg!(test_flag) {
+        return;
+    }
+
+    unreachable!();
+}

examples/flag_locations/BUILD.bazel

@@ -20,7 +20,7 @@ load(
     "crate_name_from_attr",
     "dedent",
     "determine_output_hash",
-    "expand_locations",
+    "expand_dict_value_locations",
     "find_toolchain",
 )
 
@@ -340,7 +340,7 @@ def _create_test_launcher(ctx, toolchain, output, providers):
 
     # Expand the environment variables and write them to a file
     environ_file = ctx.actions.declare_file(launcher_filename + ".launchfiles/env")
-    environ = expand_locations(
+    environ = expand_dict_value_locations(
         ctx,
         getattr(ctx.attr, "env", {}),
         data,
@@ -620,7 +620,14 @@ _common_attrs = {
         """),
     ),
     "rustc_flags": attr.string_list(
-        doc = "List of compiler flags passed to `rustc`.",
+        doc = dedent("""\
+            List of compiler flags passed to `rustc`.
+
+            These strings are subject to Make variable expansion for predefined
+            source/output path variables like `$location`, `$execpath`, and `$rootpath`.
+            This expansion is useful if you wish to pass a generated file of
+            arguments to rustc: `@$(location //package:target)`.
+        """),
     ),
     # TODO(stardoc): How do we provide additional documentation to an inherited attribute?
     # "name": attr.string(

examples/flag_locations/main.rs

@@ -21,7 +21,8 @@ load("//rust/private:common.bzl", "rust_common")
 load(
     "//rust/private:utils.bzl",
     "crate_name_from_attr",
-    "expand_locations",
+    "expand_dict_value_locations",
+    "expand_list_element_locations",
     "find_cc_toolchain",
     "get_lib_name",
     "get_preferred_artifact",
@@ -438,9 +439,16 @@ def construct_arguments(
 
     # Tell Rustc where to find the standard library
     args.add_all(rust_lib_paths, before_each = "-L", format_each = "%s")
-
     args.add_all(rust_flags)
-    args.add_all(getattr(attr, "rustc_flags", []))
+
+    data_paths = getattr(attr, "data", []) + getattr(attr, "compile_data", [])
+    args.add_all(
+        expand_list_element_locations(
+            ctx,
+            getattr(attr, "rustc_flags", []),
+            data_paths,
+        ),
+    )
     add_edition_flags(args, crate_info)
 
     # Link!
@@ -471,11 +479,10 @@ def construct_arguments(
                 env["CARGO_BIN_EXE_" + dep_crate_info.output.basename] = dep_crate_info.output.short_path
 
     # Update environment with user provided variables.
-    env.update(expand_locations(
+    env.update(expand_dict_value_locations(
         ctx,
         crate_info.rustc_env,
-        getattr(attr, "data", []) +
-        getattr(attr, "compile_data", []),
+        data_paths,
     ))
 
     # This empty value satisfies Clippy, which otherwise complains about the

rust/private/rust.bzl

@@ -152,7 +152,7 @@ def _expand_location(ctx, env, data):
             env = env.replace(directive, "${pwd}/" + directive)
     return ctx.expand_location(env, data)
 
-def expand_locations(ctx, env, data):
+def expand_dict_value_locations(ctx, env, data):
     """Performs location-macro expansion on string values.
 
     $(execroot ...) and $(location ...) are prefixed with ${pwd},
@@ -167,6 +167,8 @@ def expand_locations(ctx, env, data):
     as compilation happens in a separate sandbox folder, so when it comes time
     to read the file at runtime, the path is no longer valid.
 
+    See [`expand_location`](https://docs.bazel.build/versions/main/skylark/lib/ctx.html#expand_location) for detailed documentation.
+
     Args:
         ctx (ctx): The rule's context object
         env (dict): A dict whose values we iterate over
@@ -179,6 +181,27 @@ def expand_locations(ctx, env, data):
     """
     return dict([(k, _expand_location(ctx, v, data)) for (k, v) in env.items()])
 
+def expand_list_element_locations(ctx, args, data):
+    """Performs location-macro expansion on a list of string values.
+
+    $(execroot ...) and $(location ...) are prefixed with ${pwd},
+    which process_wrapper and build_script_runner will expand at run time
+    to the absolute path.
+
+    See [`expand_location`](https://docs.bazel.build/versions/main/skylark/lib/ctx.html#expand_location) for detailed documentation.
+
+    Args:
+        ctx (ctx): The rule's context object
+        args (list): A list we iterate over
+        data (sequence of Targets): The targets which may be referenced by
+            location macros. This is expected to be the `data` attribute of
+            the target, though may have other targets or attributes mixed in.
+
+    Returns:
+        list: A list of arguments with expanded location macros
+    """
+    return [_expand_location(ctx, arg, data) for arg in args]
+
 def name_to_crate_name(name):
     """Converts a build target's name into the name of its associated crate.
 

rust/private/rustc.bzl

@@ -0,0 +1,4 @@
+load(":location_expansion_test.bzl", "location_expansion_test_suite")
+
+############################ UNIT TESTS #############################
+location_expansion_test_suite(name = "location_expansion_test_suite")

rust/private/utils.bzl

@@ -0,0 +1,54 @@
+"""Unittest to verify location expansion in rustc flags"""
+
+load("@bazel_skylib//lib:unittest.bzl", "analysistest")
+load("//rust:defs.bzl", "rust_library")
+load("//test/unit:common.bzl", "assert_action_mnemonic", "assert_argv_contains")
+
+def _location_expansion_rustc_flags_test(ctx):
+    env = analysistest.begin(ctx)
+    tut = analysistest.target_under_test(env)
+    action = tut.actions[0]
+    argv = action.argv
+    assert_action_mnemonic(env, action, "Rustc")
+    assert_argv_contains(env, action, "test/unit/location_expansion/mylibrary.rs")
+    expected = "@${pwd}/" + ctx.bin_dir.path + "/test/unit/location_expansion/generated_flag.data"
+    assert_argv_contains(env, action, expected)
+    return analysistest.end(env)
+
+location_expansion_rustc_flags_test = analysistest.make(_location_expansion_rustc_flags_test)
+
+def _location_expansion_test():
+    native.genrule(
+        name = "flag_generator",
+        outs = ["generated_flag.data"],
+        cmd = "echo --cfg=test_flag > $@",
+    )
+
+    rust_library(
+        name = "mylibrary",
+        srcs = ["mylibrary.rs"],
+        rustc_flags = [
+            "@$(location :flag_generator)",
+        ],
+        compile_data = [":flag_generator"],
+    )
+
+    location_expansion_rustc_flags_test(
+        name = "location_expansion_rustc_flags_test",
+        target_under_test = ":mylibrary",
+    )
+
+def location_expansion_test_suite(name):
+    """Entry-point macro called from the BUILD file.
+
+    Args:
+        name: Name of the macro.
+    """
+    _location_expansion_test()
+
+    native.test_suite(
+        name = name,
+        tests = [
+            ":location_expansion_rustc_flags_test",
+        ],
+    )