Include transitive out_dirs in linking action (#3579)

If a build script compiles a file which is needed at linking time, and
adds a linker flag to add it to the search path, we need the out_dir to
be present in the link action in order for the search path flag to
actually take effect.

This was the original behaviour way back in
3a569b8 but got removed in
9426a38 which was intending to only
restrict where flags were propagated but also stopped propagating
out_dir files.

This is required because cargo uses the path to the out_dir as a
well-known-path that doesn't move between builds, so link flags from a
build script which get transitively propagated up to a link action
happen to point at the right location on disk. We need to propagate this
out_dir because the known location on disk is probably in a sandbox, so
we need to explicitly materialise the files into the sandbox.

Author: illicitonion

rust/private/rustc.bzl

@@ -1891,6 +1891,7 @@ def _create_extra_input_args(build_info, dep_info, include_link_flags = True):
     build_env_file = None
     build_flags_files = []
 
+    # We include the direct dep build_info because crates which use cargo build scripts may need to e.g. include_str! a generated file.
     if build_info:
         if build_info.out_dir:
             out_dir = build_info.out_dir.path
@@ -1904,6 +1905,12 @@ def _create_extra_input_args(build_info, dep_info, include_link_flags = True):
 
         input_depsets.append(build_info.compile_data)
 
+    # We include transitive dep build_infos because cargo build scripts may generate files which get linked into the final binary.
+    # This should probably only actually be exposed to actions which link.
+    for dep_build_info in dep_info.transitive_build_infos.to_list():
+        if dep_build_info.out_dir:
+            input_files.append(dep_build_info.out_dir)
+
     out_dir_compile_inputs = depset(
         input_files,
         transitive = [dep_info.link_search_path_files, dep_info.transitive_data] + input_depsets,

test/unit/common.bzl

@@ -70,6 +70,24 @@ def _startswith(list, prefix):
             return False
     return True
 
+def assert_list_contains(env, list, expected_element):
+    """Assert that list contains the expected element.
+
+    Args:
+          env: env from analysistest.begin(ctx).
+          list: list supposed to contain expected_element.
+          expected_element: element to be found inside list.
+    """
+    if expected_element in list:
+        return
+    unittest.fail(
+        env,
+        "Expected the to find '{expected_element}' within '{list}'".format(
+            expected_element = expected_element,
+            list = list,
+        ),
+    )
+
 def assert_list_contains_adjacent_elements(env, list_under_test, adjacent_elements):
     """Assert that list_under_test contains given adjacent flags.
 

test/unit/transitive_link_search_paths/bin.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("dummy file");
+}

test/unit/transitive_link_search_paths/transitive_link_search_paths_test.bzl

@@ -2,7 +2,8 @@
 
 load("@bazel_skylib//lib:unittest.bzl", "analysistest", "asserts")
 load("//cargo:defs.bzl", "cargo_build_script")
-load("//rust:defs.bzl", "rust_common", "rust_library", "rust_proc_macro")
+load("//rust:defs.bzl", "rust_binary", "rust_common", "rust_library", "rust_proc_macro")
+load("//test/unit:common.bzl", "assert_action_mnemonic", "assert_list_contains")
 
 def _transitive_link_search_paths_test_impl(ctx):
     env = analysistest.begin(ctx)
@@ -18,6 +19,19 @@ def _transitive_link_search_paths_test_impl(ctx):
 
 transitive_link_search_paths_test = analysistest.make(_transitive_link_search_paths_test_impl)
 
+def _transitive_out_dir_test_impl(ctx):
+    env = analysistest.begin(ctx)
+    tut = analysistest.target_under_test(env)
+    action = tut.actions[0]
+    assert_action_mnemonic(env, action, "Rustc")
+    inputs = action.inputs.to_list()
+    input_basenames = [f.basename for f in inputs]
+    assert_list_contains(env, input_basenames, "dep_build_script.out_dir")
+
+    return analysistest.end(env)
+
+transitive_out_dir_test = analysistest.make(_transitive_out_dir_test_impl)
+
 def _transitive_link_search_paths_test():
     cargo_build_script(
         name = "proc_macro_build_script",
@@ -46,11 +60,23 @@ def _transitive_link_search_paths_test():
         deps = [":dep_build_script"],
     )
 
+    rust_binary(
+        name = "bin",
+        srcs = ["bin.rs"],
+        edition = "2018",
+        deps = [":dep"],
+    )
+
     transitive_link_search_paths_test(
         name = "transitive_link_search_paths_test",
         target_under_test = ":dep",
     )
 
+    transitive_out_dir_test(
+        name = "transitive_out_dir_test",
+        target_under_test = ":bin",
+    )
+
 def transitive_link_search_paths_test_suite(name):
     """Entry-point macro called from the BUILD file.
 
@@ -63,5 +89,6 @@ def transitive_link_search_paths_test_suite(name):
         name = name,
         tests = [
             ":transitive_link_search_paths_test",
+            ":transitive_out_dir_test",
         ],
     )