Avoid duplicate native linker flags for binaries with a package library

cargo/tests/cargo_build_script/duplicate_link_flags/BUILD.bazel

@@ -0,0 +1,3 @@
+load(":duplicate_link_flags_test.bzl", "duplicate_link_flags_test_suite")
+
+duplicate_link_flags_test_suite(name = "duplicate_link_flags_test_suite")

cargo/tests/cargo_build_script/duplicate_link_flags/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rustc-link-lib=static=example");
+}

cargo/tests/cargo_build_script/duplicate_link_flags/duplicate_link_flags_test.bzl

@@ -0,0 +1,79 @@
+"""Check when a package build script's native link flags reach a Rustc action."""
+
+load("@bazel_skylib//lib:unittest.bzl", "analysistest", "asserts")
+load("//cargo:defs.bzl", "cargo_build_script")
+load("//rust:defs.bzl", "rust_binary", "rust_library")
+
+def _link_flags_test_impl(ctx):
+    env = analysistest.begin(ctx)
+    target = analysistest.target_under_test(env)
+    rustc_actions = [action for action in target.actions if action.mnemonic == "Rustc"]
+    asserts.equals(env, 1, len(rustc_actions))
+
+    if rustc_actions:
+        argv = rustc_actions[0].argv
+        link_flags = [
+            argv[i + 1]
+            for i in range(len(argv) - 1)
+            if argv[i] == "--arg-file" and argv[i + 1].endswith("shared_script.linkflags")
+        ]
+        asserts.equals(env, ctx.attr.expected_count, len(link_flags))
+
+    return analysistest.end(env)
+
+_link_flags_test = analysistest.make(
+    _link_flags_test_impl,
+    attrs = {"expected_count": attr.int(mandatory = True)},
+)
+
+def duplicate_link_flags_test_suite(name):
+    """Verify a direct library owns its package build script's native flags."""
+    cargo_build_script(
+        name = "shared_script",
+        srcs = ["build.rs"],
+    )
+
+    rust_library(
+        name = "lib",
+        srcs = ["lib.rs"],
+        deps = [":shared_script"],
+    )
+
+    rust_binary(
+        name = "bin_with_lib",
+        srcs = ["main.rs"],
+        deps = [":lib", ":shared_script"],
+    )
+
+    rust_binary(
+        name = "bin_without_lib",
+        srcs = ["main.rs"],
+        deps = [":shared_script"],
+    )
+
+    _link_flags_test(
+        name = "lib_link_flags_test",
+        target_under_test = ":lib",
+        expected_count = 1,
+    )
+
+    _link_flags_test(
+        name = "bin_with_lib_link_flags_test",
+        target_under_test = ":bin_with_lib",
+        expected_count = 0,
+    )
+
+    _link_flags_test(
+        name = "bin_without_lib_link_flags_test",
+        target_under_test = ":bin_without_lib",
+        expected_count = 1,
+    )
+
+    native.test_suite(
+        name = name,
+        tests = [
+            ":lib_link_flags_test",
+            ":bin_with_lib_link_flags_test",
+            ":bin_without_lib_link_flags_test",
+        ],
+    )

cargo/tests/cargo_build_script/duplicate_link_flags/lib.rs

@@ -0,0 +1,3 @@
+pub fn value() -> i32 {
+    1
+}

cargo/tests/cargo_build_script/duplicate_link_flags/main.rs

@@ -0,0 +1 @@
+fn main() {}

rust/private/rustc.bzl

@@ -2576,7 +2576,7 @@ def _process_build_scripts(
         build_env_file = build_info.rustc_env
         if build_info.flags:
             build_flags_files.append(build_info.flags)
-        if build_info.linker_flags and include_link_flags:
+        if build_info.linker_flags and include_link_flags and not _build_script_linked_by_library(build_info, dep_info):
             build_flags_files.append(build_info.linker_flags)
             direct_inputs.append(build_info.linker_flags)
 
@@ -2600,6 +2600,17 @@ def _process_build_scripts(
         depset(build_flags_files, transitive = [dep_info.link_search_path_files]),
     )
 
+def _build_script_linked_by_library(build_info, dep_info):
+    # Cargo applies rustc-link-lib to the package library when one exists.
+    # Binaries that depend on that library already link its native archives.
+    for crate in dep_info.direct_crates.to_list():
+        if crate.dep.type not in ("lib", "rlib", "dylib") or crate.dep.is_test:
+            continue
+        for dep in crate.dep.deps.to_list():
+            if dep.build_info == build_info and dep.build_info.linker_flags == build_info.linker_flags:
+                return True
+    return False
+
 def _compute_rpaths(toolchain, output_dir, dep_info, use_pic, link_std_dylib, output_file = None, workspace_name = ""):
     """Determine the artifact's rpaths relative to the bazel root for runtime linking of shared libraries.