feat: introduce link_deps attribute for native library linkage

  This change adds a dedicated link_deps attribute to Rust rules, allowing native symbols to be linked without exposing them as Rust crates.

  Key changes:
   - Added link_deps to _common_attrs for CcInfo/CrateInfo targets.
   - Filtered link_deps out of rust_proc_macro to avoid transition complexity.
   - Implemented transform_link_deps to extract linkage context while hiding crate metadata.
   - Added a proactive warning for native libraries incorrectly listed in deps.
   - Added unit tests in test/unit/link_deps to verify correct linkage.

Author: Suyashagarw

rust/private/rust.bzl

@@ -52,6 +52,7 @@ load(
     "get_edition",
     "get_import_macro_deps",
     "transform_deps",
+    "transform_link_deps",
     "transform_sources",
 )
 
@@ -66,20 +67,49 @@ def _assert_no_deprecated_attributes(_ctx):
     pass
 
 def _assert_correct_dep_mapping(ctx):
-    """Forces a failure if proc_macro_deps and deps are mixed inappropriately
+    """Ensures dependencies are correctly mapped between 'deps', 'proc_macro_deps', and 'link_deps'.
+
+    This function validates that procedural macros and native libraries are listed in
+    their appropriate attributes to maintain the rules_rust dependency model.
 
     Args:
         ctx (ctx): The current rule's context object
     """
     for dep in ctx.attr.deps:
-        if rust_common.crate_info in dep:
-            if dep[rust_common.crate_info].type == "proc-macro":
+        # Identify if this is a Rust-related target using any known Rust provider.
+        is_rust_target = (
+            rust_common.crate_info in dep or
+            rust_common.crate_group_info in dep or
+            rust_common.test_crate_info in dep or
+            rust_common.dep_info in dep or
+            BuildInfo in dep
+        )
+
+        if is_rust_target:
+            if rust_common.crate_info in dep and dep[rust_common.crate_info].type == "proc-macro":
                 fail(
                     "{} listed {} in its deps, but it is a proc-macro. It should instead be in the bazel property proc_macro_deps.".format(
                         ctx.label,
                         dep.label,
                     ),
                 )
+
+            continue
+
+        # If it's not a known Rust target but provides CcInfo, it's a native library
+        # that should ideally be in 'link_deps'.
+        if CcInfo in dep:
+            # buildifier: disable=print
+            print(
+                ("\nWARNING: Target {dep} in 'deps' of {target} is a C++ library. " +
+                    "Only Rust targets are allowed in 'deps'. " +
+                    "Please use 'link_deps' for manual FFI linkage or 'cc_deps' for binding generation. " +
+                    "Support for C++ libraries in 'deps' is deprecated and will be removed in a future release.").format(
+                    dep = dep.label,
+                    target = ctx.label,
+                ),
+            )
+
     for dep in ctx.attr.proc_macro_deps:
         if CrateInfo in dep:
             types = [dep[CrateInfo].type]
@@ -217,6 +247,8 @@ def _rust_library_common(ctx, crate_type):
         )
 
     deps = transform_deps(ctx.attr.deps)
+    if hasattr(ctx.attr, "link_deps"):
+        deps += transform_link_deps(ctx.attr.link_deps)
     proc_macro_deps = transform_deps(ctx.attr.proc_macro_deps + get_import_macro_deps(ctx))
 
     return rustc_compile_action(
@@ -269,6 +301,8 @@ def _rust_binary_impl(ctx):
     output = ctx.actions.declare_file(output_filename + toolchain.binary_ext)
 
     deps = transform_deps(ctx.attr.deps)
+    if hasattr(ctx.attr, "link_deps"):
+        deps += transform_link_deps(ctx.attr.link_deps)
     proc_macro_deps = transform_deps(ctx.attr.proc_macro_deps + get_import_macro_deps(ctx))
 
     crate_root = getattr(ctx.file, "crate_root", None)
@@ -356,6 +390,8 @@ def _rust_test_impl(ctx):
 
     crate_type = "bin"
     deps = transform_deps(ctx.attr.deps)
+    if hasattr(ctx.attr, "link_deps"):
+        deps += transform_link_deps(ctx.attr.link_deps)
     proc_macro_deps = transform_deps(ctx.attr.proc_macro_deps + get_import_macro_deps(ctx))
 
     if ctx.attr.crate and ctx.attr.srcs:
@@ -713,15 +749,22 @@ _COMMON_ATTRS = {
     ),
     "deps": attr.label_list(
         doc = dedent("""\
-            List of other libraries to be linked to this library target.
+            List of other Rust libraries to be linked to this library target.
 
-            These can be either other `rust_library` targets or `cc_library` targets if
-            linking a native library.
+            These must be targets that provide `CrateInfo`, such as `rust_library`.
         """),
     ),
     "edition": attr.string(
         doc = "The rust edition to use for this crate. Defaults to the edition specified in the rust_toolchain.",
     ),
+    "link_deps": attr.label_list(
+        doc = dedent("""\
+            List of other native libraries to be linked to this library target.
+
+            These are typically `cc_library` targets.
+        """),
+        providers = [[CcInfo], [rust_common.crate_info]],
+    ),
     "lint_config": attr.label(
         doc = "Set of lints to apply when building this crate.",
         providers = [LintsInfo],
@@ -1128,7 +1171,7 @@ rust_proc_macro = rule(
     # need to declare `_allowlist_function_transition`, see
     # https://docs.bazel.build/versions/main/skylark/config.html#user-defined-transitions.
     attrs = dict(
-        _COMMON_ATTRS.items(),
+        {name: value for name, value in _COMMON_ATTRS.items() if name != "link_deps"}.items(),
         _allowlist_function_transition = attr.label(
             default = Label("//tools/allowlists/function_transition_allowlist"),
         ),

rust/private/utils.bzl

@@ -530,6 +530,23 @@ def transform_deps(deps):
         crate_group_info = dep[CrateGroupInfo] if CrateGroupInfo in dep else None,
     ) for dep in deps]
 
+def transform_link_deps(link_deps):
+    """Transforms a [Target] into [DepVariantInfo] for native symbol linkage.
+
+    Args:
+        link_deps (list of Targets): Dependencies coming from ctx.attr.link_deps
+
+    Returns:
+        list of DepVariantInfos with only CcInfo populated.
+    """
+    return [DepVariantInfo(
+        crate_info = None,
+        dep_info = None,
+        build_info = None,
+        cc_info = dep[CcInfo] if CcInfo in dep else None,
+        crate_group_info = None,
+    ) for dep in link_deps]
+
 def get_import_macro_deps(ctx):
     """Returns a list of targets to be added to proc_macro_deps.
 

test/unit/link_deps/BUILD.bazel

@@ -0,0 +1,39 @@
+load("@rules_cc//cc:defs.bzl", "cc_library")
+load("//rust:defs.bzl", "rust_binary", "rust_library")
+load(":link_deps_test.bzl", "link_deps_test")
+
+# A dummy library to be used as a dependency
+rust_library(
+    name = "leaf_lib",
+    srcs = ["lib.rs"],
+)
+
+# Test 1: rust_library supports link_deps
+rust_library(
+    name = "main_lib",
+    srcs = ["lib.rs"],
+    link_deps = [":leaf_lib"],
+    tags = ["manual"],
+)
+
+link_deps_test(
+    name = "link_deps_success_test",
+    target_under_test = ":main_lib",
+)
+
+# Test 2: Build test linking a C library via link_deps
+cc_library(
+    name = "c_lib",
+    srcs = ["c_lib.c"],
+)
+
+rust_binary(
+    name = "rust_bin",
+    srcs = ["main.rs"],
+    link_deps = [":c_lib"],
+)
+
+build_test(
+    name = "rust_bin_link_deps_test",
+    target_under_test = ":rust_bin",
+)

test/unit/link_deps/c_lib.c

@@ -0,0 +1,3 @@
+int c_function() {
+    return 42;
+}

test/unit/link_deps/lib.rs

@@ -0,0 +1 @@
+

test/unit/link_deps/link_deps_test.bzl

@@ -0,0 +1,29 @@
+load("//rust/private:common.bzl", "rust_common")
+load("@rules_cc//cc/common:cc_info.bzl", "CcInfo")
+load("@bazel_skylib//lib:unittest.bzl", "analysistest", "asserts")
+
+def _link_deps_test_impl(ctx):
+    env = analysistest.begin(ctx)
+    target = analysistest.target_under_test(env)
+
+    # Verify that CcInfo (symbols) is present
+    asserts.true(env, CcInfo in target, "Target should provide CcInfo from link_deps")
+
+    dep_info = target[rust_common.dep_info]
+
+    # Verify that leaf_lib is NOT in direct_crates
+    for direct_crate in dep_info.direct_crates.to_list():
+        asserts.not_equals(env, "leaf_lib", direct_crate.name, "link_deps should not be added to direct_crates")
+
+    # Verify that transitive_noncrates specifically contains leaf_lib as a linking input
+    linker_inputs = dep_info.transitive_noncrates.to_list()
+    found_leaf_lib = False
+    for linker_input in linker_inputs:
+        if linker_input.owner == Label("//test/unit/link_deps:leaf_lib"):
+            found_leaf_lib = True
+            break
+    asserts.true(env, found_leaf_lib, "link_deps should pass leaf_lib as a linker input")
+
+    return analysistest.end(env)
+
+link_deps_test = analysistest.make(_link_deps_test_impl)

test/unit/link_deps/main.rs

@@ -0,0 +1,9 @@
+extern "C" {
+    fn c_function() -> i32;
+}
+
+fn main() {
+    unsafe {
+        assert_eq!(c_function(), 42);
+    }
+}