Introduce feature rules_rust_link_cc. (#2723)

This feature is similar to `rules_rust_unsupported_feature`. The
difference is that, `rules_rust_unsupported_feature` gets disabled if
*any* rust is involved, while this feature gets disabled if *only* rust
is involved.

My use case for this feature is to static link to c++ / objc stdlibs.
They are needed only when any C object is being linked, and will produce
invalid rust_proc_macro if linked blindly.

This implementation uses the fact that `DepInfo.transitive_noncrates`
would be empty unless there is a `cc_library` or `cc_import` in the
(transitive) deps. As a side effect, `collect_deps()` can no longer use
the output of `_are_linkstamps_supported()` so it now always returns a
populated linkstamp depset, to be discarded later if linkstamps are not
supported.

---------

Co-authored-by: scentini <[email protected]>

rust/private/clippy.bzl

@@ -99,19 +99,18 @@ def _clippy_aspect_impl(target, ctx):
     toolchain = find_toolchain(ctx)
     cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
 
-    dep_info, build_info, linkstamps = collect_deps(
+    dep_info, build_info, _ = collect_deps(
         deps = crate_info.deps,
         proc_macro_deps = crate_info.proc_macro_deps,
         aliases = crate_info.aliases,
-        # Clippy doesn't need to invoke transitive linking, therefore doesn't need linkstamps.
-        are_linkstamps_supported = False,
     )
 
     compile_inputs, out_dir, build_env_files, build_flags_files, linkstamp_outs, ambiguous_libs = collect_inputs(
         ctx,
         ctx.rule.file,
         ctx.rule.files,
-        linkstamps,
+        # Clippy doesn't need to invoke transitive linking, therefore doesn't need linkstamps.
+        depset([]),
         toolchain,
         cc_toolchain,
         feature_configuration,

rust/private/rustc.bzl

@@ -40,6 +40,12 @@ load(
 )
 load(":utils.bzl", "is_std_dylib")
 
+# This feature is disabled unless one of the dependencies is a cc_library.
+# Authors of C++ toolchains can place linker flags that should only be applied
+# when linking with C objects in a feature with this name, or require this
+# feature from other features which needs to be disabled together.
+RUST_LINK_CC_FEATURE = "rules_rust_link_cc"
+
 BuildInfo = _BuildInfo
 
 AliasableDepInfo = provider(
@@ -195,21 +201,19 @@ def _is_proc_macro(crate_info):
 def collect_deps(
         deps,
         proc_macro_deps,
-        aliases,
-        are_linkstamps_supported = False):
+        aliases):
     """Walks through dependencies and collects the transitive dependencies.
 
     Args:
         deps (list): The deps from ctx.attr.deps.
         proc_macro_deps (list): The proc_macro deps from ctx.attr.proc_macro_deps.
         aliases (dict): A dict mapping aliased targets to their actual Crate information.
-        are_linkstamps_supported (bool): Whether the current rule and the toolchain support building linkstamps..
 
     Returns:
         tuple: Returns a tuple of:
             DepInfo,
             BuildInfo,
-            linkstamps (depset[CcLinkstamp]): A depset of CcLinkstamps that need to be compiled and linked into all linked binaries.
+            linkstamps (depset[CcLinkstamp]): A depset of CcLinkstamps that need to be compiled and linked into all linked binaries when applicable.
 
     """
     direct_crates = []
@@ -249,7 +253,7 @@ def collect_deps(
         cc_info = _get_cc_info(dep)
         dep_build_info = _get_build_info(dep)
 
-        if cc_info and are_linkstamps_supported:
+        if cc_info:
             linkstamps.append(cc_info.linking_context.linkstamps())
 
         if crate_info:
@@ -1150,8 +1154,6 @@ def rustc_compile_action(
     rustc_output = crate_info_dict.get("rustc_output", None)
     rustc_rmeta_output = crate_info_dict.get("rustc_rmeta_output", None)
 
-    cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
-
     # Determine whether to use cc_common.link:
     #  * either if experimental_use_cc_common_link is 1,
     #  * or if experimental_use_cc_common_link is -1 and
@@ -1169,11 +1171,17 @@ def rustc_compile_action(
         deps = crate_info_dict["deps"],
         proc_macro_deps = crate_info_dict["proc_macro_deps"],
         aliases = crate_info_dict["aliases"],
-        are_linkstamps_supported = _are_linkstamps_supported(
-            feature_configuration = feature_configuration,
-            has_grep_includes = hasattr(ctx.attr, "_use_grep_includes"),
-        ),
     )
+    extra_disabled_features = [RUST_LINK_CC_FEATURE]
+    if crate_info.type in ["bin", "cdylib"] and dep_info.transitive_noncrates.to_list():
+        # One or more of the transitive deps is a cc_library / cc_import
+        extra_disabled_features = []
+    cc_toolchain, feature_configuration = find_cc_toolchain(ctx, extra_disabled_features)
+    if not _are_linkstamps_supported(
+        feature_configuration = feature_configuration,
+        has_grep_includes = hasattr(ctx.attr, "_use_grep_includes"),
+    ):
+        linkstamps = depset([])
 
     # Determine if the build is currently running with --stamp
     stamp = is_stamping_enabled(attr)

rust/private/rustdoc.bzl

@@ -77,7 +77,7 @@ def rustdoc_compile_action(
 
     cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
 
-    dep_info, build_info, linkstamps = collect_deps(
+    dep_info, build_info, _ = collect_deps(
         deps = crate_info.deps,
         proc_macro_deps = crate_info.proc_macro_deps,
         aliases = crate_info.aliases,
@@ -87,7 +87,7 @@ def rustdoc_compile_action(
         ctx = ctx,
         file = ctx.file,
         files = ctx.files,
-        linkstamps = linkstamps,
+        linkstamps = depset([]),
         toolchain = toolchain,
         cc_toolchain = cc_toolchain,
         feature_configuration = feature_configuration,

rust/private/unpretty.bzl

@@ -132,19 +132,18 @@ def _rust_unpretty_aspect_impl(target, ctx):
     toolchain = find_toolchain(ctx)
     cc_toolchain, feature_configuration = find_cc_toolchain(ctx)
 
-    dep_info, build_info, linkstamps = collect_deps(
+    dep_info, build_info, _ = collect_deps(
         deps = crate_info.deps,
         proc_macro_deps = crate_info.proc_macro_deps,
         aliases = crate_info.aliases,
-        # Rust expand doesn't need to invoke transitive linking, therefore doesn't need linkstamps.
-        are_linkstamps_supported = False,
     )
 
     compile_inputs, out_dir, build_env_files, build_flags_files, linkstamp_outs, ambiguous_libs = collect_inputs(
         ctx,
         ctx.rule.file,
         ctx.rule.files,
-        linkstamps,
+        # Rust expand doesn't need to invoke transitive linking, therefore doesn't need linkstamps.
+        depset([]),
         toolchain,
         cc_toolchain,
         feature_configuration,

rust/private/utils.bzl

@@ -41,11 +41,12 @@ def find_toolchain(ctx):
     """
     return ctx.toolchains[Label("//rust:toolchain_type")]
 
-def find_cc_toolchain(ctx):
+def find_cc_toolchain(ctx, extra_unsupported_features = tuple()):
     """Extracts a CcToolchain from the current target's context
 
     Args:
         ctx (ctx): The current target's rule context object
+        extra_unsupported_features (sequence of str): Extra featrures to disable
 
     Returns:
         tuple: A tuple of (CcToolchain, FeatureConfiguration)
@@ -56,7 +57,8 @@ def find_cc_toolchain(ctx):
         ctx = ctx,
         cc_toolchain = cc_toolchain,
         requested_features = ctx.features,
-        unsupported_features = UNSUPPORTED_FEATURES + ctx.disabled_features,
+        unsupported_features = UNSUPPORTED_FEATURES + ctx.disabled_features +
+                               list(extra_unsupported_features),
     )
     return cc_toolchain, feature_configuration