Fix linkstamp deps in proc macros

These were not being linked in for two reasons:

- missing `_use_grep_includes` attribute on the `rust_proc_macro` rule:
  this attribute & condition are entirely dead code now ==> delete them.
- the explicit crate type list doesn't include `proc-macro` ==> add it.

Author: sam-mccall

rust/private/rust.bzl

@@ -924,7 +924,6 @@ _rust_test_attrs = {
             E.g. `bazel test //src:rust_test --test_arg=foo::test::test_fn`.
         """),
     ),
-    "_use_grep_includes": attr.bool(default = True),
 } | _coverage_attrs | _experimental_use_cc_common_link_attrs
 
 rust_library = rule(
@@ -1085,7 +1084,6 @@ rust_shared_library = rule(
         "_allowlist_function_transition": attr.label(
             default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
         ),
-        "_use_grep_includes": attr.bool(default = True),
     },
     fragments = ["cpp"],
     cfg = _rust_shared_library_transition,
@@ -1197,7 +1195,6 @@ _rust_binary_attrs = {
         default = False,
     ),
     "stamp": _stamp_attribute(default_value = -1),
-    "_use_grep_includes": attr.bool(default = True),
 } | _experimental_use_cc_common_link_attrs
 
 def _rust_binary_transition_impl(settings, attr):

rust/private/rustc.bzl

@@ -184,14 +184,11 @@ def get_compilation_mode_opts(ctx, toolchain):
 
     return toolchain.compilation_mode_opts[comp_mode]
 
-def _are_linkstamps_supported(feature_configuration, has_grep_includes):
+def _are_linkstamps_supported(feature_configuration):
     # Are linkstamps supported by the C++ toolchain?
     return (cc_common.is_enabled(feature_configuration = feature_configuration, feature_name = "linkstamps") and
             # Is Bazel recent enough to support Starlark linkstamps?
-            hasattr(cc_common, "register_linkstamp_compile_action") and
-            # The current rule doesn't define _grep_includes attribute; this
-            # attribute is required for compiling linkstamps.
-            has_grep_includes)
+            hasattr(cc_common, "register_linkstamp_compile_action"))
 
 def _should_use_pic(cc_toolchain, feature_configuration, crate_type, compilation_mode):
     """Whether or not [PIC][pic] should be enabled
@@ -750,7 +747,7 @@ def collect_inputs(
 
     # Register linkstamps when linking with rustc (when linking with
     # cc_common.link linkstamps are handled by cc_common.link itself).
-    if not experimental_use_cc_common_link and crate_info.type in ("bin", "cdylib"):
+    if not experimental_use_cc_common_link and crate_info.type in ("bin", "cdylib", "proc-macro"):
         # There is no other way to register an action for each member of a depset than
         # flattening the depset as of 2021-10-12. Luckily, usually there is only one linkstamp
         # in a build, and we only flatten the list on binary targets that perform transitive linking,
@@ -1254,10 +1251,7 @@ def rustc_compile_action(
         # 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"),
-    ):
+    if not _are_linkstamps_supported(feature_configuration = feature_configuration):
         linkstamps = depset([])
 
     # Determine if the build is currently running with --stamp

test/linkstamp/BUILD.bazel

@@ -0,0 +1,24 @@
+load("@rules_cc//cc:defs.bzl", "cc_library")
+load(
+    "@rules_rust//rust:defs.bzl",
+    "rust_proc_macro",
+    "rust_test",
+    "rust_static_library",
+)
+
+cc_library(
+    name = "linkstamp",
+    linkstamp = "linkstamp.c",
+)
+
+rust_proc_macro(
+    name = "linkstamp_proc_macro",
+    srcs = ["linkstamp_proc_macro.rs"],
+    deps = [":linkstamp"]
+)
+
+rust_test(
+    name = "linkstamp_proc_macro_test",
+    srcs = ["linkstamp_proc_macro_test.rs"],
+    proc_macro_deps = [":linkstamp_proc_macro"],
+)

test/linkstamp/linkstamp.c

@@ -0,0 +1 @@
+int defined_by_linkstamp = 42;

test/linkstamp/linkstamp_proc_macro.rs

@@ -0,0 +1,8 @@
+extern "C" {
+    pub static defined_by_linkstamp : std::os::raw::c_int;
+}
+#[proc_macro]
+pub fn num_from_linkstamp(_: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let tt: proc_macro::TokenTree = proc_macro::Literal::u32_suffixed(unsafe { defined_by_linkstamp } as u32).into();
+    tt.into()
+}

test/linkstamp/linkstamp_proc_macro_test.rs

@@ -0,0 +1,4 @@
+#[test]
+fn test_linkstamp_proc_macro() {
+    assert_eq!(42, linkstamp_proc_macro::num_from_linkstamp!());
+}