Make lto toolchain options public and don't link proc-macros with LTO

Proc-macro crates can not be linked with lto and we should not emit bitcode either.
This fixes bazelbuild#3143

rust/private/lto.bzl

@@ -60,7 +60,7 @@ def _determine_lto_object_format(ctx, toolchain, crate_info):
     if is_exec_configuration(ctx):
         return "only_object"
 
-    mode = toolchain._lto.mode
+    mode = toolchain.lto.mode
 
     if mode in ["off", "unspecified"]:
         return "only_object"
@@ -75,9 +75,10 @@ def _determine_lto_object_format(ctx, toolchain, crate_info):
         # If we're building an 'rlib' and LTO is enabled, then we can skip
         # generating object files entirely.
         return "only_bitcode"
-    elif crate_info.type == "dylib":
+    elif crate_info.type in ["dylib", "proc-macro"]:
         # If we're a dylib and we're running LTO, then only emit object code
         # because 'rustc' doesn't currently support LTO with dylibs.
+        # proc-macros do not benefit from LTO, and cannot be dynamically linked with LTO.
         return "only_object"
     else:
         return "object_and_bitcode"
@@ -93,7 +94,7 @@ def construct_lto_arguments(ctx, toolchain, crate_info):
     Returns:
         list: A list of strings that are valid flags for 'rustc'.
     """
-    mode = toolchain._lto.mode
+    mode = toolchain.lto.mode
 
     # The user is handling LTO on their own, don't add any arguments.
     if mode == "manual":
@@ -102,7 +103,8 @@ def construct_lto_arguments(ctx, toolchain, crate_info):
     format = _determine_lto_object_format(ctx, toolchain, crate_info)
     args = []
 
-    if mode in ["thin", "fat", "off"] and not is_exec_configuration(ctx):
+    # proc-macros do not benefit from LTO, and cannot be dynamically linked with LTO.
+    if mode in ["thin", "fat", "off"] and not is_exec_configuration(ctx) and crate_info.type != "proc-macro":
         args.append("lto={}".format(mode))
 
     if format == "object_and_bitcode":

rust/toolchain.bzl

@@ -521,7 +521,7 @@ def _rust_toolchain_impl(ctx):
     third_party_dir = ctx.attr._third_party_dir[BuildSettingInfo].value
     pipelined_compilation = ctx.attr._pipelined_compilation[BuildSettingInfo].value
     no_std = ctx.attr._no_std[BuildSettingInfo].value
-    lto = ctx.attr._lto[RustLtoInfo]
+    lto = ctx.attr.lto[RustLtoInfo]
 
     experimental_use_global_allocator = ctx.attr._experimental_use_global_allocator[BuildSettingInfo].value
     if _experimental_use_cc_common_link(ctx):
@@ -672,6 +672,7 @@ def _rust_toolchain_impl(ctx):
         nostd_and_global_allocator_cc_info = _make_libstd_and_allocator_ccinfo(ctx, rust_std, ctx.attr.global_allocator_library, "no_std_with_alloc"),
         llvm_cov = ctx.file.llvm_cov,
         llvm_profdata = ctx.file.llvm_profdata,
+        lto = lto,
         make_variables = make_variable_info,
         rust_doc = sysroot.rustdoc,
         rust_std = sysroot.rust_std,
@@ -705,7 +706,6 @@ def _rust_toolchain_impl(ctx):
         _toolchain_generated_sysroot = ctx.attr._toolchain_generated_sysroot[BuildSettingInfo].value,
         _incompatible_do_not_include_data_in_compile_data = ctx.attr._incompatible_do_not_include_data_in_compile_data[IncompatibleFlagInfo].enabled,
         _no_std = no_std,
-        _lto = lto,
         _codegen_units = ctx.attr._codegen_units[BuildSettingInfo].value,
     )
     return [
@@ -808,6 +808,11 @@ rust_toolchain = rule(
             doc = "LLVM tools that are shipped with the Rust toolchain.",
             allow_files = True,
         ),
+        "lto": attr.label(
+            providers = [RustLtoInfo],
+            default = Label("//rust/settings:lto"),
+            doc = "Label to an LTO setting whether which can enable custom LTO settings",
+        ),
         "opt_level": attr.string_dict(
             doc = "Rustc optimization levels.",
             default = {
@@ -897,10 +902,6 @@ rust_toolchain = rule(
             default = Label("//rust/settings:incompatible_do_not_include_data_in_compile_data"),
             doc = "Label to a boolean build setting that controls whether to include data files in compile_data.",
         ),
-        "_lto": attr.label(
-            providers = [RustLtoInfo],
-            default = Label("//rust/settings:lto"),
-        ),
         "_no_std": attr.label(
             default = Label("//rust/settings:no_std"),
         ),

test/unit/lto/lto_test_suite.bzl

@@ -2,7 +2,7 @@
 
 load("@bazel_skylib//lib:unittest.bzl", "analysistest")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
-load("//rust:defs.bzl", "rust_library")
+load("//rust:defs.bzl", "rust_library", "rust_proc_macro")
 load(
     "//test/unit:common.bzl",
     "assert_action_mnemonic",
@@ -78,6 +78,14 @@ _lto_level_fat_test = analysistest.make(
     config_settings = {str(Label("//rust/settings:lto")): "fat"},
 )
 
+def _lto_proc_macro(ctx):
+    return _lto_test_impl(ctx, None, "no", False)
+
+_lto_proc_macro_test = analysistest.make(
+    _lto_proc_macro,
+    config_settings = {str(Label("//rust/settings:lto")): "thin"},
+)
+
 def lto_test_suite(name):
     """Entry-point macro called from the BUILD file.
 
@@ -100,6 +108,12 @@ def lto_test_suite(name):
         edition = "2021",
     )
 
+    rust_proc_macro(
+        name = "proc_macro",
+        srcs = [":lib.rs"],
+        edition = "2021",
+    )
+
     _lto_level_default_test(
         name = "lto_level_default_test",
         target_under_test = ":lib",
@@ -125,6 +139,11 @@ def lto_test_suite(name):
         target_under_test = ":lib",
     )
 
+    _lto_proc_macro_test(
+        name = "lto_proc_macro_test",
+        target_under_test = ":proc_macro",
+    )
+
     native.test_suite(
         name = name,
         tests = [
@@ -133,5 +152,6 @@ def lto_test_suite(name):
             ":lto_level_off_test",
             ":lto_level_thin_test",
             ":lto_level_fat_test",
+            ":lto_proc_macro_test",
         ],
     )