From 55ef2b8c7d306bac7519894b5f99237fb15b8b80 Mon Sep 17 00:00:00 2001
From: Parker Timmerman <parker@parkertimmerman.com>
Date: Sun, 29 Dec 2024 13:21:52 -0500
Subject: [PATCH] fix: add 'manual' option for `//rust/settings/lto` (#3120)

Related to https://github.com/bazelbuild/rules_rust/pull/3119

Adds an option named "manual" to `//rust/settings/lto` which was
introduced in https://github.com/bazelbuild/rules_rust/pull/3104. When
set to "manual" we will not automatically set any flags so users with
customized setups can configure the relevant flags on their own.

FWIW this was an oversight and IMO should have been included in the
original PR. Apologies to anyone who stumbled over this or encountered
build issues!
---
 rust/private/lto.bzl             | 14 +++++++++-----
 test/unit/lto/lto_test_suite.bzl | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/rust/private/lto.bzl b/rust/private/lto.bzl
index 3f5c16264a..04c271750d 100644
--- a/rust/private/lto.bzl
+++ b/rust/private/lto.bzl
@@ -3,6 +3,8 @@
 load("//rust/private:utils.bzl", "is_exec_configuration")
 
 _LTO_MODES = [
+    # Do nothing, let the user manually handle LTO.
+    "manual",
     # Default. No mode has been explicitly set, rustc will do "thin local" LTO
     # between the codegen units of a single crate.
     "unspecified",
@@ -64,8 +66,6 @@ def _determine_lto_object_format(ctx, toolchain, crate_info):
         return "only_object"
 
     perform_linking = crate_info.type in ["bin", "staticlib", "cdylib"]
-
-    # is_linkable = crate_info.type in ["lib", "rlib", "dylib", "proc-macro"]
     is_dynamic = crate_info.type in ["dylib", "cdylib", "proc-macro"]
     needs_object = perform_linking or is_dynamic
 
@@ -94,17 +94,21 @@ def construct_lto_arguments(ctx, toolchain, crate_info):
         list: A list of strings that are valid flags for 'rustc'.
     """
     mode = toolchain._lto.mode
-    format = _determine_lto_object_format(ctx, toolchain, crate_info)
 
+    # The user is handling LTO on their own, don't add any arguments.
+    if mode == "manual":
+        return []
+
+    format = _determine_lto_object_format(ctx, toolchain, crate_info)
     args = []
 
     if mode in ["thin", "fat", "off"] and not is_exec_configuration(ctx):
         args.append("lto={}".format(mode))
 
-    if format in ["unspecified", "object_and_bitcode"]:
+    if format == "object_and_bitcode":
         # Embedding LLVM bitcode in object files is `rustc's` default.
         args.extend([])
-    elif format in ["off", "only_object"]:
+    elif format == "only_object":
         args.extend(["embed-bitcode=no"])
     elif format == "only_bitcode":
         args.extend(["linker-plugin-lto"])
diff --git a/test/unit/lto/lto_test_suite.bzl b/test/unit/lto/lto_test_suite.bzl
index d70d3bce24..46c0b3172f 100644
--- a/test/unit/lto/lto_test_suite.bzl
+++ b/test/unit/lto/lto_test_suite.bzl
@@ -46,6 +46,14 @@ _lto_level_default_test = analysistest.make(
     config_settings = {},
 )
 
+def _lto_level_manual(ctx):
+    return _lto_test_impl(ctx, None, None, False)
+
+_lto_level_manual_test = analysistest.make(
+    _lto_level_manual,
+    config_settings = {str(Label("//rust/settings:lto")): "manual"},
+)
+
 def _lto_level_off(ctx):
     return _lto_test_impl(ctx, "off", "no", False)
 
@@ -97,6 +105,11 @@ def lto_test_suite(name):
         target_under_test = ":lib",
     )
 
+    _lto_level_manual_test(
+        name = "lto_level_manual_test",
+        target_under_test = ":lib",
+    )
+
     _lto_level_off_test(
         name = "lto_level_off_test",
         target_under_test = ":lib",
@@ -116,6 +129,7 @@ def lto_test_suite(name):
         name = name,
         tests = [
             ":lto_level_default_test",
+            ":lto_level_manual_test",
             ":lto_level_off_test",
             ":lto_level_thin_test",
             ":lto_level_fat_test",