rustc_llvm: Emit module summaries when using -Clto=fat

[PiJoules]

Currently, module summaries are only emitted with thin lto. If we would link full/fat lto'd rust code against lto'd c++ code built with CFI (or WPD), those passes would fail during the link step because the participating rust modules are missing module summaries. Rust code does not know at compile-time if it would be participating in some special link which may require module summaries, so this PR ensures module summaries are unconditionally emitted for full/fat lto, just like with thin lto.

The WriteBitcodeToFile function just invokes the normal BitcodeWriterPass under the hood, but doesn't provide a way to set the argument for emitting module summaries. So this patch just adds the pass directly and sets that argument.

[rustbot]

r? @cuviper

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[PiJoules]

cc @ilovepi

More context at #general > Emitting module summaries under -Clto=fat

[rust-log-analyzer]

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All checks passed!
checking python file formatting
29 files already formatted
checking C++ file formatting
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:929:77: error: code should be clang-formatted [-Wclang-format-violations]
      // Note the bitcode writer will only emit the full LTO block ID if the "ThinLTO"
                                                                            ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:929:87: error: code should be clang-formatted [-Wclang-format-violations]
      // Note the bitcode writer will only emit the full LTO block ID if the "ThinLTO"
                                                                                      ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:930:7: error: code should be clang-formatted [-Wclang-format-violations]
      // metadata is defined and explicitly set to zero. Otherwise, the thin LTO block
      ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:930:68: error: code should be clang-formatted [-Wclang-format-violations]
---
                                                                                      ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:931:7: error: code should be clang-formatted [-Wclang-format-violations]
      // ID will be emitted.
      ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:932:19: error: code should be clang-formatted [-Wclang-format-violations]
      auto *Zero = ConstantInt::get(Type::getInt32Ty(TheModule->getContext()), 0);
                  ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:934:51: error: code should be clang-formatted [-Wclang-format-violations]
      MPM.addPass(BitcodeWriterPass(ThinLTODataOS, /*ShouldPreserveUseListOrder=*/false,
                                                  ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:1470:79: error: code should be clang-formatted [-Wclang-format-violations]
        // Note the bitcode writer will only emit the full LTO block ID if the "ThinLTO"
                                                                              ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:1470:89: error: code should be clang-formatted [-Wclang-format-violations]
        // Note the bitcode writer will only emit the full LTO block ID if the "ThinLTO"
                                                                                        ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:1471:9: error: code should be clang-formatted [-Wclang-format-violations]
        // metadata is defined and explicitly set to zero. Otherwise, the thin LTO block
        ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:1471:70: error: code should be clang-formatted [-Wclang-format-violations]
---
                                                                                        ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:1472:9: error: code should be clang-formatted [-Wclang-format-violations]
        // ID will be emitted.
        ^
/checkout/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp:1473:21: error: code should be clang-formatted [-Wclang-format-violations]
        auto *Zero = ConstantInt::get(Type::getInt32Ty(unwrap(M)->getContext()), 0);
                    ^

clang-format linting failed! Printing diff suggestions:
rerun tidy with `--extra-checks=cpp:fmt --bless` to reformat C++ code
tidy [extra_checks:cpp_fmt]: checks with external tool 'clang-format' failed
tidy [extra_checks:cpp_fmt]: FAIL
spellchecking files
building external tool typos from package typos-cli@1.38.1
finished building tool typos
yarn install v1.22.22
warning package.json: No license field
---
info: ✓ ES-Check passed! All files are ES10 compatible.
typechecking javascript files
tidy: The following check failed: extra_checks:cpp_fmt
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1624:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1511:29

--- BACKTRACE vvv
   0: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   1: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   2: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   3: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   4: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1511:29
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::Tidy>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1596:36
   6: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1433:21
   7: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
   8: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:232:18
   9: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1139:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1118:14
  11: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  12: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  13: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:206:18
  16: <&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:287:21
  17: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
  18: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:544:19
  19: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


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  local time: Thu Jun 18 18:44:34 UTC 2026
  network time: Thu, 18 Jun 2026 18:44:34 GMT
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