change the type of the argument of drop_in_place lang item to &mut _

[WaffleLapkin]

View all comments

We used to special case core::ptr::drop_in_place when computing LLVM argument attributes with this hack:

rust/compiler/rustc_ty_utils/src/abi.rs

Lines 383 to 392 in db5e2dc

	let drop_target_pointee_info = drop_target_pointee.and_then(|pointee| {
	assert_eq!(pointee, layout.ty.builtin_deref(true).unwrap());
	assert_eq!(offset, Size::ZERO);
	// The argument to `drop_in_place` is semantically equivalent to a mutable reference.
	let mutref = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, pointee);
	let layout = cx.layout_of(mutref).unwrap();
	layout.pointee_info_at(&cx, offset)
	});
	if let Some(pointee) = drop_target_pointee_info.or_else(|| layout.pointee_info_at(&cx, offset))

This is because even though drop_in_place takes a *mut T it is semantically a &mut T (remember how &mut Self is passed to Drop::drop). This is apparently relevant for perf.

This PR replaces this hack with a simpler solution -- it makes drop_in_place a thin wrapper around newly added core::ptr::drop_glue, which is the actual lang item and takes a &mut T:

rust/library/core/src/ptr/mod.rs

Lines 810 to 833 in d2563d5

	pub const unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T)
	where
	T: [const] Destruct,
	{
	// Due to historic reasons, `drop_in_place` takes a pointer rather than a reference,
	// which results in worse codegen since we don't apply noalias/dereferenceable llvm
	// attributes to pointer arguments. To workaround this without breaking public
	// interface, `drop_in_place` calls the lang item, rather than being one directly.
	// SAFETY:
	// - compiler glue has the same safety requirements as this function
	// - the pointer must be valid as per the safety requirement of this function
	unsafe { drop_glue(&mut *to_drop) }
	}
	/// Helper function for `drop_in_place`.
	#[lang = "drop_glue"]
	const unsafe fn drop_glue<T: PointeeSized>(_: &mut T)
	where
	T: [const] Destruct,
	{
	// Code here does not matter - this is replaced by the
	// real drop glue by the compiler.
	}

---

The rest of the PR is blessing tests and cleaning up things which are not necessary after this change.

One thing that is a bit awkward is that now that drop_glue is the actual lang item, a lot of the comments referring to drop_in_place are outdated. Should I try fixing that?

I've also changed async_drop_in_place to take a &mut T, and it simplified the code handling it a bit. (since it's unstable we don't need to introduce a wrapper)

---

cc @RalfJung
Closes #154274

[RalfJung]

Oh wow I hadn't expected my wish to be fulfilled before I could even finish my PR that motivated me to express the wish in the first place. :-) ❤️

[RalfJung]

I like the interpreter changes and renames. Not sure what the status of the rest of the PR is, but if you submit just those as a separate PR I'll r+.

[RalfJung]

LGTM apart from the problem that CI already pointed out. :)

[RalfJung]

Suggested change

	Rvalue::Use(Operand::Move(Place::from(cor_ref_tmp_local)), WithRetag::No),
	Rvalue::Use(Operand::Move(Place::from(cor_ref_tmp_local)), WithRetag::Yes),

All assignments generated during MIR building should have retags.

View changes since the review

[WaffleLapkin]

Ugh, got confused with other changes. Fixed.

Is WithRetags::No for cases where an assignment did not exist in the original MIR, but got introduced by an optimization, and thus doing a retag would introduce UB, which did not exist in the original MIR?

[RalfJung]

Yes, that's one major usecase for WithRetags::No.

[RalfJung]

r=me with CI green

[WaffleLapkin]

@bors r=RalfJung,scottmcm,saethlin

[rust-bors]

📌 Commit e0ecfbb has been approved by RalfJung,scottmcm,saethlin

It is now in the queue for this repository.

[rust-log-analyzer]

The job dist-x86_64-mingw failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0522`.
Command `D:\a\rust\rust\build\x86_64-pc-windows-gnu\stage0\bin\rustc.exe --cfg bootstrap --target x86_64-pc-windows-gnu --emit=obj -o D:\a\rust\rust\build\x86_64-pc-windows-gnu\native\rtstartup\rsbegin.o D:\a\rust\rust\library\rtstartup\rsbegin.rs` failed with exit code 1
Created at: src\bootstrap\src\core\build_steps\compile.rs:933:31
Executed at: src\bootstrap\src\core\build_steps\compile.rs:945:22

--- BACKTRACE vvv
   0: std::backtrace_rs::backtrace::win64::trace
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src\..\..\backtrace\src\backtrace/win64.rs:85:14
   1: std::backtrace_rs::backtrace::trace_unsynchronized::<<std::backtrace::Backtrace>::create::{closure#0}>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src\..\..\backtrace\src\backtrace/mod.rs:66:14
   2: <std::backtrace::Backtrace>::create
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/backtrace.rs:331:13
   3: <std::backtrace::Backtrace>::capture
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/backtrace.rs:296:9
   4: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at .\src\bootstrap\src\utils\exec.rs:939:17
   5: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at .\src\bootstrap\src\utils\exec.rs:831:21
   6: <bootstrap::utils::exec::ExecutionContext>::run
             at .\src\bootstrap\src\utils\exec.rs:741:45
   7: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at .\src\bootstrap\src\utils\exec.rs:339:27
   8: <bootstrap::core::build_steps::compile::StartupObjects as bootstrap::core::builder::Step>::run
             at .\src\bootstrap\src\core\build_steps\compile.rs:945:22
   9: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::compile::StartupObjects>
             at .\src\bootstrap\src\core\builder\mod.rs:1595:36
  10: <bootstrap::core::build_steps::compile::Std as bootstrap::core::builder::Step>::run
             at .\src\bootstrap\src\core\build_steps\compile.rs:213:39
  11: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::compile::Std>
             at .\src\bootstrap\src\core\builder\mod.rs:1595:36
  12: <bootstrap::core::builder::Builder>::std
             at .\src\bootstrap\src\core\builder\mod.rs:1294:18
  13: bootstrap::core::build_steps::doc::prepare_doc_compiler
             at .\src\bootstrap\src\core\build_steps\doc.rs:840:13
  14: <bootstrap::core::build_steps::doc::TheBook as bootstrap::core::builder::Step>::make_run
             at .\src\bootstrap\src\core\build_steps\doc.rs:231:29
  15: <bootstrap::core::builder::StepDescription>::maybe_run
             at .\src\bootstrap\src\core\builder\mod.rs:476:13
  16: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at .\src\bootstrap\src\core\builder\cli_paths.rs:141:22
  17: <bootstrap::core::builder::Builder>::run_step_descriptions
             at .\src\bootstrap\src\core\builder\mod.rs:1138:9
  18: <bootstrap::core::builder::Builder>::run_default_doc_steps
             at .\src\bootstrap\src\core\builder\mod.rs:1122:14
  19: <bootstrap::core::build_steps::dist::Docs as bootstrap::core::builder::Step>::run
             at .\src\bootstrap\src\core\build_steps\dist.rs:88:17
  20: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::dist::Docs>
             at .\src\bootstrap\src\core\builder\mod.rs:1595:36
  21: <bootstrap::core::build_steps::dist::Docs as bootstrap::core::builder::Step>::make_run
             at .\src\bootstrap\src\core\build_steps\dist.rs:79:21
  22: <bootstrap::core::builder::StepDescription>::maybe_run
             at .\src\bootstrap\src\core\builder\mod.rs:476:13
  23: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at .\src\bootstrap\src\core\builder\cli_paths.rs:141:22
  24: <bootstrap::core::builder::Builder>::run_step_descriptions
             at .\src\bootstrap\src\core\builder\mod.rs:1138:9
  25: <bootstrap::core::builder::Builder>::execute_cli
             at .\src\bootstrap\src\core\builder\mod.rs:1117:14
  26: <bootstrap::Build>::build
             at .\src\bootstrap\src\lib.rs:803:25
  27: bootstrap::main
             at .\src\bootstrap\src\bin\main.rs:130:11
  28: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\core\src\ops\function.rs:250:5
  29: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src\sys\backtrace.rs:166:18
  30: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src\rt.rs:206:18
  31: <&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\core\src\ops/function.rs:287:21
  32: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/panicking.rs:581:40
  33: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/panicking.rs:544:19
  34: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/panic.rs:359:14
  35: std::rt::lang_start_internal::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/rt.rs:175:24
  36: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library\std\src/panicking.rs:581:40
---
  44: <unknown>
  45: <unknown>


Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `dist bootstrap --include-default-paths`
Build completed unsuccessfully in 0:50:08
  local time: Tue May  5 00:50:25 CUT 2026
  network time: Tue, 05 May 2026 00:50:25 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[rust-bors]

💔 Test for 2e5d870 failed: CI. Failed job:

• auto - dist-x86_64-mingw (web logs, enhanced plaintext logs)