Expand async drops during drop elaboration

[cjgillot]

Async drops implemented by #123948 are expanded in await loops inside StateTransform. I find this odd, as StateTransform is tasked with removing yields from MIR.

This PR moves this async drop -> loop transformation to drop elaboration.

The first few commits create many mir-opt tests to gauge the effect of following commits. I can easily trim those.
The next few commits are from #156422.
Then come a few commits performing elementary simplifications to drop elaboration code.

The most important commit is Expand async drops during drop elaboration. It performs the main change and attempts to document what I had to understand doing that.

The last commit is a cleanup.

This is a large PR, but most of the diff is in generated files. I can easily split them, each commit compiles and passes tests on its own.

cc @zetanumbers

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

The Cranelift subtree was changed

cc @bjorn3

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

r? @JohnTitor

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The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 16 candidates

[bjorn3]

Is there some relation to #156495 or would that PR be orthogonal to this one?

[cjgillot]

Is there some relation to #156495 or would that PR be orthogonal to this one?

It should be orthogonal.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

    Checking rustc_interface v0.0.0 (/checkout/compiler/rustc_interface)
warning: unclosed HTML tag `ObjTy`
   --> compiler/rustc_mir_transform/src/elaborate_drop.rs:203:36
    |
203 |     /// * #1:pin_obj_bb:   call Pin<ObjTy>::new_unchecked(&mut obj)
    |                                    ^^^^^^^
    |
    = note: `#[warn(rustdoc::invalid_html_tags)]` on by default
help: try marking as source code
    |
203 |     /// * #1:pin_obj_bb:   call `Pin<ObjTy>::new_unchecked`(&mut obj)
    |                                 +                         +

warning: unclosed HTML tag `T`
   --> compiler/rustc_mir_transform/src/elaborate_drop.rs:204:90
    |
204 |     /// * #2:call_drop_bb: fut = call obj.<AsyncDrop::drop>() OR call async_drop_in_place<T>(obj)
    |                                                                                          ^^^
    |
help: try marking as source code
    |
204 |     /// * #2:call_drop_bb: fut = call obj.<AsyncDrop::drop>() OR call `async_drop_in_place<T>`(obj)
    |                                                                       +                      +

[RUSTC-TIMING] rustc_interface test:false 0.827
    Checking rustc_driver_impl v0.0.0 (/checkout/compiler/rustc_driver_impl)
[RUSTC-TIMING] rustc_driver_impl test:false 0.482
---
error: `rustc_mir_transform` (lib doc) generated 2 warnings
error: warnings are denied by `build.warnings` configuration
warning: build failed, waiting for other jobs to finish...
Bootstrap failed while executing `doc compiler --stage 1`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo doc -Zwarnings --target x86_64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --features 'llvm rustc' --manifest-path /checkout/compiler/rustc/Cargo.toml -Zskip-rustdoc-fingerprint --no-deps -Zrustdoc-map -p rustc-main -p rustc_abi -p rustc_arena -p rustc_ast -p rustc_ast_ir -p rustc_ast_lowering -p rustc_ast_passes -p rustc_ast_pretty -p rustc_attr_parsing -p rustc_baked_icu_data -p rustc_borrowck -p rustc_builtin_macros -p rustc_codegen_llvm -p rustc_codegen_ssa -p rustc_const_eval -p rustc_data_structures -p rustc_driver -p rustc_driver_impl -p rustc_error_codes -p rustc_error_messages -p rustc_errors -p rustc_expand -p rustc_feature -p rustc_fs_util -p rustc_graphviz -p rustc_hashes -p rustc_hir -p rustc_hir_analysis -p rustc_hir_id -p rustc_hir_pretty -p rustc_hir_typeck -p rustc_incremental -p rustc_index -p rustc_index_macros -p rustc_infer -p rustc_interface -p rustc_lexer -p rustc_lint -p rustc_lint_defs -p rustc_llvm -p rustc_log -p rustc_macros -p rustc_metadata -p rustc_middle -p rustc_mir_build -p rustc_mir_dataflow -p rustc_mir_transform -p rustc_monomorphize -p rustc_next_trait_solver -p rustc_parse -p rustc_parse_format -p rustc_passes -p rustc_pattern_analysis -p rustc_privacy -p rustc_proc_macro -p rustc_public -p rustc_public_bridge -p rustc_query_impl -p rustc_resolve -p rustc_sanitizers -p rustc_serialize -p rustc_session -p rustc_span -p rustc_symbol_mangling -p rustc_target -p rustc_thread_pool -p rustc_trait_selection -p rustc_traits -p rustc_transmute -p rustc_ty_utils -p rustc_type_ir -p rustc_type_ir_macros -p rustc_windows_rc [workdir=/checkout]` failed with exit code 101
Created at: src/bootstrap/src/core/build_steps/doc.rs:916:25
Executed at: src/bootstrap/src/core/build_steps/doc.rs:977:26

--- 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::doc::Rustc as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/doc.rs:977:26
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::doc::Rustc>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1595:36
   6: <bootstrap::core::build_steps::doc::Rustc as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/doc.rs:886:21
   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:1138:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1117:14
  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/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:01:30
  local time: Sat May 16 20:58:06 UTC 2026
  network time: Sat, 16 May 2026 20:58:06 GMT
##[error]Process completed with exit code 1.
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