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Add MaybeDangling#149614

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WaffleLapkin:dangle-maybe
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Add MaybeDangling#149614
WaffleLapkin wants to merge 5 commits intorust-lang:mainfrom
WaffleLapkin:dangle-maybe

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@WaffleLapkin WaffleLapkin commented Dec 3, 2025

cc #118166

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 3, 2025
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@WaffleLapkin WaffleLapkin force-pushed the dangle-maybe branch 2 times, most recently from c428102 to 5bd5888 Compare December 3, 2025 20:47
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theemathas
theemathas reviewed Dec 4, 2025
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library/core/src/mem/manually_drop.rs Outdated
Comment on lines 187 to 192
#[stable(feature = "manually_drop", since = "1.20.0")]
impl<T: ?Sized + PartialEq> PartialEq for ManuallyDrop<T> {
fn eq(&self, other: &Self) -> bool {
self.value.as_ref().eq(other.value.as_ref())
}
}
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You also need to implement StructuralPartialEq, since ManuallyDrop in a const can currently be used in patterns on stable.

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joboet commented Dec 5, 2025

I don't know how much this is still an experiment, but if you're looking for more coverage, you might want to replace the MaybeUninit-based manual MaybeDangling implementation in std:

rust/library/std/src/thread/lifecycle.rs

Lines 61 to 82 in 864339a

// Pass `f` in `MaybeUninit` because actually that closure might *run longer than the lifetime of `F`*.
// See <https://github.com/rust-lang/rust/issues/101983> for more details.
// To prevent leaks we use a wrapper that drops its contents.
#[repr(transparent)]
struct MaybeDangling<T>(MaybeUninit<T>);
impl<T> MaybeDangling<T> {
fn new(x: T) -> Self {
MaybeDangling(MaybeUninit::new(x))
}
fn into_inner(self) -> T {
// Make sure we don't drop.
let this = ManuallyDrop::new(self);
// SAFETY: we are always initialized.
unsafe { this.0.assume_init_read() }
}
}
impl<T> Drop for MaybeDangling<T> {
fn drop(&mut self) {
// SAFETY: we are always initialized.
unsafe { self.0.assume_init_drop() };
}
}

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@WaffleLapkin WaffleLapkin force-pushed the dangle-maybe branch 2 times, most recently from ac4154d to 259187e Compare December 13, 2025 13:16
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@WaffleLapkin WaffleLapkin mentioned this pull request Dec 13, 2025
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matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Dec 15, 2025
@matthiaskrgr
Rollup merge of rust-lang#149950 - WaffleLapkin:inlines-ur-mu-into-as…
b8218da 
…m, r=jdonszelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Dec 15, 2025
@matthiaskrgr
Rollup merge of rust-lang#149950 - WaffleLapkin:inlines-ur-mu-into-as…
a18e0ae 
…m, r=jdonszelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
Zalathar added a commit to Zalathar/rust that referenced this pull request Dec 16, 2025
@Zalathar
Rollup merge of rust-lang#149950 - WaffleLapkin:inlines-ur-mu-into-as…
6fb1ad7 
…m, r=jdonszelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
Zalathar added a commit to Zalathar/rust that referenced this pull request Dec 16, 2025
@Zalathar
Rollup merge of rust-lang#149950 - WaffleLapkin:inlines-ur-mu-into-as…
461d0d0 
…m, r=jdonszelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
rust-timer added a commit that referenced this pull request Dec 16, 2025
@rust-timer
Unrolled build for #149950
3e82c40 
Rollup merge of #149950 - WaffleLapkin:inlines-ur-mu-into-asm, r=jdonszelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from #149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
@WaffleLapkin WaffleLapkin force-pushed the dangle-maybe branch 2 times, most recently from ed7011f to 97cda84 Compare December 17, 2025 21:41
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bjorn3 pushed a commit to rust-lang/rustc_codegen_cranelift that referenced this pull request Dec 18, 2025
@Zalathar
Rollup merge of #149950 - WaffleLapkin:inlines-ur-mu-into-asm, r=jdon…
d57ccad 
…szelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang/rust#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
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bors commented Dec 24, 2025

☔ The latest upstream changes (presumably #150334) made this pull request unmergeable. Please resolve the merge conflicts.

@WaffleLapkin WaffleLapkin force-pushed the dangle-maybe branch 5 times, most recently from a4b6adc to f7213a1 Compare December 27, 2025 21:22
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rust-log-analyzer commented Dec 27, 2025

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) 

---- [codegen] tests/codegen-llvm/manually_drop_refs.rs stdout ----
------FileCheck stdout------------------------------

------FileCheck stderr------------------------------
/checkout/tests/codegen-llvm/manually_drop_refs.rs:6:11: error: CHECK: expected string not found in input
// CHECK: define noundef nonnull ptr @f(ptr noundef nonnull readnone returned {{(captures\(ret: address, provenance\) )?}}%x) unnamed_addr
          ^
/checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll:1:1: note: scanning from here
; ModuleID = 'manually_drop_refs.f6bd3c5d90b64e2d-cgu.0'
^
/checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll:7:11: note: possible intended match here
define dso_local noundef nonnull ptr @f(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 {
          ^

Input file: /checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll
Check file: /checkout/tests/codegen-llvm/manually_drop_refs.rs

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           1: ; ModuleID = 'manually_drop_refs.f6bd3c5d90b64e2d-cgu.0' 
check:6'0     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
           2: source_filename = "manually_drop_refs.f6bd3c5d90b64e2d-cgu.0" 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           3: target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-i128:128-n32:64-S128-ni:1:10:20" 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           4: target triple = "wasm32-unknown-wasip1" 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           5:  
check:6'0     ~
           6: ; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           7: define dso_local noundef nonnull ptr @f(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 { 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
check:6'1               ?                                                                                                                             possible intended match
           8: start: 
check:6'0     ~~~~~~~
           9:  ret ptr %x 
check:6'0     ~~~~~~~~~~~~
          10: } 
check:6'0     ~~
          11:  
check:6'0     ~
          12: ; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          13: define dso_local noundef nonnull ptr @g(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 { 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          14: start: 
check:6'0     ~~~~~~~
          15:  ret ptr %x 
check:6'0     ~~~~~~~~~~~~
          16: } 
check:6'0     ~~
          17:  
check:6'0     ~
          18: ; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          19: define dso_local noundef nonnull ptr @h(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 { 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          20: start: 
check:6'0     ~~~~~~~
          21:  ret ptr %x 
check:6'0     ~~~~~~~~~~~~
          22: } 
check:6'0     ~~
          23:  
check:6'0     ~
          24: attributes #0 = { mustprogress nofree norecurse nosync nounwind willreturn memory(none) "target-cpu"="generic" } 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          25:  
check:6'0     ~
          26: !llvm.ident = !{!0} 
check:6'0     ~~~~~~~~~~~~~~~~~~~~
          27:  
check:6'0     ~
          28: !0 = !{!"rustc version 1.94.0-nightly (bfc66f8cd 2025-12-27)"} 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>>

------------------------------------------

error: verification with 'FileCheck' failed
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin/FileCheck" "--input-file" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll" "/checkout/tests/codegen-llvm/manually_drop_refs.rs" "--check-prefix=CHECK" "--allow-unused-prefixes" "--dump-input-context" "100"
stdout: none
--- stderr -------------------------------
/checkout/tests/codegen-llvm/manually_drop_refs.rs:6:11: error: CHECK: expected string not found in input
// CHECK: define noundef nonnull ptr @f(ptr noundef nonnull readnone returned {{(captures\(ret: address, provenance\) )?}}%x) unnamed_addr
          ^
/checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll:1:1: note: scanning from here
; ModuleID = 'manually_drop_refs.f6bd3c5d90b64e2d-cgu.0'
^
/checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll:7:11: note: possible intended match here
define dso_local noundef nonnull ptr @f(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 {
          ^

Input file: /checkout/obj/build/x86_64-unknown-linux-gnu/test/codegen-llvm/manually_drop_refs/manually_drop_refs.ll
Check file: /checkout/tests/codegen-llvm/manually_drop_refs.rs

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           1: ; ModuleID = 'manually_drop_refs.f6bd3c5d90b64e2d-cgu.0' 
check:6'0     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
           2: source_filename = "manually_drop_refs.f6bd3c5d90b64e2d-cgu.0" 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           3: target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-i128:128-n32:64-S128-ni:1:10:20" 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           4: target triple = "wasm32-unknown-wasip1" 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           5:  
check:6'0     ~
           6: ; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           7: define dso_local noundef nonnull ptr @f(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 { 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
check:6'1               ?                                                                                                                             possible intended match
           8: start: 
check:6'0     ~~~~~~~
           9:  ret ptr %x 
check:6'0     ~~~~~~~~~~~~
          10: } 
check:6'0     ~~
          11:  
check:6'0     ~
          12: ; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          13: define dso_local noundef nonnull ptr @g(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 { 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          14: start: 
check:6'0     ~~~~~~~
          15:  ret ptr %x 
check:6'0     ~~~~~~~~~~~~
          16: } 
check:6'0     ~~
          17:  
check:6'0     ~
          18: ; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          19: define dso_local noundef nonnull ptr @h(ptr noundef nonnull readnone returned captures(ret: address, provenance) %x) unnamed_addr #0 { 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          20: start: 
check:6'0     ~~~~~~~
          21:  ret ptr %x 
check:6'0     ~~~~~~~~~~~~
          22: } 
check:6'0     ~~
          23:  
check:6'0     ~
          24: attributes #0 = { mustprogress nofree norecurse nosync nounwind willreturn memory(none) "target-cpu"="generic" } 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          25:  
check:6'0     ~
          26: !llvm.ident = !{!0} 
check:6'0     ~~~~~~~~~~~~~~~~~~~~
          27:  
check:6'0     ~
          28: !0 = !{!"rustc version 1.94.0-nightly (bfc66f8cd 2025-12-27)"} 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>>
------------------------------------------

---- [codegen] tests/codegen-llvm/manually_drop_refs.rs stdout end ----

Kobzol pushed a commit to Kobzol/rustc_codegen_cranelift that referenced this pull request Dec 29, 2025
@Zalathar
Rollup merge of #149950 - WaffleLapkin:inlines-ur-mu-into-asm, r=jdon…
1114ce2 
…szelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang/rust#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
christian-schilling pushed a commit to christian-schilling/rustc_codegen_cranelift that referenced this pull request Jan 27, 2026
@Zalathar
Rollup merge of #149950 - WaffleLapkin:inlines-ur-mu-into-asm, r=jdon…
a824d0a 
…szelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang/rust#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
christian-schilling pushed a commit to christian-schilling/rustc_codegen_cranelift that referenced this pull request Jan 27, 2026
@Zalathar
Rollup merge of #149950 - WaffleLapkin:inlines-ur-mu-into-asm, r=jdon…
612bb4c 
…szelmann

Simplify how inline asm handles `MaybeUninit`

This is just better, but this is also allows it to handle changes from rust-lang/rust#149614 (i.e. `ManuallyDrop` containing `MaybeDangle`).
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