Add `MaybeDangling` by WaffleLapkin · Pull Request #149614 · rust-lang/rust

WaffleLapkin · 2025-12-03T20:03:54Z

theemathas · 2025-12-04T03:33:34Z

library/core/src/mem/manually_drop.rs

+#[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())
+    }
 }


You also need to implement StructuralPartialEq, since ManuallyDrop in a const can currently be used in patterns on stable.

joboet · 2025-12-05T14:04:46Z

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() }; 
        
               } 
        
           }

…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`).

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`).

…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`).

bors · 2025-12-24T13:59:02Z

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

…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`).

Make `size`/`align` always correct rather than conditionally on the `safe` field. This makes it less error prone and easier to work with for `MaybeDangling` / potential future pointer kinds like `Aligned<_>`.

Instead of defaulting to `None` it now defaults to `Align::ONE` i.e. no alignment restriction. Codegen test changes are due to us now skipping `align 1` annotations (they are useless; not skipping them makes all the raw pointers gain an `align 1` annotation which doesn't seem any good)

rust-log-analyzer · 2026-03-05T12:13:03Z

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

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

/dev/sda15      105M  6.2M   99M   6% /boot/efi
tmpfs           1.6G   12K  1.6G   1% /run/user/1001
================================================================================

Sufficient disk space available (95102764KB >= 52428800KB). Skipping cleanup.
##[group]Run echo "[CI_PR_NUMBER=$num]"
echo "[CI_PR_NUMBER=$num]"
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
---
tests/pass/shims/x86/rounding-error.rs ... ok
tests/pass/shims/x86/intrinsics-x86-gfni.rs ... ok

FAILED TEST: tests/pass/0weak_memory/extra_cpp.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-e894q0" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/miri" "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass/0weak_memory" "tests/pass/0weak_memory/extra_cpp.rs" "-Zmiri-ignore-leaks" "--edition" "2021"

error: test got exit status: 1, but expected 0
 = note: compilation failed, but was expected to succeed

error: no output was expected
Execute `./miri test --bless` to update `tests/pass/0weak_memory/extra_cpp.stderr` to the actual output
+++ <stderr output>
error: Undefined Behavior: trying to retag from <2713> for Unique permission at alloc202[0x0], but that tag does not exist in the borrow stack for this location
##[error]  --> /checkout/library/core/src/mem/maybe_dangling.rs:109:9
   |
LL |         x
   |         ^
   |         |
   |         this error occurs as part of retag (of a reference/box inside this compound value) at alloc202[0x0..0x4]
   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <2713> was created by a Unique retag at offsets [0x0..0x4]
  --> /checkout/library/core/src/mem/maybe_dangling.rs:107:26
   |
LL |         let x = unsafe { ptr::read(&self.0) };
   |                          ^^^^^^^^^^^^^^^^^^
help: <2713> was later invalidated at offsets [0x0..0x4] by a Unique retag (of a reference/box inside this compound value)
  --> /checkout/library/core/src/mem/maybe_dangling.rs:108:21
   |
LL |         mem::forget(self);
   |                     ^^^^
   = note: this is on thread `unnamed-1`
   = note: stack backtrace:
           0: std::mem::MaybeDangling::<{closure@tests/pass/0weak_memory/extra_cpp.rs:47:20: 47:27}>::into_inner
               at /checkout/library/core/src/mem/maybe_dangling.rs:109:9: 109:10
           1: std::thread::lifecycle::spawn_unchecked::<'_, {closure@tests/pass/0weak_memory/extra_cpp.rs:47:20: 47:27}, u32>::{closure#1}
               at /checkout/library/std/src/thread/lifecycle.rs:67:17: 67:31
           2: <{closure@std::thread::lifecycle::spawn_unchecked<'_, {closure@tests/pass/0weak_memory/extra_cpp.rs:47:20: 47:27}, u32>::{closure#1}} as std::ops::FnOnce<()>>::call_once - shim(vtable)
               at /checkout/library/core/src/ops/function.rs:250:5: 250:71
           3: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
               at /checkout/library/alloc/src/boxed.rs:2240:9: 2240:52
           4: std::sys::thread::unix::Thread::new::thread_start
               at /checkout/library/std/src/sys/thread/unix.rs:118:17: 118:29
note: the last function in that backtrace got called indirectly due to this code
  --> tests/pass/0weak_memory/extra_cpp.rs:47:14
   |
LL |     let j1 = spawn(move || x.load(Relaxed));
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error



full stderr:
error: Undefined Behavior: trying to retag from <2713> for Unique permission at alloc202[0x0], but that tag does not exist in the borrow stack for this location
##[error]  --> /checkout/library/core/src/mem/maybe_dangling.rs:109:9
   |
LL |         x
   |         ^
   |         |
   |         this error occurs as part of retag (of a reference/box inside this compound value) at alloc202[0x0..0x4]
   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <2713> was created by a Unique retag at offsets [0x0..0x4]
  --> /checkout/library/core/src/mem/maybe_dangling.rs:107:26
   |
LL |         let x = unsafe { ptr::read(&self.0) };
   |                          ^^^^^^^^^^^^^^^^^^
help: <2713> was later invalidated at offsets [0x0..0x4] by a Unique retag (of a reference/box inside this compound value)
  --> /checkout/library/core/src/mem/maybe_dangling.rs:108:21
   |
LL |         mem::forget(self);
   |                     ^^^^
   = note: this is on thread `unnamed-1`
   = note: stack backtrace:
           0: std::mem::MaybeDangling::<{closure@tests/pass/0weak_memory/extra_cpp.rs:47:20: 47:27}>::into_inner
               at /checkout/library/core/src/mem/maybe_dangling.rs:109:9: 109:10
           1: std::thread::lifecycle::spawn_unchecked::<'_, {closure@tests/pass/0weak_memory/extra_cpp.rs:47:20: 47:27}, u32>::{closure#1}
               at /checkout/library/std/src/thread/lifecycle.rs:67:17: 67:31
           2: <{closure@std::thread::lifecycle::spawn_unchecked<'_, {closure@tests/pass/0weak_memory/extra_cpp.rs:47:20: 47:27}, u32>::{closure#1}} as std::ops::FnOnce<()>>::call_once - shim(vtable)
               at /checkout/library/core/src/ops/function.rs:250:5: 250:71
           3: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
               at /checkout/library/alloc/src/boxed.rs:2240:9: 2240:52
           4: std::sys::thread::unix::Thread::new::thread_start
               at /checkout/library/std/src/sys/thread/unix.rs:118:17: 118:29
note: the last function in that backtrace got called indirectly due to this code
  --> tests/pass/0weak_memory/extra_cpp.rs:47:14
   |
LL |     let j1 = spawn(move || x.load(Relaxed));
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error


full stdout:



FAILED TEST: tests/pass/concurrency/scope.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-e894q0" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/miri" "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass/concurrency" "tests/pass/concurrency/scope.rs" "--edition" "2021"

error: test got exit status: 1, but expected 0
 = note: compilation failed, but was expected to succeed

error: no output was expected
Execute `./miri test --bless` to update `tests/pass/concurrency/scope.stderr` to the actual output
+++ <stderr output>
error: Undefined Behavior: trying to retag from <5269> for Unique permission at alloc272[0x0], but that tag does not exist in the borrow stack for this location
##[error]  --> /checkout/library/core/src/mem/maybe_dangling.rs:109:9
   |
LL |         x
   |         ^
   |         |
   |         this error occurs as part of retag (of a reference/box inside this compound value) at alloc272[0x0..0x8]
   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <5269> was created by a Unique retag at offsets [0x0..0x8]
  --> /checkout/library/core/src/mem/maybe_dangling.rs:107:26
   |
LL |         let x = unsafe { ptr::read(&self.0) };
   |                          ^^^^^^^^^^^^^^^^^^
help: <5269> was later invalidated at offsets [0x0..0x8] by a Unique retag (of a reference/box inside this compound value)
  --> /checkout/library/core/src/mem/maybe_dangling.rs:108:21
   |
LL |         mem::forget(self);
   |                     ^^^^
   = note: this is on thread `unnamed-2`
   = note: stack backtrace:
           0: std::mem::MaybeDangling::<{closure@tests/pass/concurrency/scope.rs:12:17: 12:19}>::into_inner
               at /checkout/library/core/src/mem/maybe_dangling.rs:109:9: 109:10
           1: std::thread::lifecycle::spawn_unchecked::<'_, {closure@tests/pass/concurrency/scope.rs:12:17: 12:19}, ()>::{closure#1}
               at /checkout/library/std/src/thread/lifecycle.rs:67:17: 67:31
           2: <{closure@std::thread::lifecycle::spawn_unchecked<'_, {closure@tests/pass/concurrency/scope.rs:12:17: 12:19}, ()>::{closure#1}} as std::ops::FnOnce<()>>::call_once - shim(vtable)
               at /checkout/library/core/src/ops/function.rs:250:5: 250:71
           3: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
               at /checkout/library/alloc/src/boxed.rs:2240:9: 2240:52
           4: std::sys::thread::unix::Thread::new::thread_start
               at /checkout/library/std/src/sys/thread/unix.rs:118:17: 118:29
note: the last function in that backtrace got called indirectly due to this code
  --> tests/pass/concurrency/scope.rs:12:9
   |
LL | /         s.spawn(|| {
LL | |             let _s = format!("hello from the second scoped thread");
...  |
LL | |         });
   | |__________^

error: aborting due to 1 previous error



full stderr:
error: Undefined Behavior: trying to retag from <5269> for Unique permission at alloc272[0x0], but that tag does not exist in the borrow stack for this location
##[error]  --> /checkout/library/core/src/mem/maybe_dangling.rs:109:9
   |
LL |         x
   |         ^
   |         |
   |         this error occurs as part of retag (of a reference/box inside this compound value) at alloc272[0x0..0x8]
   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <5269> was created by a Unique retag at offsets [0x0..0x8]
  --> /checkout/library/core/src/mem/maybe_dangling.rs:107:26
   |
LL |         let x = unsafe { ptr::read(&self.0) };
   |                          ^^^^^^^^^^^^^^^^^^
help: <5269> was later invalidated at offsets [0x0..0x8] by a Unique retag (of a reference/box inside this compound value)
  --> /checkout/library/core/src/mem/maybe_dangling.rs:108:21
   |
LL |         mem::forget(self);
   |                     ^^^^
   = note: this is on thread `unnamed-2`
   = note: stack backtrace:
           0: std::mem::MaybeDangling::<{closure@tests/pass/concurrency/scope.rs:12:17: 12:19}>::into_inner
               at /checkout/library/core/src/mem/maybe_dangling.rs:109:9: 109:10
           1: std::thread::lifecycle::spawn_unchecked::<'_, {closure@tests/pass/concurrency/scope.rs:12:17: 12:19}, ()>::{closure#1}
               at /checkout/library/std/src/thread/lifecycle.rs:67:17: 67:31
           2: <{closure@std::thread::lifecycle::spawn_unchecked<'_, {closure@tests/pass/concurrency/scope.rs:12:17: 12:19}, ()>::{closure#1}} as std::ops::FnOnce<()>>::call_once - shim(vtable)
               at /checkout/library/core/src/ops/function.rs:250:5: 250:71
           3: <std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once
               at /checkout/library/alloc/src/boxed.rs:2240:9: 2240:52
           4: std::sys::thread::unix::Thread::new::thread_start
               at /checkout/library/std/src/sys/thread/unix.rs:118:17: 118:29
note: the last function in that backtrace got called indirectly due to this code
  --> tests/pass/concurrency/scope.rs:12:9
   |
LL | /         s.spawn(|| {
LL | |             let _s = format!("hello from the second scoped thread");
...  |
LL | |         });
   | |__________^

error: aborting due to 1 previous error
---
Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.30.3/src/lib.rs:365

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ BACKTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
   1: <color_eyre[1ba11ab6ce8f36c8]::config::EyreHook>::into_eyre_hook::{closure#0}<unknown>
      at <unknown source file>:<unknown line>
   2: <eyre[a72c708637b034ae]::Report>::from_adhoc::<&str><unknown>
      at <unknown source file>:<unknown line>
   3: ui_test[575ce783bed580f9]::run_tests_generic::<ui_test[575ce783bed580f9]::default_file_filter, ui[bf4bfb1bff49271e]::run_tests::{closure#1}, alloc[ee4e7bd47497d396]::boxed::Box<dyn ui_test[575ce783bed580f9]::status_emitter::StatusEmitter>><unknown>
      at <unknown source file>:<unknown line>
   4: ui[bf4bfb1bff49271e]::ui<unknown>
      at <unknown source file>:<unknown line>
   5: ui[bf4bfb1bff49271e]::main<unknown>
      at <unknown source file>:<unknown line>
   6: std[7f9a3599c25af3b3]::sys::backtrace::__rust_begin_short_backtrace::<fn() -> core[c2c1cf217ae1ec70]::result::Result<(), eyre[a72c708637b034ae]::Report>, core[c2c1cf217ae1ec70]::result::Result<(), eyre[a72c708637b034ae]::Report>><unknown>
      at <unknown source file>:<unknown line>
   7: std[7f9a3599c25af3b3]::rt::lang_start::<core[c2c1cf217ae1ec70]::result::Result<(), eyre[a72c708637b034ae]::Report>>::{closure#0}<unknown>
      at <unknown source file>:<unknown line>
   8: std[7f9a3599c25af3b3]::rt::lang_start_internal<unknown>
      at <unknown source file>:<unknown line>
   9: main<unknown>
      at <unknown source file>:<unknown line>
  10: __libc_start_main<unknown>
      at <unknown source file>:<unknown line>
  11: _start<unknown>
      at <unknown source file>:<unknown line>

Run with COLORBT_SHOW_HIDDEN=1 environment variable to disable frame filtering.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/deps/ui-967339ea3dcc1e54` (exit status: 1)
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo test -Zwarnings --target x86_64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --manifest-path /checkout/src/tools/miri/Cargo.toml -- [workdir=/checkout]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:191:21
Executed at: src/bootstrap/src/core/build_steps/test.rs:739:19

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test --stage 2 src/tools/miri src/tools/miri/cargo-miri`
Build completed unsuccessfully in 0:47:12
  local time: Thu Mar  5 12:12:49 UTC 2026
  network time: Thu, 05 Mar 2026 12:12:49 GMT
##[error]Process completed with exit code 1.

rust-bors · 2026-03-20T11:11:22Z

☔ The latest upstream changes (presumably #154123) made this pull request unmergeable. Please resolve the merge conflicts.