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implement CovariantUnsafeCell#159738

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WaffleLapkin:covariant-unsafe-cell
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implement CovariantUnsafeCell#159738
WaffleLapkin wants to merge 2 commits into
rust-lang:mainfrom
WaffleLapkin:covariant-unsafe-cell

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@WaffleLapkin WaffleLapkin commented Jul 22, 2026

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. 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 Jul 22, 2026
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r? @jhpratt

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WaffleLapkin
WaffleLapkin commented Jul 22, 2026
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Comment thread library/core/src/cell/covariant_unsafe_cell.rs
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WaffleLapkin force-pushed the covariant-unsafe-cell branch 2 times, most recently from b2a7ea2 to c37239a Compare July 22, 2026 19:53
@ChayimFriedman2

ChayimFriedman2 commented Jul 23, 2026

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This implements CovariantUnsafeCell on top of UnsafeCell, but it seems more natural to do the opposite - then UnsafeCell can be a normal library type, without any special-casing in the compiler (the diff will be larger though, and it may also have compile time perf implications).

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WaffleLapkin commented Jul 24, 2026 •
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@ChayimFriedman2 I call this out explicitly in code:

rust/library/core/src/cell/covariant_unsafe_cell.rs

Lines 10 to 18 in c37239a

// Implementation note:
//
// We could make `CovariantUnsafeCell` be the canonical lang item and make `UnsafeCell` a wrapper
// over it, with `PhantomData<*mut T>`. That would however be a huge compiler change, without clear
// benefit.
//
// As such, `CovariantUnsafeCell` is wrapping `UnsafeCell` instead. It is a lang-item only to
// hardcode its variance to be **co**variant in `T`, even though it is wrapping `UnsafeCell` which
// is **in**variant in `T`.

If nothing else, I don't want to have to change all compiler comments from referring to UnsafeCell to referring to CovariantUnsafeCell.

This was referenced Jul 24, 2026
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@jhpratt

jhpratt commented Jul 25, 2026

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Let's handle any major change like that in a followup, presumably with team input on whether it's worthwhile.

@bors r+

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rust-bors Bot commented Jul 25, 2026

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📌 Commit c37239a has been approved by jhpratt

It is now in the queue for this repository.

@rust-bors rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 25, 2026
jhpratt added a commit to jhpratt/rust that referenced this pull request Jul 25, 2026
@jhpratt
Rollup merge of rust-lang#159738 - WaffleLapkin:covariant-unsafe-cell…
b0d8666 
…, r=jhpratt

implement `CovariantUnsafeCell`

Implements rust-lang#159735 / rust-lang/libs-team#815.
@jhpratt jhpratt mentioned this pull request Jul 25, 2026
Closed
rust-bors Bot pushed a commit that referenced this pull request Jul 25, 2026
@bors
Auto merge of #159888 - jhpratt:rollup-EWfAsjw, r=jhpratt
336190f 
Rollup of 20 pull requests

Successful merges:

 - #138618 (Support using const pointers in asm `const` operand)
 - #157962 (Lower paths to functions in const args as ConstKind::Error)
 - #158404 (trait_solver: normalize next-gen region constraints)
 - #158709 (rustdoc: warn on improperly interleaved HTML/MD)
 - #159720 (document #[global_allocator] constraints)
 - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled)
 - #159738 (implement `CovariantUnsafeCell`)
 - #159740 (reuse regular exported_non_generic_symbols logic in Miri)
 - #159780 (check `extern "custom"` function pointers)
 - #159786 (rustdoc-js: ignore editor temp files in test folder discovery)
 - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new)
 - #155388 (stepping into where-clauses during normalization may be productive)
 - #155914 (when bailing on ambiguity, don't force other results to ambig)
 - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`)
 - #159676 (Update wasm-component-ld to 0.5.27)
 - #159730 (allow accessing the contents of UnsafeCell without going through get)
 - #159809 (Avoid `#[target_features]`)
 - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate)
 - #159853 (Updated expect messages for `CString` struct and method documentation)
 - #159877 (Revert "Export `derive` at `core::derive` and `std::derive`")
rust-bors Bot pushed a commit that referenced this pull request Jul 25, 2026
@bors
Auto merge of #159888 - jhpratt:rollup-EWfAsjw, r=jhpratt
b882550 
Rollup of 20 pull requests

Successful merges:

 - #138618 (Support using const pointers in asm `const` operand)
 - #157962 (Lower paths to functions in const args as ConstKind::Error)
 - #158404 (trait_solver: normalize next-gen region constraints)
 - #158709 (rustdoc: warn on improperly interleaved HTML/MD)
 - #159720 (document #[global_allocator] constraints)
 - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled)
 - #159738 (implement `CovariantUnsafeCell`)
 - #159740 (reuse regular exported_non_generic_symbols logic in Miri)
 - #159780 (check `extern "custom"` function pointers)
 - #159786 (rustdoc-js: ignore editor temp files in test folder discovery)
 - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new)
 - #155388 (stepping into where-clauses during normalization may be productive)
 - #155914 (when bailing on ambiguity, don't force other results to ambig)
 - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`)
 - #159676 (Update wasm-component-ld to 0.5.27)
 - #159730 (allow accessing the contents of UnsafeCell without going through get)
 - #159809 (Avoid `#[target_features]`)
 - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate)
 - #159853 (Updated expect messages for `CString` struct and method documentation)
 - #159877 (Revert "Export `derive` at `core::derive` and `std::derive`")
GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this pull request Jul 25, 2026
@GuillaumeGomez
Rollup merge of rust-lang#159738 - WaffleLapkin:covariant-unsafe-cell…
843b254 
…, r=jhpratt

implement `CovariantUnsafeCell`

Implements rust-lang#159735 / rust-lang/libs-team#815.
@GuillaumeGomez GuillaumeGomez mentioned this pull request Jul 25, 2026
Closed
rust-bors Bot pushed a commit that referenced this pull request Jul 25, 2026
@bors
Auto merge of #159903 - GuillaumeGomez:rollup-C5Wk3Te, r=GuillaumeGomez
91fd797 
Rollup of 23 pull requests

Successful merges:

 - #159720 (document #[global_allocator] constraints)
 - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled)
 - #159738 (implement `CovariantUnsafeCell`)
 - #159740 (reuse regular exported_non_generic_symbols logic in Miri)
 - #159780 (check `extern "custom"` function pointers)
 - #159786 (rustdoc-js: ignore editor temp files in test folder discovery)
 - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new)
 - #155388 (stepping into where-clauses during normalization may be productive)
 - #155914 (when bailing on ambiguity, don't force other results to ambig)
 - #159204 (Add support to caller_location to rustc_public)
 - #159411 ([rustdoc] Correctly handle output options with --show-coverage)
 - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`)
 - #159676 (Update wasm-component-ld to 0.5.27)
 - #159730 (allow accessing the contents of UnsafeCell without going through get)
 - #159809 (Avoid `#[target_features]`)
 - #159810 (Add tuple never coercion collection regression test)
 - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate)
 - #159853 (Updated expect messages for `CString` struct and method documentation)
 - #159877 (Revert "Export `derive` at `core::derive` and `std::derive`")
 - #159878 (bootstrap: Remove obsolete option `build.compiletest-use-stage0-libtest`)
 - #159882 (Update expect messages in library/alloc/boxed.rs and library/alloc/string.rs to follow the style guide)
 - #159891 (Split multiline derives into std/rustc macros)
 - #159895 (rustc-dev-guide subtree update)
GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this pull request Jul 25, 2026
@GuillaumeGomez
Rollup merge of rust-lang#159738 - WaffleLapkin:covariant-unsafe-cell…
88965dd 
…, r=jhpratt

implement `CovariantUnsafeCell`

Implements rust-lang#159735 / rust-lang/libs-team#815.
@GuillaumeGomez GuillaumeGomez mentioned this pull request Jul 25, 2026
Open
rust-bors Bot pushed a commit that referenced this pull request Jul 25, 2026
@bors
Auto merge of #159905 - GuillaumeGomez:rollup-oGAHN5d, r=GuillaumeGomez
0725aa3 
Rollup of 23 pull requests

Successful merges:

 - #159673 (bootstrap: forward -fdebug-prefix-map when using cc)
 - #159720 (document #[global_allocator] constraints)
 - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled)
 - #159738 (implement `CovariantUnsafeCell`)
 - #159740 (reuse regular exported_non_generic_symbols logic in Miri)
 - #159780 (check `extern "custom"` function pointers)
 - #159786 (rustdoc-js: ignore editor temp files in test folder discovery)
 - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new)
 - #155388 (stepping into where-clauses during normalization may be productive)
 - #155914 (when bailing on ambiguity, don't force other results to ambig)
 - #159204 (Add support to caller_location to rustc_public)
 - #159411 ([rustdoc] Correctly handle output options with --show-coverage)
 - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`)
 - #159676 (Update wasm-component-ld to 0.5.27)
 - #159730 (allow accessing the contents of UnsafeCell without going through get)
 - #159809 (Avoid `#[target_features]`)
 - #159810 (Add tuple never coercion collection regression test)
 - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate)
 - #159853 (Updated expect messages for `CString` struct and method documentation)
 - #159878 (bootstrap: Remove obsolete option `build.compiletest-use-stage0-libtest`)
 - #159882 (Update expect messages in library/alloc/boxed.rs and library/alloc/string.rs to follow the style guide)
 - #159891 (Split multiline derives into std/rustc macros)
 - #159895 (rustc-dev-guide subtree update)
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