Make `FlatMapInPlaceVec` an unsafe trait. by nnethercote · Pull Request #155951 · rust-lang/rust

nnethercote · 2026-04-29T00:18:28Z

Because the memory safety of FlatMapInPlace::flat_map_in_place depends on FlatMapInPlaceVec impls behaving correctly.

r? @chenyukang

Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly.

nnethercote · 2026-04-29T00:19:01Z

Thanks to @programmerjake for pointing this out.

Kivooeo

looks fine to me, feel free to r=me

View changes since this review

nnethercote · 2026-04-29T02:26:29Z

@bors r=kivooeo rollup

rust-bors · 2026-04-29T02:26:32Z

📌 Commit a940270 has been approved by kivooeo

It is now in the queue for this repository.

Nikolay-Sitnikov · 2026-04-29T02:54:27Z

...relies on impls of this trait implementing all the operations correctly.

Is the "correct way to implement this trait" explained in more detail somewhere else, or is that statement just referring to the obvious limitation of "operations must be pure/idempotent outside of the expected mutation"? It seems strange to have a comment requiring you to promise that you implemented the trait correctly, but no comments about what "correctly" means for this trait.

nnethercote · 2026-04-29T03:18:35Z

The meaning of correct operation is left implicit because it's obvious. E.g. len should return the actual length not usize::MAX, set_len should actually set the len and not be a no-op, as_ptr should return a pointer to the start of the buffer and not to some random place, etc.

@chenyukang

…ec, r=kivooeo Make `FlatMapInPlaceVec` an unsafe trait. Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly. r? @chenyukang

@chenyukang

…ec, r=kivooeo Make `FlatMapInPlaceVec` an unsafe trait. Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly. r? @chenyukang

Rollup of 9 pull requests Successful merges: - #151742 (Remove redundant information in `rustc_abi::Variants`) - #155856 (std_detect: support detecting more features on aarch64 Windows) - #155861 (Suggest `[const] Trait` bounds in more places) - #155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - #155916 (Update with new LLVM 22 target for `wasm32-wali-linux-musl` target) - #155935 (remap OUT_DIR paths to fix build script path leakage in crate metadata. ) - #155950 (use the new `//@ needs-asm-mnemonic: ret` more) - #155949 (Update `opt_ast_lowering_delayed_lints` query to allow "stealing" lints, allowing to use `FnOnce` instead of `Fn`) - #155951 (Make `FlatMapInPlaceVec` an unsafe trait.)

@chenyukang

…ec, r=kivooeo Make `FlatMapInPlaceVec` an unsafe trait. Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly. r? @chenyukang

…uwer Rollup of 12 pull requests Successful merges: - #155189 (simd_reduce_min/max: remove float support) - #155721 (When archive format is wrong produce an error instead of ICE) - #155794 (privacy: share effective visibility initialization) - #155856 (std_detect: support detecting more features on aarch64 Windows) - #155861 (Suggest `[const] Trait` bounds in more places) - #155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - #155916 (Update with new LLVM 22 target for `wasm32-wali-linux-musl` target) - #155935 (remap OUT_DIR paths to fix build script path leakage in crate metadata. ) - #155950 (use the new `//@ needs-asm-mnemonic: ret` more) - #155958 (ci(free-disk-space): remove more tools and fix warnings) - #155949 (Update `opt_ast_lowering_delayed_lints` query to allow "stealing" lints, allowing to use `FnOnce` instead of `Fn`) - #155951 (Make `FlatMapInPlaceVec` an unsafe trait.)

@chenyukang

…ec, r=kivooeo Make `FlatMapInPlaceVec` an unsafe trait. Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly. r? @chenyukang

…uwer Rollup of 22 pull requests Successful merges: - #154149 (resolve: Extend `ambiguous_import_visibilities` deprecation lint to glob-vs-glob ambiguities) - #155189 (simd_reduce_min/max: remove float support) - #155453 (apply Cortex-A53 errata 843419 mitigation to the AArch64 Linux targets) - #155562 (Add a missing `GenericTypeVisitable`, and avoid having interner traits for `FnSigKind` and `Abi`) - #155608 (rustc_middle: Implement the `partial_cmp` operation for `DefId`s) - #155721 (When archive format is wrong produce an error instead of ICE) - #155794 (privacy: share effective visibility initialization) - #155832 (c-variadic: more precise compatibility check in const-eval) - #155856 (std_detect: support detecting more features on aarch64 Windows) - #155861 (Suggest `[const] Trait` bounds in more places) - #155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - #155916 (Update with new LLVM 22 target for `wasm32-wali-linux-musl` target) - #155935 (remap OUT_DIR paths to fix build script path leakage in crate metadata. ) - #155950 (use the new `//@ needs-asm-mnemonic: ret` more) - #155958 (ci(free-disk-space): remove more tools and fix warnings) - #155966 (miri subtree update) - #155711 (bump curl-sys and openssl-sys to support OpenSSL 4.0.x) - #155831 (Add `AcceptContext::expect_key_value`) - #155877 (Avoid misleading return-type note for foreign `Fn` callees) - #155949 (Update `opt_ast_lowering_delayed_lints` query to allow "stealing" lints, allowing to use `FnOnce` instead of `Fn`) - #155951 (Make `FlatMapInPlaceVec` an unsafe trait.) - #155967 (Fix `doc_cfg` feature for extern items)

…uwer Rollup of 21 pull requests Successful merges: - #155966 (miri subtree update) - #154149 (resolve: Extend `ambiguous_import_visibilities` deprecation lint to glob-vs-glob ambiguities) - #155189 (simd_reduce_min/max: remove float support) - #155562 (Add a missing `GenericTypeVisitable`, and avoid having interner traits for `FnSigKind` and `Abi`) - #155608 (rustc_middle: Implement the `partial_cmp` operation for `DefId`s) - #155721 (When archive format is wrong produce an error instead of ICE) - #155794 (privacy: share effective visibility initialization) - #155832 (c-variadic: more precise compatibility check in const-eval) - #155856 (std_detect: support detecting more features on aarch64 Windows) - #155861 (Suggest `[const] Trait` bounds in more places) - #155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - #155916 (Update with new LLVM 22 target for `wasm32-wali-linux-musl` target) - #155935 (remap OUT_DIR paths to fix build script path leakage in crate metadata. ) - #155950 (use the new `//@ needs-asm-mnemonic: ret` more) - #155958 (ci(free-disk-space): remove more tools and fix warnings) - #155711 (bump curl-sys and openssl-sys to support OpenSSL 4.0.x) - #155831 (Add `AcceptContext::expect_key_value`) - #155877 (Avoid misleading return-type note for foreign `Fn` callees) - #155949 (Update `opt_ast_lowering_delayed_lints` query to allow "stealing" lints, allowing to use `FnOnce` instead of `Fn`) - #155951 (Make `FlatMapInPlaceVec` an unsafe trait.) - #155967 (Fix `doc_cfg` feature for extern items)

@chenyukang

Rollup merge of #155951 - nnethercote:unsafe-FlatMapInPlaceVec, r=kivooeo Make `FlatMapInPlaceVec` an unsafe trait. Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly. r? @chenyukang

JonathanBrouwer · 2026-04-30T06:21:49Z

@rust-timer build a14ea70

…uwer Rollup of 21 pull requests Successful merges: - rust-lang/rust#155966 (miri subtree update) - rust-lang/rust#154149 (resolve: Extend `ambiguous_import_visibilities` deprecation lint to glob-vs-glob ambiguities) - rust-lang/rust#155189 (simd_reduce_min/max: remove float support) - rust-lang/rust#155562 (Add a missing `GenericTypeVisitable`, and avoid having interner traits for `FnSigKind` and `Abi`) - rust-lang/rust#155608 (rustc_middle: Implement the `partial_cmp` operation for `DefId`s) - rust-lang/rust#155721 (When archive format is wrong produce an error instead of ICE) - rust-lang/rust#155794 (privacy: share effective visibility initialization) - rust-lang/rust#155832 (c-variadic: more precise compatibility check in const-eval) - rust-lang/rust#155856 (std_detect: support detecting more features on aarch64 Windows) - rust-lang/rust#155861 (Suggest `[const] Trait` bounds in more places) - rust-lang/rust#155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - rust-lang/rust#155916 (Update with new LLVM 22 target for `wasm32-wali-linux-musl` target) - rust-lang/rust#155935 (remap OUT_DIR paths to fix build script path leakage in crate metadata. ) - rust-lang/rust#155950 (use the new `//@ needs-asm-mnemonic: ret` more) - rust-lang/rust#155958 (ci(free-disk-space): remove more tools and fix warnings) - rust-lang/rust#155711 (bump curl-sys and openssl-sys to support OpenSSL 4.0.x) - rust-lang/rust#155831 (Add `AcceptContext::expect_key_value`) - rust-lang/rust#155877 (Avoid misleading return-type note for foreign `Fn` callees) - rust-lang/rust#155949 (Update `opt_ast_lowering_delayed_lints` query to allow "stealing" lints, allowing to use `FnOnce` instead of `Fn`) - rust-lang/rust#155951 (Make `FlatMapInPlaceVec` an unsafe trait.) - rust-lang/rust#155967 (Fix `doc_cfg` feature for extern items)

rust-timer · 2026-04-30T09:07:47Z

Finished benchmarking commit (a14ea70): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 0.7%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.9%	[2.9%, 2.9%]	1
Regressions ❌ (secondary)	2.7%	[1.7%, 3.4%]	3
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-2.5%	[-2.7%, -2.4%]	3
All ❌✅ (primary)	0.7%	[-1.5%, 2.9%]	2

Cycles

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Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 487.214s -> 482.531s (-0.96%)
Artifact size: 390.94 MiB -> 390.92 MiB (-0.00%)

Make FlatMapInPlaceVec an unsafe trait.

a940270

Because the memory safety of `FlatMapInPlace::flat_map_in_place` depends on `FlatMapInPlaceVec` impls behaving correctly.

rustbot assigned chenyukang Apr 29, 2026

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. labels Apr 29, 2026

Kivooeo approved these changes Apr 29, 2026

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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 Apr 29, 2026

jhpratt mentioned this pull request Apr 29, 2026

Rollup of 7 pull requests #155956

Closed

jhpratt mentioned this pull request Apr 29, 2026

Rollup of 9 pull requests #155960

Closed

JonathanBrouwer mentioned this pull request Apr 29, 2026

Rollup of 12 pull requests #155964

Closed

JonathanBrouwer mentioned this pull request Apr 29, 2026

Rollup of 22 pull requests #155976

Closed

JonathanBrouwer mentioned this pull request Apr 29, 2026

Rollup of 21 pull requests #155979

Merged

rust-bors Bot merged commit 4663607 into rust-lang:main Apr 30, 2026
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rustbot added this to the 1.97.0 milestone Apr 30, 2026

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Make `FlatMapInPlaceVec` an unsafe trait.#155951

Make `FlatMapInPlaceVec` an unsafe trait.#155951
rust-bors[bot] merged 1 commit intorust-lang:mainfrom
nnethercote:unsafe-FlatMapInPlaceVec

nnethercote commented Apr 29, 2026

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