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Remove special-case handling of vec.split_off(0)
#119917
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r? @cuviper (rustbot has picked a reviewer for you, use r? to override) |
That isn't really a cost since it's the same old allocation. I'd say the extra cost is the
😮💨 I wish people stopped documenting implementation details that aren't necessary to the function of a method. I think we should ping @richkadel though, perhaps he can give arguments in favor of the existing code. Also note that we now have |
The “cost” that I have in mind is not just the larger allocation for In many cases, that cost won't matter. The returned vector might happen to be more than half-full, or it might be very short-lived, or it might be only a negligible fraction of the overall program's memory usage. But if (If the user knows this quirk of the implementation, and wants to avoid it, they may end up having to copy the returned vector into a new vector with a more appropriate capacity. Doing so completely undermines the benefit of trying to avoid the copy in the first place.) If the standard library decides to incur that extra cost on the user's behalf in the (I think it's also significant that this choice of implementation only comes up for the
Curiously, Calling |
Thank you for identifying this issue and raising some great points! I really appreciate that @Zalathar took the time to read through my original defense in #76682, so I don't need to reiterate everything. I do want to raise awareness (for anyone that didn't read through that PR) that there were rudimentary tests done to show the performance benefit of the change, for the use case I was trying to solve. So to revert it now will negatively impact existing code that currently depends on the behavior from 2021 until now. In the PR, we did discuss the fact that the rustdoc for I did mention So to simply revert it puts us back in what I think is still a less than ideal place. But I 100% agree that the capacity issues of both the initial value and the returned value are problematic. My opinion is just mine, so I hope you get input from a few others, including @Mark-Simulacrum who approved the original change, but I suggest:
If you change it, I think it's especially important to update the rustdoc this time around. Recommending the |
There are other possible approaches such as only moving the allocation when it's not significantly oversized. |
Yeah, regardless of whether this PR is accepted or not, it would be good to add suggestions for other similar options, especially in the
Also as mentioned above, the current implementation is inconsistent with the “newly allocated vector” wording, so that tension ought to be resolved in some way. |
I have also found that https://doc.rust-lang.org/std/collections/index.html#sequences lists the performance of |
As long as there is no call to realloc happening capacity shouldn't factor into performance since requesting uninitialized memory from the OS doesn't depend on the size of the allocation. |
I think this makes sense, especially with doc additions to discuss the various capacity options. But it's enough of a change to warrant talking as a team. @rustbot +I-libs-nominated |
This was raised in https://rust-lang.zulipchat.com/#narrow/stream/259402-t-libs.2Fmeetings/topic/Meeting.202024-01-17/near/416062684, but that particular discussion didn't reach a specific conclusion/consensus before moving on to other nominated issues. |
…s, r=dtolnay Document some alternatives to `Vec::split_off` One of the discussion points that came up in rust-lang#119917 is that some people use `Vec::split_off` in cases where they probably shouldn't, because the alternatives (like `mem::take`) are hard to discover. This PR adds some suggestions to the documentation of `split_off` that should point people towards alternatives that might be more appropriate for their use-case. I've deliberately tried to keep these changes as simple and uncontroversial as possible, so that they don't depend on how the team decides to handle the concerns raised in rust-lang#119917. That's why I haven't touched the existing documentation for `split_off`, and haven't added links to `split_off` to the documentation of other methods.
…s, r=dtolnay Document some alternatives to `Vec::split_off` One of the discussion points that came up in rust-lang#119917 is that some people use `Vec::split_off` in cases where they probably shouldn't, because the alternatives (like `mem::take`) are hard to discover. This PR adds some suggestions to the documentation of `split_off` that should point people towards alternatives that might be more appropriate for their use-case. I've deliberately tried to keep these changes as simple and uncontroversial as possible, so that they don't depend on how the team decides to handle the concerns raised in rust-lang#119917. That's why I haven't touched the existing documentation for `split_off`, and haven't added links to `split_off` to the documentation of other methods.
Rollup merge of rust-lang#120180 - Zalathar:vec-split-off-alternatives, r=dtolnay Document some alternatives to `Vec::split_off` One of the discussion points that came up in rust-lang#119917 is that some people use `Vec::split_off` in cases where they probably shouldn't, because the alternatives (like `mem::take`) are hard to discover. This PR adds some suggestions to the documentation of `split_off` that should point people towards alternatives that might be more appropriate for their use-case. I've deliberately tried to keep these changes as simple and uncontroversial as possible, so that they don't depend on how the team decides to handle the concerns raised in rust-lang#119917. That's why I haven't touched the existing documentation for `split_off`, and haven't added links to `split_off` to the documentation of other methods.
After a followup discussion, and with the docs already added in #120180, I think we're good! @bors r+ |
…iaskrgr Rollup of 9 pull requests Successful merges: - rust-lang#117420 (Make `#![allow_internal_unstable(..)]` work with `stmt_expr_attributes`) - rust-lang#117678 (Stabilize `slice_group_by`) - rust-lang#119917 (Remove special-case handling of `vec.split_off(0)`) - rust-lang#120117 (Update `std::io::Error::downcast` return type) - rust-lang#120329 (RFC 3349 precursors) - rust-lang#120339 (privacy: Refactor top-level visiting in `NamePrivacyVisitor`) - rust-lang#120345 (Clippy subtree update) - rust-lang#120360 (Don't fire `OPAQUE_HIDDEN_INFERRED_BOUND` on sized return of AFIT) - rust-lang#120372 (Fix outdated comment on Box) r? `@ghost` `@rustbot` modify labels: rollup
Rollup merge of rust-lang#119917 - Zalathar:split-off, r=cuviper Remove special-case handling of `vec.split_off(0)` rust-lang#76682 added special handling to `Vec::split_off` for the case where `at == 0`. Instead of copying the vector's contents into a freshly-allocated vector and returning it, the special-case code steals the old vector's allocation, and replaces it with a new (empty) buffer with the same capacity. That eliminates the need to copy the existing elements, but comes at a surprising cost, as seen in rust-lang#119913. The returned vector's capacity is no longer determined by the size of its contents (as would be expected for a freshly-allocated vector), and instead uses the full capacity of the old vector. In cases where the capacity is large but the size is small, that results in a much larger capacity than would be expected from reading the documentation of `split_off`. This is especially bad when `split_off` is called in a loop (to recycle a buffer), and the returned vectors have a wide variety of lengths. I believe it's better to remove the special-case code, and treat `at == 0` just like any other value: - The current documentation states that `split_off` returns a “newly allocated vector”, which is not actually true in the current implementation when `at == 0`. - If the value of `at` could be non-zero at runtime, then the caller has already agreed to the cost of a full memcpy of the taken elements in the general case. Avoiding that copy would be nice if it were close to free, but the different handling of capacity means that it is not. - If the caller specifically wants to avoid copying in the case where `at == 0`, they can easily implement that behaviour themselves using `mem::replace`. Fixes rust-lang#119913.
Pkgsrc changes: * Adapt checksums and patches. Upstream chnages: Version 1.77.0 (2024-03-21) ========================== - [Reveal opaque types within the defining body for exhaustiveness checking.] (rust-lang/rust#116821) - [Stabilize C-string literals.] (rust-lang/rust#117472) - [Stabilize THIR unsafeck.] (rust-lang/rust#117673) - [Add lint `static_mut_refs` to warn on references to mutable statics.] (rust-lang/rust#117556) - [Support async recursive calls (as long as they have indirection).] (rust-lang/rust#117703) - [Undeprecate lint `unstable_features` and make use of it in the compiler.] (rust-lang/rust#118639) - [Make inductive cycles in coherence ambiguous always.] (rust-lang/rust#118649) - [Get rid of type-driven traversal in const-eval interning] (rust-lang/rust#119044), only as a [future compatiblity lint] (rust-lang/rust#122204) for now. - [Deny braced macro invocations in let-else.] (rust-lang/rust#119062) Compiler -------- - [Include lint `soft_unstable` in future breakage reports.] (rust-lang/rust#116274) - [Make `i128` and `u128` 16-byte aligned on x86-based targets.] (rust-lang/rust#116672) - [Use `--verbose` in diagnostic output.] (rust-lang/rust#119129) - [Improve spacing between printed tokens.] (rust-lang/rust#120227) - [Merge the `unused_tuple_struct_fields` lint into `dead_code`.] (rust-lang/rust#118297) - [Error on incorrect implied bounds in well-formedness check] (rust-lang/rust#118553), with a temporary exception for Bevy. - [Fix coverage instrumentation/reports for non-ASCII source code.] (rust-lang/rust#119033) - [Fix `fn`/`const` items implied bounds and well-formedness check.] (rust-lang/rust#120019) - [Promote `riscv32{im|imafc}-unknown-none-elf` targets to tier 2.] (rust-lang/rust#118704) - Add several new tier 3 targets: - [`aarch64-unknown-illumos`] (rust-lang/rust#112936) - [`hexagon-unknown-none-elf`] (rust-lang/rust#117601) - [`riscv32imafc-esp-espidf`] (rust-lang/rust#119738) - [`riscv32im-risc0-zkvm-elf`] (rust-lang/rust#117958) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [Implement `From<&[T; N]>` for `Cow<[T]>`.] (rust-lang/rust#113489) - [Remove special-case handling of `vec.split_off (0)`.](rust-lang/rust#119917) Stabilized APIs --------------- - [`array::each_ref`] (https://doc.rust-lang.org/stable/std/primitive.array.html#method.each_ref) - [`array::each_mut`] (https://doc.rust-lang.org/stable/std/primitive.array.html#method.each_mut) - [`core::net`] (https://doc.rust-lang.org/stable/core/net/index.html) - [`f32::round_ties_even`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.round_ties_even) - [`f64::round_ties_even`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.round_ties_even) - [`mem::offset_of!`] (https://doc.rust-lang.org/stable/std/mem/macro.offset_of.html) - [`slice::first_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first_chunk) - [`slice::first_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first_chunk_mut) - [`slice::split_first_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first_chunk) - [`slice::split_first_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first_chunk_mut) - [`slice::last_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last_chunk) - [`slice::last_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last_chunk_mut) - [`slice::split_last_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last_chunk) - [`slice::split_last_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last_chunk_mut) - [`slice::chunk_by`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.chunk_by) - [`slice::chunk_by_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.chunk_by_mut) - [`Bound::map`] (https://doc.rust-lang.org/stable/std/ops/enum.Bound.html#method.map) - [`File::create_new`] (https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.create_new) - [`Mutex::clear_poison`] (https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html#method.clear_poison) - [`RwLock::clear_poison`] (https://doc.rust-lang.org/stable/std/sync/struct.RwLock.html#method.clear_poison) Cargo ----- - [Extend the build directive syntax with `cargo::`.] (rust-lang/cargo#12201) - [Stabilize metadata `id` format as `PackageIDSpec`.] (rust-lang/cargo#12914) - [Pull out as `cargo-util-schemas` as a crate.] (rust-lang/cargo#13178) - [Strip all debuginfo when debuginfo is not requested.] (rust-lang/cargo#13257) - [Inherit jobserver from env for all kinds of runners.] (rust-lang/cargo#12776) - [Deprecate rustc plugin support in cargo.] (rust-lang/cargo#13248) Rustdoc ----- - [Allows links in markdown headings.] (rust-lang/rust#117662) - [Search for tuples and unit by type with `()`.] (rust-lang/rust#118194) - [Clean up the source sidebar's hide button.] (rust-lang/rust#119066) - [Prevent JS injection from `localStorage`.] (rust-lang/rust#120250) Misc ---- - [Recommend version-sorting for all sorting in style guide.] (rust-lang/rust#115046) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Add more weirdness to `weird-exprs.rs`.] (rust-lang/rust#119028)
This is based on the pkgsrc-wip rust180 package, retaining the main pkgsrc changes as best as I could. Pkgsrc changes: * Adapt checksums and patches. * Make this work again on big-endian aarch64 (at least on NetBSD). * Make the choice of GCC = 12 work for sparc64 by testing options after options.mk is included (which is required...). Makes this work on NetBSD/sparc64 10.0 again. Upstream chnages: Version 1.80.1 (2024-08-08) =========================== - [Fix miscompilation in the jump threading MIR optimization when comparing floats] (rust-lang/rust#128271) - [Revert changes to the `dead_code` lint from 1.80.0] (rust-lang/rust#128618) Version 1.80.0 (2024-07-25) ========================== Language -------- - [Document maximum allocation size] (rust-lang/rust#116675) - [Allow zero-byte offsets and ZST read/writes on arbitrary pointers] (rust-lang/rust#117329) - [Support C23's variadics without a named parameter] (rust-lang/rust#124048) - [Stabilize `exclusive_range_pattern` feature] (rust-lang/rust#124459) - [Guarantee layout and ABI of `Result` in some scenarios] (rust-lang/rust#124870) Compiler -------- - [Update cc crate to v1.0.97 allowing additional spectre mitigations on MSVC targets] (rust-lang/rust#124892) - [Allow field reordering on types marked `repr(packed(1))`] (rust-lang/rust#125360) - [Add a lint against never type fallback affecting unsafe code] (rust-lang/rust#123939) - [Disallow cast with trailing braced macro in let-else] (rust-lang/rust#125049) - [Expand `for_loops_over_fallibles` lint to lint on fallibles behind references.] (rust-lang/rust#125156) - [self-contained linker: retry linking without `-fuse-ld=lld` on CCs that don't support it] (rust-lang/rust#125417) - [Do not parse CVarArgs (`...`) as a type in trait bounds] (rust-lang/rust#125863) - Improvements to LLDB formatting [#124458] (rust-lang/rust#124458) [#124500] (rust-lang/rust#124500) - [For the wasm32-wasip2 target default to PIC and do not use `-fuse-ld=lld`] (rust-lang/rust#124858) - [Add x86_64-unknown-linux-none as a tier 3 target] (rust-lang/rust#125023) - [Lint on `foo.into_iter()` resolving to `&Box<[T]>: IntoIterator`] (rust-lang/rust#124097) Libraries --------- - [Add `size_of` and `size_of_val` and `align_of` and `align_of_val` to the prelude] (rust-lang/rust#123168) - [Abort a process when FD ownership is violated] (rust-lang/rust#124210) - [io::Write::write_fmt: panic if the formatter fails when the stream does not fail] (rust-lang/rust#125012) - [Panic if `PathBuf::set_extension` would add a path separator] (rust-lang/rust#125070) - [Add assert_unsafe_precondition to unchecked_{add,sub,neg,mul,shl,shr} methods] (rust-lang/rust#121571) - [Update `c_char` on AIX to use the correct type] (rust-lang/rust#122986) - [`offset_of!` no longer returns a temporary] (rust-lang/rust#124484) - [Handle sigma in `str.to_lowercase` correctly] (rust-lang/rust#124773) - [Raise `DEFAULT_MIN_STACK_SIZE` to at least 64KiB] (rust-lang/rust#126059) Stabilized APIs --------------- - [`impl Default for Rc<CStr>`] (https://doc.rust-lang.org/beta/alloc/rc/struct.Rc.html#impl-Default-for-Rc%3CCStr%3E) - [`impl Default for Rc<str>`] (https://doc.rust-lang.org/beta/alloc/rc/struct.Rc.html#impl-Default-for-Rc%3Cstr%3E) - [`impl Default for Rc<[T]>`] (https://doc.rust-lang.org/beta/alloc/rc/struct.Rc.html#impl-Default-for-Rc%3C%5BT%5D%3E) - [`impl Default for Arc<str>`] (https://doc.rust-lang.org/beta/alloc/sync/struct.Arc.html#impl-Default-for-Arc%3Cstr%3E) - [`impl Default for Arc<CStr>`] (https://doc.rust-lang.org/beta/alloc/sync/struct.Arc.html#impl-Default-for-Arc%3CCStr%3E) - [`impl Default for Arc<[T]>`] (https://doc.rust-lang.org/beta/alloc/sync/struct.Arc.html#impl-Default-for-Arc%3C%5BT%5D%3E) - [`impl IntoIterator for Box<[T]>`] (https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#impl-IntoIterator-for-Box%3C%5BI%5D,+A%3E) - [`impl FromIterator<String> for Box<str>`] (https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#impl-FromIterator%3CString%3E-for-Box%3Cstr%3E) - [`impl FromIterator<char> for Box<str>`] (https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#impl-FromIterator%3Cchar%3E-for-Box%3Cstr%3E) - [`LazyCell`] (https://doc.rust-lang.org/beta/core/cell/struct.LazyCell.html) - [`LazyLock`] (https://doc.rust-lang.org/beta/std/sync/struct.LazyLock.html) - [`Duration::div_duration_f32`] (https://doc.rust-lang.org/beta/std/time/struct.Duration.html#method.div_duration_f32) - [`Duration::div_duration_f64`] (https://doc.rust-lang.org/beta/std/time/struct.Duration.html#method.div_duration_f64) - [`Option::take_if`] (https://doc.rust-lang.org/beta/std/option/enum.Option.html#method.take_if) - [`Seek::seek_relative`] (https://doc.rust-lang.org/beta/std/io/trait.Seek.html#method.seek_relative) - [`BinaryHeap::as_slice`] (https://doc.rust-lang.org/beta/std/collections/struct.BinaryHeap.html#method.as_slice) - [`NonNull::offset`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.offset) - [`NonNull::byte_offset`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.byte_offset) - [`NonNull::add`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.add) - [`NonNull::byte_add`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.byte_add) - [`NonNull::sub`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.sub) - [`NonNull::byte_sub`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.byte_sub) - [`NonNull::offset_from`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.offset_from) - [`NonNull::byte_offset_from`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.byte_offset_from) - [`NonNull::read`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.read) - [`NonNull::read_volatile`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.read_volatile) - [`NonNull::read_unaligned`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.read_unaligned) - [`NonNull::write`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.write) - [`NonNull::write_volatile`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.write_volatile) - [`NonNull::write_unaligned`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.write_unaligned) - [`NonNull::write_bytes`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.write_bytes) - [`NonNull::copy_to`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.copy_to) - [`NonNull::copy_to_nonoverlapping`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.copy_to_nonoverlapping) - [`NonNull::copy_from`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.copy_from) - [`NonNull::copy_from_nonoverlapping`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.copy_from_nonoverlapping) - [`NonNull::replace`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.replace) - [`NonNull::swap`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.swap) - [`NonNull::drop_in_place`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.drop_in_place) - [`NonNull::align_offset`] (https://doc.rust-lang.org/beta/std/ptr/struct.NonNull.html#method.align_offset) - [`<[T]>::split_at_checked`] (https://doc.rust-lang.org/beta/std/primitive.slice.html#method.split_at_checked) - [`<[T]>::split_at_mut_checked`] (https://doc.rust-lang.org/beta/std/primitive.slice.html#method.split_at_mut_checked) - [`str::split_at_checked`] (https://doc.rust-lang.org/beta/std/primitive.str.html#method.split_at_checked) - [`str::split_at_mut_checked`] (https://doc.rust-lang.org/beta/std/primitive.str.html#method.split_at_mut_checked) - [`str::trim_ascii`] (https://doc.rust-lang.org/beta/std/primitive.str.html#method.trim_ascii) - [`str::trim_ascii_start`] (https://doc.rust-lang.org/beta/std/primitive.str.html#method.trim_ascii_start) - [`str::trim_ascii_end`] (https://doc.rust-lang.org/beta/std/primitive.str.html#method.trim_ascii_end) - [`<[u8]>::trim_ascii`] (https://doc.rust-lang.org/beta/core/primitive.slice.html#method.trim_ascii) - [`<[u8]>::trim_ascii_start`] (https://doc.rust-lang.org/beta/core/primitive.slice.html#method.trim_ascii_start) - [`<[u8]>::trim_ascii_end`] (https://doc.rust-lang.org/beta/core/primitive.slice.html#method.trim_ascii_end) - [`Ipv4Addr::BITS`] (https://doc.rust-lang.org/beta/core/net/struct.Ipv4Addr.html#associatedconstant.BITS) - [`Ipv4Addr::to_bits`] (https://doc.rust-lang.org/beta/core/net/struct.Ipv4Addr.html#method.to_bits) - [`Ipv4Addr::from_bits`] (https://doc.rust-lang.org/beta/core/net/struct.Ipv4Addr.html#method.from_bits) - [`Ipv6Addr::BITS`] (https://doc.rust-lang.org/beta/core/net/struct.Ipv6Addr.html#associatedconstant.BITS) - [`Ipv6Addr::to_bits`] (https://doc.rust-lang.org/beta/core/net/struct.Ipv6Addr.html#method.to_bits) - [`Ipv6Addr::from_bits`] (https://doc.rust-lang.org/beta/core/net/struct.Ipv6Addr.html#method.from_bits) - [`Vec::<[T; N]>::into_flattened`] (https://doc.rust-lang.org/beta/alloc/vec/struct.Vec.html#method.into_flattened) - [`<[[T; N]]>::as_flattened`] (https://doc.rust-lang.org/beta/core/primitive.slice.html#method.as_flattened) - [`<[[T; N]]>::as_flattened_mut`] (https://doc.rust-lang.org/beta/core/primitive.slice.html#method.as_flattened_mut) These APIs are now stable in const contexts: - [`<[T]>::last_chunk`] (https://doc.rust-lang.org/beta/core/primitive.slice.html#method.last_chunk) - [`BinaryHeap::new`] (https://doc.rust-lang.org/beta/std/collections/struct.BinaryHeap.html#method.new) Cargo ----- - [Stabilize `-Zcheck-cfg` as always enabled] (rust-lang/cargo#13571) - [Warn, rather than fail publish, if a target is excluded] (rust-lang/cargo#13713) - [Add special `check-cfg` lint config for the `unexpected_cfgs` lint] (rust-lang/cargo#13913) - [Stabilize `cargo update --precise <yanked>`] (rust-lang/cargo#13974) - [Don't change file permissions on `Cargo.toml` when using `cargo add`] (rust-lang/cargo#13898) - [Support using `cargo fix` on IPv6-only networks] (rust-lang/cargo#13907) Rustdoc ----- - [Allow searching for references] (rust-lang/rust#124148) - [Stabilize `custom_code_classes_in_docs` feature] (rust-lang/rust#124577) - [fix: In cross-crate scenarios show enum variants on type aliases of enums] (rust-lang/rust#125300) Compatibility Notes ------------------- - [rustfmt estimates line lengths differently when using non-ascii characters] (rust-lang/rustfmt#6203) - [Type aliases are now handled correctly in orphan check] (rust-lang/rust#117164) - [Allow instructing rustdoc to read from stdin via `-`] (rust-lang/rust#124611) - [`std::env::{set_var, remove_var}` can no longer be converted to safe function pointers and no longer implement the `Fn` family of traits] (rust-lang/rust#124636) - [Warn (or error) when `Self` constructor from outer item is referenced in inner nested item] (rust-lang/rust#124187) - [Turn `indirect_structural_match` and `pointer_structural_match` lints into hard errors] (rust-lang/rust#124661) - [Make `where_clause_object_safety` lint a regular object safety violation] (rust-lang/rust#125380) - [Turn `proc_macro_back_compat` lint into a hard error.] (rust-lang/rust#125596) - [Detect unused structs even when implementing private traits] (rust-lang/rust#122382) - [`std::sync::ReentrantLockGuard<T>` is no longer `Sync` if `T: !Sync`] (rust-lang/rust#125527) which means [`std::io::StdoutLock` and `std::io::StderrLock` are no longer Sync] (rust-lang/rust#127340) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - Misc improvements to size of generated html by rustdoc e.g. [#124738] (rust-lang/rust#124738) and [#123734] (rust-lang/rust#123734) - [MSVC targets no longer depend on libc] (rust-lang/rust#124050) Version 1.79.0 (2024-06-13) ========================== Language -------- - [Stabilize inline `const {}` expressions.] (rust-lang/rust#104087) - [Prevent opaque types being instantiated twice with different regions within the same function.] (rust-lang/rust#116935) - [Stabilize WebAssembly target features that are in phase 4 and 5.] (rust-lang/rust#117457) - [Add the `redundant_lifetimes` lint to detect lifetimes which are semantically redundant.] (rust-lang/rust#118391) - [Stabilize the `unnameable_types` lint for public types that can't be named.] (rust-lang/rust#120144) - [Enable debuginfo in macros, and stabilize `-C collapse-macro-debuginfo` and `#[collapse_debuginfo]`.] (rust-lang/rust#120845) - [Propagate temporary lifetime extension into `if` and `match` expressions.] (rust-lang/rust#121346) - [Restrict promotion of `const fn` calls.] (rust-lang/rust#121557) - [Warn against refining impls of crate-private traits with `refining_impl_trait` lint.] (rust-lang/rust#121720) - [Stabilize associated type bounds (RFC 2289).] (rust-lang/rust#122055) - [Stabilize importing `main` from other modules or crates.] (rust-lang/rust#122060) - [Check return types of function types for well-formedness] (rust-lang/rust#115538) - [Rework `impl Trait` lifetime inference] (rust-lang/rust#116891) - [Change inductive trait solver cycles to be ambiguous] (rust-lang/rust#122791) Compiler -------- - [Define `-C strip` to only affect binaries, not artifacts like `.pdb`.] (rust-lang/rust#115120) - [Stabilize `-Crelro-level` for controlling runtime link hardening.] (rust-lang/rust#121694) - [Stabilize checking of `cfg` names and values at compile-time with `--check-cfg`.] (rust-lang/rust#123501) *Note that this only stabilizes the compiler part, the Cargo part is still unstable in this release.* - [Add `aarch64-apple-visionos` and `aarch64-apple-visionos-sim` tier 3 targets.] (rust-lang/rust#121419) - [Add `riscv32ima-unknown-none-elf` tier 3 target.] (rust-lang/rust#122696) - [Promote several Windows targets to tier 2] (rust-lang/rust#121712): `aarch64-pc-windows-gnullvm`, `i686-pc-windows-gnullvm`, and `x86_64-pc-windows-gnullvm`. Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [Implement `FromIterator` for `(impl Default + Extend, impl Default + Extend)`.] (rust-lang/rust#107462) - [Implement `{Div,Rem}Assign<NonZero<X>>` on `X`.] (rust-lang/rust#121952) - [Document overrides of `clone_from()` in core/std.] (rust-lang/rust#122201) - [Link MSVC default lib in core.] (rust-lang/rust#122268) - [Caution against using `transmute` between pointers and integers.] (rust-lang/rust#122379) - [Enable frame pointers for the standard library.] (rust-lang/rust#122646) Stabilized APIs --------------- - [`{integer}::unchecked_add`] (https://doc.rust-lang.org/stable/core/primitive.i32.html#method.unchecked_add) - [`{integer}::unchecked_mul`] (https://doc.rust-lang.org/stable/core/primitive.i32.html#method.unchecked_mul) - [`{integer}::unchecked_sub`] (https://doc.rust-lang.org/stable/core/primitive.i32.html#method.unchecked_sub) - [`<[T]>::split_at_unchecked`] (https://doc.rust-lang.org/stable/core/primitive.slice.html#method.split_at_unchecked) - [`<[T]>::split_at_mut_unchecked`] (https://doc.rust-lang.org/stable/core/primitive.slice.html#method.split_at_mut_unchecked) - [`<[u8]>::utf8_chunks`] (https://doc.rust-lang.org/stable/core/primitive.slice.html#method.utf8_chunks) - [`str::Utf8Chunks`] (https://doc.rust-lang.org/stable/core/str/struct.Utf8Chunks.html) - [`str::Utf8Chunk`] (https://doc.rust-lang.org/stable/core/str/struct.Utf8Chunk.html) - [`<*const T>::is_aligned`] (https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.is_aligned) - [`<*mut T>::is_aligned`] (https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.is_aligned-1) - [`NonNull::is_aligned`] (https://doc.rust-lang.org/stable/core/ptr/struct.NonNull.html#method.is_aligned) - [`<*const [T]>::len`] (https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.len) - [`<*mut [T]>::len`] (https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.len-1) - [`<*const [T]>::is_empty`] (https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.is_empty) - [`<*mut [T]>::is_empty`] (https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.is_empty-1) - [`NonNull::<[T]>::is_empty`] (https://doc.rust-lang.org/stable/core/ptr/struct.NonNull.html#method.is_empty) - [`CStr::count_bytes`] (https://doc.rust-lang.org/stable/core/ffi/c_str/struct.CStr.html#method.count_bytes) - [`io::Error::downcast`] (https://doc.rust-lang.org/stable/std/io/struct.Error.html#method.downcast) - [`num::NonZero<T>`] (https://doc.rust-lang.org/stable/core/num/struct.NonZero.html) - [`path::absolute`] (https://doc.rust-lang.org/stable/std/path/fn.absolute.html) - [`proc_macro::Literal::byte_character`] (https://doc.rust-lang.org/stable/proc_macro/struct.Literal.html#method.byte_character) - [`proc_macro::Literal::c_string`] (https://doc.rust-lang.org/stable/proc_macro/struct.Literal.html#method.c_string) These APIs are now stable in const contexts: - [`Atomic*::into_inner`] (https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicUsize.html#method.into_inner) - [`io::Cursor::new`] (https://doc.rust-lang.org/stable/std/io/struct.Cursor.html#method.new) - [`io::Cursor::get_ref`] (https://doc.rust-lang.org/stable/std/io/struct.Cursor.html#method.get_ref) - [`io::Cursor::position`] (https://doc.rust-lang.org/stable/std/io/struct.Cursor.html#method.position) - [`io::empty`] (https://doc.rust-lang.org/stable/std/io/fn.empty.html) - [`io::repeat`] (https://doc.rust-lang.org/stable/std/io/fn.repeat.html) - [`io::sink`] (https://doc.rust-lang.org/stable/std/io/fn.sink.html) - [`panic::Location::caller`] (https://doc.rust-lang.org/stable/std/panic/struct.Location.html#method.caller) - [`panic::Location::file`] (https://doc.rust-lang.org/stable/std/panic/struct.Location.html#method.file) - [`panic::Location::line`] (https://doc.rust-lang.org/stable/std/panic/struct.Location.html#method.line) - [`panic::Location::column`] (https://doc.rust-lang.org/stable/std/panic/struct.Location.html#method.column) Cargo ----- - [Prevent dashes in `lib.name`, always normalizing to `_`.] (rust-lang/cargo#12783) - [Stabilize MSRV-aware version requirement selection in `cargo add`.] (rust-lang/cargo#13608) - [Switch to using `gitoxide` by default for listing files.] (rust-lang/cargo#13696) Rustdoc ----- - [Always display stability version even if it's the same as the containing item.] (rust-lang/rust#118441) - [Show a single search result for items with multiple paths.] (rust-lang/rust#119912) - [Support typing `/` in docs to begin a search.] (rust-lang/rust#123355) Misc ---- Compatibility Notes ------------------- - [Update the minimum external LLVM to 17.] (rust-lang/rust#122649) - [`RustcEncodable` and `RustcDecodable` are soft-destabilized, to be removed from the prelude in next edition.] (rust-lang/rust#116016) - [The `wasm_c_abi` future-incompatibility lint will warn about use of the non-spec-compliant C ABI.] (rust-lang/rust#117918) Use `wasm-bindgen v0.2.88` to generate forward-compatible bindings. - [Check return types of function types for well-formedness] (rust-lang/rust#115538) Version 1.78.0 (2024-05-02) =========================== Language -------- - [Stabilize `#[cfg(target_abi = ...)]`] (rust-lang/rust#119590) - [Stabilize the `#[diagnostic]` namespace and `#[diagnostic::on_unimplemented]` attribute] (rust-lang/rust#119888) - [Make async-fn-in-trait implementable with concrete signatures] (rust-lang/rust#120103) - [Make matching on NaN a hard error, and remove the rest of `illegal_floating_point_literal_pattern`] (rust-lang/rust#116284) - [static mut: allow mutable reference to arbitrary types, not just slices and arrays] (rust-lang/rust#117614) - [Extend `invalid_reference_casting` to include references casting to bigger memory layout] (rust-lang/rust#118983) - [Add `non_contiguous_range_endpoints` lint for singleton gaps after exclusive ranges] (rust-lang/rust#118879) - [Add `wasm_c_abi` lint for use of older wasm-bindgen versions] (rust-lang/rust#117918) This lint currently only works when using Cargo. - [Update `indirect_structural_match` and `pointer_structural_match` lints to match RFC] (rust-lang/rust#120423) - [Make non-`PartialEq`-typed consts as patterns a hard error] (rust-lang/rust#120805) - [Split `refining_impl_trait` lint into `_reachable`, `_internal` variants] (rust-lang/rust#121720) - [Remove unnecessary type inference when using associated types inside of higher ranked `where`-bounds] (rust-lang/rust#119849) - [Weaken eager detection of cyclic types during type inference] (rust-lang/rust#119989) - [`trait Trait: Auto {}`: allow upcasting from `dyn Trait` to `dyn Auto`] (rust-lang/rust#119338) Compiler -------- - [Made `INVALID_DOC_ATTRIBUTES` lint deny by default] (rust-lang/rust#111505) - [Increase accuracy of redundant `use` checking] (rust-lang/rust#117772) - [Suggest moving definition if non-found macro_rules! is defined later] (rust-lang/rust#121130) - [Lower transmutes from int to pointer type as gep on null] (rust-lang/rust#121282) Target changes: - [Windows tier 1 targets now require at least Windows 10] (rust-lang/rust#115141) - [Enable CMPXCHG16B, SSE3, SAHF/LAHF and 128-bit Atomics in tier 1 Windows] (rust-lang/rust#120820) - [Add `wasm32-wasip1` tier 2 (without host tools) target] (rust-lang/rust#120468) - [Add `wasm32-wasip2` tier 3 target] (rust-lang/rust#119616) - [Rename `wasm32-wasi-preview1-threads` to `wasm32-wasip1-threads`] (rust-lang/rust#122170) - [Add `arm64ec-pc-windows-msvc` tier 3 target] (rust-lang/rust#119199) - [Add `armv8r-none-eabihf` tier 3 target for the Cortex-R52] (rust-lang/rust#110482) - [Add `loongarch64-unknown-linux-musl` tier 3 target] (rust-lang/rust#121832) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [Bump Unicode to version 15.1.0, regenerate tables] (rust-lang/rust#120777) - [Make align_offset, align_to well-behaved in all cases] (rust-lang/rust#121201) - [PartialEq, PartialOrd: document expectations for transitive chains] (rust-lang/rust#115386) - [Optimize away poison guards when std is built with panic=abort] (rust-lang/rust#100603) - [Replace pthread `RwLock` with custom implementation] (rust-lang/rust#110211) - [Implement unwind safety for Condvar on all platforms] (rust-lang/rust#121768) - [Add ASCII fast-path for `char::is_grapheme_extended`] (rust-lang/rust#121138) Stabilized APIs --------------- - [`impl Read for &Stdin`] (https://doc.rust-lang.org/stable/std/io/struct.Stdin.html#impl-Read-for-%26Stdin) - [Accept non `'static` lifetimes for several `std::error::Error` related implementations] (rust-lang/rust#113833) - [Make `impl<Fd: AsFd>` impl take `?Sized`] (rust-lang/rust#114655) - [`impl From<TryReserveError> for io::Error`] (https://doc.rust-lang.org/stable/std/io/struct.Error.html#impl-From%3CTryReserveError%3E-for-Error) These APIs are now stable in const contexts: - [`Barrier::new()`] (https://doc.rust-lang.org/stable/std/sync/struct.Barrier.html#method.new) Cargo ----- - [Stabilize lockfile v4](rust-lang/cargo#12852) - [Respect `rust-version` when generating lockfile] (rust-lang/cargo#12861) - [Control `--charset` via auto-detecting config value] (rust-lang/cargo#13337) - [Support `target.<triple>.rustdocflags` officially] (rust-lang/cargo#13197) - [Stabilize global cache data tracking] (rust-lang/cargo#13492) Misc ---- - [rustdoc: add `--test-builder-wrapper` arg to support wrappers such as RUSTC_WRAPPER when building doctests] (rust-lang/rust#114651) Compatibility Notes ------------------- - [Many unsafe precondition checks now run for user code with debug assertions enabled] (rust-lang/rust#120594) This change helps users catch undefined behavior in their code, though the details of how much is checked are generally not stable. - [riscv only supports split_debuginfo=off for now] (rust-lang/rust#120518) - [Consistently check bounds on hidden types of `impl Trait`] (rust-lang/rust#121679) - [Change equality of higher ranked types to not rely on subtyping] (rust-lang/rust#118247) - [When called, additionally check bounds on normalized function return type] (rust-lang/rust#118882) - [Expand coverage for `arithmetic_overflow` lint] (rust-lang/rust#119432) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Update to LLVM 18](rust-lang/rust#120055) - [Build `rustc` with 1CGU on `x86_64-pc-windows-msvc`] (rust-lang/rust#112267) - [Build `rustc` with 1CGU on `x86_64-apple-darwin`] (rust-lang/rust#112268) - [Introduce `run-make` V2 infrastructure, a `run_make_support` library and port over 2 tests as example] (rust-lang/rust#113026) - [Windows: Implement condvar, mutex and rwlock using futex] (rust-lang/rust#121956) Version 1.77.0 (2024-03-21) ========================== - [Reveal opaque types within the defining body for exhaustiveness checking.] (rust-lang/rust#116821) - [Stabilize C-string literals.] (rust-lang/rust#117472) - [Stabilize THIR unsafeck.] (rust-lang/rust#117673) - [Add lint `static_mut_refs` to warn on references to mutable statics.] (rust-lang/rust#117556) - [Support async recursive calls (as long as they have indirection).] (rust-lang/rust#117703) - [Undeprecate lint `unstable_features` and make use of it in the compiler.] (rust-lang/rust#118639) - [Make inductive cycles in coherence ambiguous always.] (rust-lang/rust#118649) - [Get rid of type-driven traversal in const-eval interning] (rust-lang/rust#119044), only as a [future compatiblity lint] (rust-lang/rust#122204) for now. - [Deny braced macro invocations in let-else.] (rust-lang/rust#119062) Compiler -------- - [Include lint `soft_unstable` in future breakage reports.] (rust-lang/rust#116274) - [Make `i128` and `u128` 16-byte aligned on x86-based targets.] (rust-lang/rust#116672) - [Use `--verbose` in diagnostic output.] (rust-lang/rust#119129) - [Improve spacing between printed tokens.] (rust-lang/rust#120227) - [Merge the `unused_tuple_struct_fields` lint into `dead_code`.] (rust-lang/rust#118297) - [Error on incorrect implied bounds in well-formedness check] (rust-lang/rust#118553), with a temporary exception for Bevy. - [Fix coverage instrumentation/reports for non-ASCII source code.] (rust-lang/rust#119033) - [Fix `fn`/`const` items implied bounds and well-formedness check.] (rust-lang/rust#120019) - [Promote `riscv32{im|imafc}-unknown-none-elf` targets to tier 2.] (rust-lang/rust#118704) - Add several new tier 3 targets: - [`aarch64-unknown-illumos`] (rust-lang/rust#112936) - [`hexagon-unknown-none-elf`] (rust-lang/rust#117601) - [`riscv32imafc-esp-espidf`] (rust-lang/rust#119738) - [`riscv32im-risc0-zkvm-elf`] (rust-lang/rust#117958) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [Implement `From<&[T; N]>` for `Cow<[T]>`.] (rust-lang/rust#113489) - [Remove special-case handling of `vec.split_off (0)`.](rust-lang/rust#119917) Stabilized APIs --------------- - [`array::each_ref`] (https://doc.rust-lang.org/stable/std/primitive.array.html#method.each_ref) - [`array::each_mut`] (https://doc.rust-lang.org/stable/std/primitive.array.html#method.each_mut) - [`core::net`] (https://doc.rust-lang.org/stable/core/net/index.html) - [`f32::round_ties_even`] (https://doc.rust-lang.org/stable/std/primitive.f32.html#method.round_ties_even) - [`f64::round_ties_even`] (https://doc.rust-lang.org/stable/std/primitive.f64.html#method.round_ties_even) - [`mem::offset_of!`] (https://doc.rust-lang.org/stable/std/mem/macro.offset_of.html) - [`slice::first_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first_chunk) - [`slice::first_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first_chunk_mut) - [`slice::split_first_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first_chunk) - [`slice::split_first_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first_chunk_mut) - [`slice::last_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last_chunk) - [`slice::last_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last_chunk_mut) - [`slice::split_last_chunk`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last_chunk) - [`slice::split_last_chunk_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last_chunk_mut) - [`slice::chunk_by`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.chunk_by) - [`slice::chunk_by_mut`] (https://doc.rust-lang.org/stable/std/primitive.slice.html#method.chunk_by_mut) - [`Bound::map`] (https://doc.rust-lang.org/stable/std/ops/enum.Bound.html#method.map) - [`File::create_new`] (https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.create_new) - [`Mutex::clear_poison`] (https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html#method.clear_poison) - [`RwLock::clear_poison`] (https://doc.rust-lang.org/stable/std/sync/struct.RwLock.html#method.clear_poison) Cargo ----- - [Extend the build directive syntax with `cargo::`.] (rust-lang/cargo#12201) - [Stabilize metadata `id` format as `PackageIDSpec`.] (rust-lang/cargo#12914) - [Pull out as `cargo-util-schemas` as a crate.] (rust-lang/cargo#13178) - [Strip all debuginfo when debuginfo is not requested.] (rust-lang/cargo#13257) - [Inherit jobserver from env for all kinds of runners.] (rust-lang/cargo#12776) - [Deprecate rustc plugin support in cargo.] (rust-lang/cargo#13248) Rustdoc ----- - [Allows links in markdown headings.] (rust-lang/rust#117662) - [Search for tuples and unit by type with `()`.] (rust-lang/rust#118194) - [Clean up the source sidebar's hide button.] (rust-lang/rust#119066) - [Prevent JS injection from `localStorage`.] (rust-lang/rust#120250) Misc ---- - [Recommend version-sorting for all sorting in style guide.] (rust-lang/rust#115046) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Add more weirdness to `weird-exprs.rs`.] (rust-lang/rust#119028)
#76682 added special handling to
Vec::split_off
for the case whereat == 0
. Instead of copying the vector's contents into a freshly-allocated vector and returning it, the special-case code steals the old vector's allocation, and replaces it with a new (empty) buffer with the same capacity.That eliminates the need to copy the existing elements, but comes at a surprising cost, as seen in #119913. The returned vector's capacity is no longer determined by the size of its contents (as would be expected for a freshly-allocated vector), and instead uses the full capacity of the old vector.
In cases where the capacity is large but the size is small, that results in a much larger capacity than would be expected from reading the documentation of
split_off
. This is especially bad whensplit_off
is called in a loop (to recycle a buffer), and the returned vectors have a wide variety of lengths.I believe it's better to remove the special-case code, and treat
at == 0
just like any other value:split_off
returns a “newly allocated vector”, which is not actually true in the current implementation whenat == 0
.at
could be non-zero at runtime, then the caller has already agreed to the cost of a full memcpy of the taken elements in the general case. Avoiding that copy would be nice if it were close to free, but the different handling of capacity means that it is not.at == 0
, they can easily implement that behaviour themselves usingmem::replace
.Fixes #119913.