Prefer `-1` for `None` by scottmcm · Pull Request #155473 · rust-lang/rust

scottmcm · 2026-04-18T06:03:52Z

Currently we pick "weird" numbers like 1114112 for None::<char>. While that's not wrong, it's kinda unnatural -- a human wouldn't make that choice.

This PR instead picks -1 for thinge like None::<char> -- like clang's WEOF -- and such.

Any enums with more than one niched value (so not Result nor Option) remain as they were before. Also we continue to use 0 when that's possible -- -1 is only preferred when zero isn't possible.

Inspired when someone in discord posted an example like this https://rust.godbolt.org/z/W94s9qdYW and I thought it was odd that we're currently picking -9223372036854775808 to be the value to store to mark an Option<Vec<_>> as None. (Especially since that needs an 8-byte immediate on x64, and writing -1 is only a 4-byte immediate.)

scottmcm · 2026-04-18T06:04:54Z

            // zero is unavailable because wrapping occurs
            move_end(v)


Note that this arm just kinda gave up before; it never even considered whether putting it next to start might be better.

View changes since the review

scottmcm · 2026-04-18T06:14:19Z

@bors try @rust-timer queue

Prefer `-1` for `None`

scottmcm · 2026-04-18T06:26:30Z

It helps to remember to git add all the changes 🤦

scottmcm · 2026-04-18T08:11:23Z

@bors try @rust-timer queue

Prefer `-1` for `None`

rust-bors · 2026-04-18T10:24:25Z

☀️ Try build successful (CI)
Build commit: ed76e05 (ed76e056cbb36453c64da4dd8ffccac4e093f819, parent: 2f201bccb3a7fb5a85b0fcfcc0a020a946d6d58a)

rust-timer · 2026-04-18T11:36:03Z

Finished benchmarking commit (ed76e05): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	0.5%	[0.0%, 1.5%]	9
Improvements ✅ (primary)	-0.3%	[-2.9%, -0.2%]	86
Improvements ✅ (secondary)	-0.3%	[-1.8%, -0.0%]	62
All ❌✅ (primary)	-0.3%	[-2.9%, 0.3%]	87

Max RSS (memory usage)

Results (secondary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	6.5%	[6.5%, 6.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.4%	[-4.4%, -4.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.9%, secondary -4.2%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-2.9%	[-3.0%, -2.9%]	2
Improvements ✅ (secondary)	-6.2%	[-7.6%, -3.4%]	3
All ❌✅ (primary)	-2.9%	[-3.0%, -2.9%]	2

Binary size

Results (primary -0.2%, secondary -0.4%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.0%, 0.6%]	2
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.9%	[-2.1%, -0.3%]	3
All ❌✅ (primary)	-0.2%	[-0.6%, -0.1%]	5

Bootstrap: 491.618s -> 493.312s (0.34%)
Artifact size: 394.25 MiB -> 396.31 MiB (0.52%)

scottmcm · 2026-04-18T16:25:15Z

Wow, those results are way better than I expected 😅

Seems pretty clearly a net win to me.
@rustbot label: +perf-regression-triaged

rustbot · 2026-04-18T16:25:21Z

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Only exclude the #155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc #155473

…=mati865 Only exclude the rust-lang#155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc rust-lang#155473

…uwer Rollup of 14 pull requests Successful merges: - #155850 (Only exclude the #155473 change for 1-byte bool-likes) - #155923 (Subtree sync for rustc_codegen_cranelift) - #151994 (switch to v0 mangling by default on stable) - #154325 (Tweak irrefutable let else warning output) - #155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - #155273 (Lock stable_crate_ids once in create_crate_num) - #155361 (Document that CFI diverges from Rust wrt. ABI-compatibility rules) - #155692 (disable naked-dead-code-elimination test if no RET mnemonic is available) - #155747 (Update documentation for `wasm32-wali-linux-musl` after integrating n…) - #155768 (compiletest: Overhaul the code for running an incremental test revision) - #155907 (Handle hkl const closures) - #155910 (misc stuff from reading borrowck again :)) - #155913 (Delete the 12 year old fixme) - #155920 (remove review queue triagebot mentions)

…=mati865 Only exclude the rust-lang#155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc rust-lang#155473

…uwer Rollup of 15 pull requests Successful merges: - #155923 (Subtree sync for rustc_codegen_cranelift) - #155930 (Sync from portable simd 2026 04 28) - #155850 (Only exclude the #155473 change for 1-byte bool-likes) - #151994 (switch to v0 mangling by default on stable) - #154325 (Tweak irrefutable let else warning output) - #155273 (Lock stable_crate_ids once in create_crate_num) - #155361 (Document that CFI diverges from Rust wrt. ABI-compatibility rules) - #155692 (disable naked-dead-code-elimination test if no RET mnemonic is available) - #155747 (Update documentation for `wasm32-wali-linux-musl` after integrating n…) - #155768 (compiletest: Overhaul the code for running an incremental test revision) - #155907 (Handle hkl const closures) - #155910 (misc stuff from reading borrowck again :)) - #155913 (Delete the 12 year old fixme) - #155920 (remove review queue triagebot mentions) - #155936 (Rename `SharedContext::emit_dyn_lint*` into `emit_lint*`)

Rollup merge of #155850 - scottmcm:tweak_niche_assignment, r=mati865 Only exclude the #155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc #155473

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#155923 (Subtree sync for rustc_codegen_cranelift) - rust-lang/rust#155930 (Sync from portable simd 2026 04 28) - rust-lang/rust#155850 (Only exclude the rust-lang/rust#155473 change for 1-byte bool-likes) - rust-lang/rust#151994 (switch to v0 mangling by default on stable) - rust-lang/rust#154325 (Tweak irrefutable let else warning output) - rust-lang/rust#155273 (Lock stable_crate_ids once in create_crate_num) - rust-lang/rust#155361 (Document that CFI diverges from Rust wrt. ABI-compatibility rules) - rust-lang/rust#155692 (disable naked-dead-code-elimination test if no RET mnemonic is available) - rust-lang/rust#155747 (Update documentation for `wasm32-wali-linux-musl` after integrating n…) - rust-lang/rust#155768 (compiletest: Overhaul the code for running an incremental test revision) - rust-lang/rust#155907 (Handle hkl const closures) - rust-lang/rust#155910 (misc stuff from reading borrowck again :)) - rust-lang/rust#155913 (Delete the 12 year old fixme) - rust-lang/rust#155920 (remove review queue triagebot mentions) - rust-lang/rust#155936 (Rename `SharedContext::emit_dyn_lint*` into `emit_lint*`)

apiraino · 2026-05-29T10:31:20Z

We are observing some breakage in a number of crates (see linked issues).
There is no layout guarantees so this is technically not a breaking change. This said, should we give some visibility about this in the release notes?

scottmcm · 2026-05-29T22:03:47Z

@apiraino Agreed, a compatibility note sounds like a good idea.

BrianIAm · 2026-06-08T15:19:04Z

Just a small note: 1114112 isn't an arbitrary "weird" number, it's 0x110000, the first value above the Unicode codespace (valid scalar values are 0x0000..=0x10FFFF), which makes it a natural niche for char specifically. It's the tightest possible niche that still lies outside the valid range.

That said, -1 is arguably more ergonomic in practice (smaller immediate on x86-64, matches WEOF convention, easier to spot in a debugger), so the change may still be the right call.

If the value does stay as -1, it might be worth a comment explaining why 0x110000 was the previous choice, so the next reader doesn't have to work it out. And if it stays as 0x110000, a comment like // 0x110000 - one past the end of valid Unicode scalar values would prevent the "weird number" confusion from recurring.

This MR contains the following updates: | Package | Update | Change | |---|---|---| | [rust](https://github.com/rust-lang/rust) | minor | `1.96.1` → `1.97.0` | MR created with the help of [el-capitano/tools/renovate-bot](https://gitlab.com/el-capitano/tools/renovate-bot). **Proposed changes to behavior should be submitted there as MRs.** --- ### Release Notes <details> <summary>rust-lang/rust (rust)</summary> ### [`v1.97.0`](https://github.com/rust-lang/rust/blob/HEAD/RELEASES.md#Version-1970-2026-07-09) [Compare Source](rust-lang/rust@1.96.1...1.97.0) \========================== <a id="1.97.0-Language"></a> ## Language - [Consider `Result<T, Uninhabited>` and `ControlFlow<Uninhabited, T>` to be equivalent to `T` for must use lint](rust-lang/rust#148214) - [Add allow-by-default `dead_code_pub_in_binary` lint for unused pub items in binary crates](rust-lang/rust#149509) - [Stabilize the `div32`, `lam-bh`, `lamcas`, `ld-seq-sa` and `scq` target features](rust-lang/rust#154510) - [Stabilize `cfg(target_has_atomic_primitive_alignment)`](rust-lang/rust#155006) - [Allow trailing `self` in imports in more cases](rust-lang/rust#155137) <a id="1.97.0-Platform-Support"></a> ## Platform Support - [nvptx64-nvidia-cuda: drop support for old architectures and old ISAs](rust-lang/rust#152443) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. [platform-support-doc]: https://doc.rust-lang.org/rustc/platform-support.html <a id="1.97.0-Stabilized-APIs"></a> ## Stabilized APIs - [`Default for RepeatN`](https://doc.rust-lang.org/stable/std/iter/struct.RepeatN.html#impl-Default-for-RepeatN%3CA%3E) - [`Copy for ffi::FromBytesUntilNulError`](https://doc.rust-lang.org/stable/std/ffi/struct.FromBytesUntilNulError.html#impl-Copy-for-FromBytesUntilNulError) - [`Send for std::fs::File` on UEFI](rust-lang/rust#154003) - [`<{integer}>::isolate_highest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.isolate_highest_one) - [`<{integer}>::isolate_lowest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.isolate_lowest_one) - [`<{integer}>::highest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.highest_one) - [`<{integer}>::lowest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.lowest_one) - [`<{integer}>::bit_width`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.bit_width) - [`NonZero<{integer}>::isolate_highest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.isolate_highest_one) - [`NonZero<{integer}>::isolate_lowest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.isolate_lowest_one) - [`NonZero<{integer}>::highest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.highest_one) - [`NonZero<{integer}>::lowest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.lowest_one) - [`NonZero<{integer}>::bit_width`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.bit_width) These previously stable APIs are now stable in const contexts: - [`char::is_control`](https://doc.rust-lang.org/stable/std/primitive.char.html#method.is_control) <a id="1.97.0-Cargo"></a> ## Cargo - [Stabilize `build.warnings` config.](rust-lang/cargo#16796) This controls how lint warnings from local packages are treated. Useful for enforcing a warning-free build in CI, replacing `-Dwarnings`. [docs](https://doc.rust-lang.org/nightly/cargo/reference/config.html#buildwarnings) - [Stabilize `resolver.lockfile-path` config.](rust-lang/cargo#16694) This allows specifying the path to the lockfile to use when resolving dependencies. Useful when working with read-only source directories. [docs](https://doc.rust-lang.org/nightly/cargo/reference/config.html#resolverlockfile-path) - [cargo-clean: Error when `--target-dir` doesn't look like a Cargo target directory.](rust-lang/cargo#16712) This prevents accidental deletion of non-target directories. - [Add `-m` shorthand for `--manifest-path`](rust-lang/cargo#16858) - [Remove `curl` dependency from `crates-io` crate](rust-lang/cargo#16936) <a id="1.97.0-Rustdoc"></a> ## Rustdoc - [Stabilize `--emit` flag](rust-lang/rust#146220) - [Stabilize `--remap-path-prefix`](rust-lang/rust#155307) <a id="1.97.0-Compatibility-Notes"></a> ## Compatibility Notes - [Emit a future-compatibility warning when relying on `f32: From<{float}>` to constrain `{float}`](rust-lang/rust#139087) - [Rust will use the v0 symbol mangling scheme by default.](rust-lang/rust#151994) This may cause some tools (such as debuggers or profilers, especially with old versions) to fail to demangle symbols emitted by Rust. It may also cause the formatting of text in backtraces to change. - [Prevent deref coercions in `pin!`, in order to prevent unsoundness.](rust-lang/rust#153457) The most likely case where this might impact users is: writing `pin!(x)` where `x` has type `&mut T` will now always correctly produce a value of type `Pin<&mut &mut T>`, instead of sometimes allowing a coercion that produces a value of type `Pin<&mut T>`. This coercion was previously incorrectly allowed since Rust 1.88.0. - [Deprecate `std::char` constants and functions](rust-lang/rust#153873) - [Warn on linker output by default](rust-lang/rust#153968) - [Remove hidden `f64` methods which have been deprecated since 1.0](rust-lang/rust#153975) - [report the `varargs_without_pattern` lint in deps](rust-lang/rust#154599) - [Forbid passing generic arguments to module path segments even if the module reexports a generic enum variant](rust-lang/rust#154971) - [Error on invalid macho `link_section` specifier](rust-lang/rust#155065) - The encoding of certain `enum`s [have changed](rust-lang/rust#155473). This is not a breaking change, as it only applies to `enum`s without layout guarantees, but is noted here as we've seen people impacted from having made assumptions about the layout algorithm. - [Error on `#[export_name = "..."]` where the name is empty](rust-lang/rust#155515) - [Syntactically reject tuple index shorthands in struct patterns](rust-lang/rust#155698) - [validate `#[link_name = "..."]` & `#[link(name = "...")]` parameters](rust-lang/rust#155817) - On Windows, after calling `shutdown` on a socket to shut down the write side, attempting to write to the socket will now produce a `BrokenPipe` error rather than `Other`. [Map `WSAESHUTDOWN` to `io::ErrorKind::BrokenPipe`](rust-lang/rust#156063) </details> --- ### Configuration 📅 **Schedule**: (UTC) - Branch creation - At any time (no schedule defined) - Automerge - At any time (no schedule defined) 🚦 **Automerge**: Disabled by config. 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This PR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [rust](https://github.com/rust-lang/rust) | | minor | `1.96.1` → `1.97.0` | | rust | stage | minor | `1.96-bookworm` → `1.97-bookworm` | --- ### Release Notes <details> <summary>rust-lang/rust (rust)</summary> ### [`v1.97.0`](https://github.com/rust-lang/rust/blob/HEAD/RELEASES.md#Version-1970-2026-07-09) [Compare Source](rust-lang/rust@1.96.1...1.97.0) \========================== <a id="1.97.0-Language"></a> ## Language - [Consider `Result<T, Uninhabited>` and `ControlFlow<Uninhabited, T>` to be equivalent to `T` for must use lint](rust-lang/rust#148214) - [Add allow-by-default `dead_code_pub_in_binary` lint for unused pub items in binary crates](rust-lang/rust#149509) - [Stabilize the `div32`, `lam-bh`, `lamcas`, `ld-seq-sa` and `scq` target features](rust-lang/rust#154510) - [Stabilize `cfg(target_has_atomic_primitive_alignment)`](rust-lang/rust#155006) - [Allow trailing `self` in imports in more cases](rust-lang/rust#155137) <a id="1.97.0-Platform-Support"></a> ## Platform Support - [nvptx64-nvidia-cuda: drop support for old architectures and old ISAs](rust-lang/rust#152443) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. [platform-support-doc]: https://doc.rust-lang.org/rustc/platform-support.html <a id="1.97.0-Stabilized-APIs"></a> ## Stabilized APIs - [`Default for RepeatN`](https://doc.rust-lang.org/stable/std/iter/struct.RepeatN.html#impl-Default-for-RepeatN%3CA%3E) - [`Copy for ffi::FromBytesUntilNulError`](https://doc.rust-lang.org/stable/std/ffi/struct.FromBytesUntilNulError.html#impl-Copy-for-FromBytesUntilNulError) - [`Send for std::fs::File` on UEFI](rust-lang/rust#154003) - [`<{integer}>::isolate_highest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.isolate_highest_one) - [`<{integer}>::isolate_lowest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.isolate_lowest_one) - [`<{integer}>::highest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.highest_one) - [`<{integer}>::lowest_one`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.lowest_one) - [`<{integer}>::bit_width`](https://doc.rust-lang.org/stable/std/primitive.u32.html#method.bit_width) - [`NonZero<{integer}>::isolate_highest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.isolate_highest_one) - [`NonZero<{integer}>::isolate_lowest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.isolate_lowest_one) - [`NonZero<{integer}>::highest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.highest_one) - [`NonZero<{integer}>::lowest_one`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.lowest_one) - [`NonZero<{integer}>::bit_width`](https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.bit_width) These previously stable APIs are now stable in const contexts: - [`char::is_control`](https://doc.rust-lang.org/stable/std/primitive.char.html#method.is_control) <a id="1.97.0-Cargo"></a> ## Cargo - [Stabilize `build.warnings` config.](rust-lang/cargo#16796) This controls how lint warnings from local packages are treated. Useful for enforcing a warning-free build in CI, replacing `-Dwarnings`. [docs](https://doc.rust-lang.org/nightly/cargo/reference/config.html#buildwarnings) - [Stabilize `resolver.lockfile-path` config.](rust-lang/cargo#16694) This allows specifying the path to the lockfile to use when resolving dependencies. Useful when working with read-only source directories. [docs](https://doc.rust-lang.org/nightly/cargo/reference/config.html#resolverlockfile-path) - [cargo-clean: Error when `--target-dir` doesn't look like a Cargo target directory.](rust-lang/cargo#16712) This prevents accidental deletion of non-target directories. - [Add `-m` shorthand for `--manifest-path`](rust-lang/cargo#16858) - [Remove `curl` dependency from `crates-io` crate](rust-lang/cargo#16936) <a id="1.97.0-Rustdoc"></a> ## Rustdoc - [Stabilize `--emit` flag](rust-lang/rust#146220) - [Stabilize `--remap-path-prefix`](rust-lang/rust#155307) <a id="1.97.0-Compatibility-Notes"></a> ## Compatibility Notes - [Emit a future-compatibility warning when relying on `f32: From<{float}>` to constrain `{float}`](rust-lang/rust#139087) - [Rust will use the v0 symbol mangling scheme by default.](rust-lang/rust#151994) This may cause some tools (such as debuggers or profilers, especially with old versions) to fail to demangle symbols emitted by Rust. It may also cause the formatting of text in backtraces to change. - [Prevent deref coercions in `pin!`, in order to prevent unsoundness.](rust-lang/rust#153457) The most likely case where this might impact users is: writing `pin!(x)` where `x` has type `&mut T` will now always correctly produce a value of type `Pin<&mut &mut T>`, instead of sometimes allowing a coercion that produces a value of type `Pin<&mut T>`. This coercion was previously incorrectly allowed since Rust 1.88.0. - [Deprecate `std::char` constants and functions](rust-lang/rust#153873) - [Warn on linker output by default](rust-lang/rust#153968) - [Remove hidden `f64` methods which have been deprecated since 1.0](rust-lang/rust#153975) - [report the `varargs_without_pattern` lint in deps](rust-lang/rust#154599) - [Forbid passing generic arguments to module path segments even if the module reexports a generic enum variant](rust-lang/rust#154971) - [Error on invalid macho `link_section` specifier](rust-lang/rust#155065) - The encoding of certain `enum`s [have changed](rust-lang/rust#155473). This is not a breaking change, as it only applies to `enum`s without layout guarantees, but is noted here as we've seen people impacted from having made assumptions about the layout algorithm. - [Error on `#[export_name = "..."]` where the name is empty](rust-lang/rust#155515) - [Syntactically reject tuple index shorthands in struct patterns](rust-lang/rust#155698) - [validate `#[link_name = "..."]` & `#[link(name = "...")]` parameters](rust-lang/rust#155817) - On Windows, after calling `shutdown` on a socket to shut down the write side, attempting to write to the socket will now produce a `BrokenPipe` error rather than `Other`. [Map `WSAESHUTDOWN` to `io::ErrorKind::BrokenPipe`](rust-lang/rust#156063) </details> --- ### Configuration 📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined). 🚦 **Automerge**: Disabled by config. 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Package updates relative to rust196: * Version & checksum changes * Applied patches to updated vendored crates Upstream changes: Version 1.97.0 (2026-07-09) ========================== Language -------- - [Consider `Result<T, Uninhabited>` and `ControlFlow<Uninhabited, T>` to be equivalent to `T` for must use lint] (rust-lang/rust#148214) - [Add allow-by-default `dead_code_pub_in_binary` lint for unused pub items in binary crates] (rust-lang/rust#149509) - [Stabilize the `div32`, `lam-bh`, `lamcas`, `ld-seq-sa` and `scq` target features] (rust-lang/rust#154510) - [Stabilize `cfg(target_has_atomic_primitive_alignment)`] (rust-lang/rust#155006) - [Allow trailing `self` in imports in more cases] (rust-lang/rust#155137) Platform Support ---------------- - [nvptx64-nvidia-cuda: drop support for old architectures and old ISAs] (rust-lang/rust#152443) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. [platform-support-doc]: https://doc.rust-lang.org/rustc/platform-support.html Stabilized APIs --------------- - [`Default for RepeatN`] (https://doc.rust-lang.org/stable/std/iter/struct.RepeatN.html#impl-Default-for-RepeatN%3CA%3E) - [`Copy for ffi::FromBytesUntilNulError`] (https://doc.rust-lang.org/stable/std/ffi/struct.FromBytesUntilNulError.html#impl-Copy-for-FromBytesUntilNulError) - [`Send for std::fs::File` on UEFI] (rust-lang/rust#154003) - [`<{integer}>::isolate_highest_one`] (https://doc.rust-lang.org/stable/std/primitive.u32.html#method.isolate_highest_one) - [`<{integer}>::isolate_lowest_one`] (https://doc.rust-lang.org/stable/std/primitive.u32.html#method.isolate_lowest_one) - [`NonZero<{integer}>::isolate_highest_one`] (https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.isolate_highest_one) - [`NonZero<{integer}>::isolate_lowest_one`] (https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.isolate_lowest_one) - [`<{integer}>::bit_width`] (https://doc.rust-lang.org/stable/std/primitive.u32.html#method.bit_width) - [`<{integer}>::lowest_one`] (https://doc.rust-lang.org/stable/std/primitive.u32.html#method.lowest_one) - [`<{integer}>::highest_one`] (https://doc.rust-lang.org/stable/std/primitive.u32.html#method.highest_one) - [`NonZero<{integer}>::bit_width`] (https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.bit_width) - [`NonZero<{integer}>::highest_one`] (https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.highest_one) - [`NonZero<{integer}>::lowest_one`] (https://doc.rust-lang.org/stable/std/num/struct.NonZero.html#method.lowest_one) These previously stable APIs are now stable in const contexts: - [`char::is_control`] (https://doc.rust-lang.org/stable/std/primitive.char.html#method.is_control) Cargo ----- - [Stabilize `build.warnings` config.] (rust-lang/cargo#16796) This controls how lint warnings from local packages are treated. Useful for enforcing a warning-free build in CI, replacing `-Dwarnings`. [docs] (https://doc.rust-lang.org/nightly/cargo/reference/config.html#buildwarnings) - [Stabilize `resolver.lockfile-path` config.] (rust-lang/cargo#16694) This allows specifying the path to the lockfile to use when resolving dependencies. Useful when working with read-only source directories. [docs] (https://doc.rust-lang.org/nightly/cargo/reference/config.html#resolverlockfile-path) - [cargo-clean: Error when `--target-dir` doesn't look like a Cargo target directory.] (rust-lang/cargo#16712) This prevents accidental deletion of non-target directories. - [Add `-m` shorthand for `--manifest-path`] (rust-lang/cargo#16858) - [Remove `curl` dependency from `crates-io` crate] (rust-lang/cargo#16936) Rustdoc ----- - [Stabilize `--emit` flag] (rust-lang/rust#146220) - [Stabilize `--remap-path-prefix`] (rust-lang/rust#155307) Compatibility Notes ------------------- - [Emit a future-compatibility warning when relying on `f32: From<{float}>` to constrain `{float}`] (rust-lang/rust#139087) - [Rust will use the v0 symbol mangling scheme by default.] (rust-lang/rust#151994) This may cause some tools (such as debuggers or profilers, especially with old versions) to fail to demangle symbols emitted by Rust. It may also cause the formatting of text in backtraces to change. - [Prevent deref coercions in `pin!`, in order to prevent unsoundness.] (rust-lang/rust#153457) The most likely case where this might impact users is: writing `pin!(x)` where `x` has type `&mut T` will now always correctly produce a value of type `Pin<&mut &mut T>`, instead of sometimes allowing a coercion that produces a value of type `Pin<&mut T>`. This coercion was previously incorrectly allowed since Rust 1.88.0. - [Deprecate `std::char` constants and functions] (rust-lang/rust#153873) - [Warn on linker output by default] (rust-lang/rust#153968) - [Remove hidden `f64` methods which have been deprecated since 1.0] (rust-lang/rust#153975) - [report the `varargs_without_pattern` lint in deps] (rust-lang/rust#154599) - [Forbid passing generic arguments to module path segments even if the module reexports a generic enum variant] (rust-lang/rust#154971) - [Error on invalid macho `link_section` specifier] (rust-lang/rust#155065) - The encoding of certain `enum`s [have changed] (rust-lang/rust#155473). This is not a breaking change, as it only applies to `enum`s without layout guarantees, but is noted here as we've seen people impacted from having made assumptions about the layout algorithm. - [Error on `#[export_name = "..."]` where the name is empty] (rust-lang/rust#155515) - [Syntactically reject tuple index shorthands in struct patterns] (rust-lang/rust#155698) - [validate `#[link_name = "..."]` & `#[link(name = "...")]` parameters] (rust-lang/rust#155817) - On Windows, after calling `shutdown` on a socket to shut down the write side, attempting to write to the socket will now produce a `BrokenPipe` error rather than `Other`. [Map `WSAESHUTDOWN` to `io::ErrorKind::BrokenPipe`] (rust-lang/rust#156063)

scottmcm · 2026-07-13T16:59:11Z

@BrianIAm To anyone editing the layout code, that's already pretty clear. The code is choosing between "pick next available value after or before the current range". If the enum is using -A..=B, we're either going to use -A-1 or B+1.

I had "weird" in scare-quotes because it was never an arbitrary choice (and it's "not wrong" as I also put there), just one where when you look at in in godbolt you kinda wish it was different, since it'd be easier to read and could pick better instructions.

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. labels Apr 18, 2026

scottmcm commented Apr 18, 2026

View reviewed changes