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Implement coherence checks for negative trait impls #90104
Implement coherence checks for negative trait impls #90104
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Awaiting bors try build completion. @rustbot label: +S-waiting-on-perf |
⌛ Trying commit 68d444f with merge 2bf0405fabe34f62185aa4b41247aa4eed8b9785... |
rustdoc is failing, this is not unexpected :) and it's probably related to the handling of |
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@bors r- CI failed? :( |
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📌 Commit 3287f72 has been approved by |
☀️ Test successful - checks-actions |
Finished benchmarking commit (aa5740c): comparison url. Summary: This benchmark run did not return any relevant changes. If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf. @rustbot label: -perf-regression |
Pkgsrc changes: * Bump available bootstraps to 1.57.0. * For some reason, the vendor/libc checksums don't need fixing. * Bump required external LLVM to 12.0, according to upstream change log. * Adapt the Darwin linker patch. (For some reason I've not figured out yet, cargo is a lot more verbose while building, echoes the rustc invocation.) Upstream changes: Version 1.58.1 (2022-01-19) =========================== * Fix race condition in `std::fs::remove_dir_all` ([CVE-2022-21658]) * [Handle captured arguments in the `useless_format` Clippy lint][clippy/8295] * [Move `non_send_fields_in_send_ty` Clippy lint to nursery][clippy/8075] * [Fix wrong error message displayed when some imports are missing][91254] * [Fix rustfmt not formatting generated files from stdin][92912] [CVE-2022-21658]: https://www.cve.org/CVERecord?id=CVE-2022-21658] [91254]: rust-lang/rust#91254 [92912]: rust-lang/rust#92912 [clippy/8075]: rust-lang/rust-clippy#8075 [clippy/8295]: rust-lang/rust-clippy#8295 Version 1.58.0 (2022-01-13) ========================== Language -------- - [Format strings can now capture arguments simply by writing `{ident}` in the string.][90473] This works in all macros accepting format strings. Support for this in `panic!` (`panic!("{ident}")`) requires the 2021 edition; panic invocations in previous editions that appear to be trying to use this will result in a warning lint about not having the intended effect. - [`*const T` pointers can now be dereferenced in const contexts.][89551] - [The rules for when a generic struct implements `Unsize` have been relaxed.][90417] Compiler -------- - [Add LLVM CFI support to the Rust compiler][89652] - [Stabilize -Z strip as -C strip][90058]. Note that while release builds already don't add debug symbols for the code you compile, the compiled standard library that ships with Rust includes debug symbols, so you may want to use the `strip` option to remove these symbols to produce smaller release binaries. Note that this release only includes support in rustc, not directly in cargo. - [Add support for LLVM coverage mapping format versions 5 and 6][91207] - [Emit LLVM optimization remarks when enabled with `-Cremark`][90833] - [Update the minimum external LLVM to 12][90175] - [Add `x86_64-unknown-none` at Tier 3*][89062] - [Build musl dist artifacts with debuginfo enabled][90733]. When building release binaries using musl, you may want to use the newly stabilized strip option to remove these debug symbols, reducing the size of your binaries. - [Don't abort compilation after giving a lint error][87337] - [Error messages point at the source of trait bound obligations in more places][89580] \* Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [All remaining functions in the standard library have `#[must_use]` annotations where appropriate][89692], producing a warning when ignoring their return value. This helps catch mistakes such as expecting a function to mutate a value in place rather than return a new value. - [Paths are automatically canonicalized on Windows for operations that support it][89174] - [Re-enable debug checks for `copy` and `copy_nonoverlapping`][90041] - [Implement `RefUnwindSafe` for `Rc<T>`][87467] - [Make RSplit<T, P>: Clone not require T: Clone][90117] - [Implement `Termination` for `Result<Infallible, E>`][88601]. This allows writing `fn main() -> Result<Infallible, ErrorType>`, for a program whose successful exits never involve returning from `main` (for instance, a program that calls `exit`, or that uses `exec` to run another program). Stabilized APIs --------------- - [`Metadata::is_symlink`] - [`Path::is_symlink`] - [`{integer}::saturating_div`] - [`Option::unwrap_unchecked`] - [`Result::unwrap_unchecked`] - [`Result::unwrap_err_unchecked`] - [`NonZero{unsigned}::is_power_of_two`] - [`File::options`] These APIs are now usable in const contexts: - [`Duration::new`] - [`Duration::checked_add`] - [`Duration::saturating_add`] - [`Duration::checked_sub`] - [`Duration::saturating_sub`] - [`Duration::checked_mul`] - [`Duration::saturating_mul`] - [`Duration::checked_div`] - [`MaybeUninit::as_ptr`] - [`MaybeUninit::as_mut_ptr`] - [`MaybeUninit::assume_init`] - [`MaybeUninit::assume_init_ref`] Cargo ----- - [Add --message-format for install command][cargo/10107] - [Warn when alias shadows external subcommand][cargo/10082] Rustdoc ------- - [Show all Deref implementations recursively in rustdoc][90183] - [Use computed visibility in rustdoc][88447] Compatibility Notes ------------------- - [Try all stable method candidates first before trying unstable ones][90329]. This change ensures that adding new nightly-only methods to the Rust standard library will not break code invoking methods of the same name from traits outside the standard library. - Windows: [`std::process::Command` will no longer search the current directory for executables.][87704] - [All proc-macro backward-compatibility lints are now deny-by-default.][88041] - [proc_macro: Append .0 to unsuffixed float if it would otherwise become int token][90297] - [Refactor weak symbols in std::sys::unix][90846]. This optimizes accesses to glibc functions, by avoiding the use of dlopen. This does not increase the [minimum expected version of glibc](https://doc.rust-lang.org/nightly/rustc/platform-support.html). However, software distributions that use symbol versions to detect library dependencies, and which take weak symbols into account in that analysis, may detect rust binaries as requiring newer versions of glibc. - [rustdoc now rejects some unexpected semicolons in doctests][91026] Internal Changes ---------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Implement coherence checks for negative trait impls][90104] - [Add rustc lint, warning when iterating over hashmaps][89558] - [Optimize live point computation][90491] - [Enable verification for 1/32nd of queries loaded from disk][90361] - [Implement version of normalize_erasing_regions that allows for normalization failure][91255] [87337]: rust-lang/rust#87337 [87467]: rust-lang/rust#87467 [87704]: rust-lang/rust#87704 [88041]: rust-lang/rust#88041 [88300]: rust-lang/rust#88300 [88447]: rust-lang/rust#88447 [88601]: rust-lang/rust#88601 [88624]: rust-lang/rust#88624 [89062]: rust-lang/rust#89062 [89174]: rust-lang/rust#89174 [89542]: rust-lang/rust#89542 [89551]: rust-lang/rust#89551 [89558]: rust-lang/rust#89558 [89580]: rust-lang/rust#89580 [89652]: rust-lang/rust#89652 [89677]: rust-lang/rust#89677 [89951]: rust-lang/rust#89951 [90041]: rust-lang/rust#90041 [90058]: rust-lang/rust#90058 [90104]: rust-lang/rust#90104 [90117]: rust-lang/rust#90117 [90175]: rust-lang/rust#90175 [90183]: rust-lang/rust#90183 [90297]: rust-lang/rust#90297 [90329]: rust-lang/rust#90329 [90361]: rust-lang/rust#90361 [90417]: rust-lang/rust#90417 [90473]: rust-lang/rust#90473 [90491]: rust-lang/rust#90491 [90733]: rust-lang/rust#90733 [90833]: rust-lang/rust#90833 [90846]: rust-lang/rust#90846 [90896]: rust-lang/rust#90896 [91026]: rust-lang/rust#91026 [91207]: rust-lang/rust#91207 [91255]: rust-lang/rust#91255 [91301]: rust-lang/rust#91301 [cargo/10082]: rust-lang/cargo#10082 [cargo/10107]: rust-lang/cargo#10107 [`Metadata::is_symlink`]: https://doc.rust-lang.org/stable/std/fs/struct.Metadata.html#method.is_symlink [`Path::is_symlink`]: https://doc.rust-lang.org/stable/std/path/struct.Path.html#method.is_symlink [`{integer}::saturating_div`]: https://doc.rust-lang.org/stable/std/primitive.i8.html#method.saturating_div [`Option::unwrap_unchecked`]: https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.unwrap_unchecked [`Result::unwrap_unchecked`]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.unwrap_unchecked [`Result::unwrap_err_unchecked`]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.unwrap_err_unchecked [`NonZero{unsigned}::is_power_of_two`]: https://doc.rust-lang.org/stable/std/num/struct.NonZeroU8.html#method.is_power_of_two [`File::options`]: https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.options [`unix::process::ExitStatusExt::core_dumped`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.core_dumped [`unix::process::ExitStatusExt::stopped_signal`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.stopped_signal [`unix::process::ExitStatusExt::continued`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.continued [`unix::process::ExitStatusExt::into_raw`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.into_raw [`Duration::new`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.new [`Duration::checked_add`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_add [`Duration::saturating_add`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.saturating_add [`Duration::checked_sub`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_sub [`Duration::saturating_sub`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.saturating_sub [`Duration::checked_mul`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_mul [`Duration::saturating_mul`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.saturating_mul [`Duration::checked_div`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_div [`Duration::as_secs_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.as_secs_f64 [`Duration::as_secs_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.as_secs_f32 [`Duration::from_secs_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.from_secs_f64 [`Duration::from_secs_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.from_secs_f32 [`Duration::mul_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.mul_f64 [`Duration::mul_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.mul_f32 [`Duration::div_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_f64 [`Duration::div_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_f32 [`Duration::div_duration_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_duration_f64 [`Duration::div_duration_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_duration_f32 [`MaybeUninit::as_ptr`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_ptr [`MaybeUninit::as_mut_ptr`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_mut_ptr [`MaybeUninit::assume_init`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init [`MaybeUninit::assume_init_ref`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init_ref
Pkgsrc changes: * Bump bootstrap kit version to 1.57.0. * Bump require external LLVM to 12.0, according to upstream change log. * Adjust patches as needed, adjust line numbers. * Update checksum adjustments. For some reason the vendor/libc checksum doesn't need fixing, apparently, it remains as commented out. * Add makefile to do all the NetBSD boostrap/cross builds (do-cross.mk). Allow passing in additions to CONFIGURE_ARGS via ADD_CONFIGURE_ARGS. Upstream changes: Version 1.58.1 (2022-01-19) =========================== * Fix race condition in `std::fs::remove_dir_all` ([CVE-2022-21658]) * [Handle captured arguments in the `useless_format` Clippy lint][clippy/8295] * [Move `non_send_fields_in_send_ty` Clippy lint to nursery][clippy/8075] * [Fix wrong error message displayed when some imports are missing][91254] * [Fix rustfmt not formatting generated files from stdin][92912] [CVE-2022-21658]: https://www.cve.org/CVERecord?id=CVE-2022-21658] [91254]: rust-lang/rust#91254 [92912]: rust-lang/rust#92912 [clippy/8075]: rust-lang/rust-clippy#8075 [clippy/8295]: rust-lang/rust-clippy#8295 Version 1.58.0 (2022-01-13) ========================== Language -------- - [Format strings can now capture arguments simply by writing `{ident}` in the string.][90473] This works in all macros accepting format strings. Support for this in `panic!` (`panic!("{ident}")`) requires the 2021 edition; panic invocations in previous editions that appear to be trying to use this will result in a warning lint about not having the intended effect. - [`*const T` pointers can now be dereferenced in const contexts.][89551] - [The rules for when a generic struct implements `Unsize` have been relaxed.][90417] Compiler -------- - [Add LLVM CFI support to the Rust compiler][89652] - [Stabilize -Z strip as -C strip][90058]. Note that while release builds already don't add debug symbols for the code you compile, the compiled standard library that ships with Rust includes debug symbols, so you may want to use the `strip` option to remove these symbols to produce smaller release binaries. Note that this release only includes support in rustc, not directly in cargo. - [Add support for LLVM coverage mapping format versions 5 and 6][91207] - [Emit LLVM optimization remarks when enabled with `-Cremark`][90833] - [Update the minimum external LLVM to 12][90175] - [Add `x86_64-unknown-none` at Tier 3*][89062] - [Build musl dist artifacts with debuginfo enabled][90733]. When building release binaries using musl, you may want to use the newly stabilized strip option to remove these debug symbols, reducing the size of your binaries. - [Don't abort compilation after giving a lint error][87337] - [Error messages point at the source of trait bound obligations in more places][89580] \* Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [All remaining functions in the standard library have `#[must_use]` annotations where appropriate][89692], producing a warning when ignoring their return value. This helps catch mistakes such as expecting a function to mutate a value in place rather than return a new value. - [Paths are automatically canonicalized on Windows for operations that support it][89174] - [Re-enable debug checks for `copy` and `copy_nonoverlapping`][90041] - [Implement `RefUnwindSafe` for `Rc<T>`][87467] - [Make RSplit<T, P>: Clone not require T: Clone][90117] - [Implement `Termination` for `Result<Infallible, E>`][88601]. This allows writing `fn main() -> Result<Infallible, ErrorType>`, for a program whose successful exits never involve returning from `main` (for instance, a program that calls `exit`, or that uses `exec` to run another program). Stabilized APIs --------------- - [`Metadata::is_symlink`] - [`Path::is_symlink`] - [`{integer}::saturating_div`] - [`Option::unwrap_unchecked`] - [`Result::unwrap_unchecked`] - [`Result::unwrap_err_unchecked`] - [`NonZero{unsigned}::is_power_of_two`] - [`File::options`] These APIs are now usable in const contexts: - [`Duration::new`] - [`Duration::checked_add`] - [`Duration::saturating_add`] - [`Duration::checked_sub`] - [`Duration::saturating_sub`] - [`Duration::checked_mul`] - [`Duration::saturating_mul`] - [`Duration::checked_div`] - [`MaybeUninit::as_ptr`] - [`MaybeUninit::as_mut_ptr`] - [`MaybeUninit::assume_init`] - [`MaybeUninit::assume_init_ref`] Cargo ----- - [Add --message-format for install command][cargo/10107] - [Warn when alias shadows external subcommand][cargo/10082] Rustdoc ------- - [Show all Deref implementations recursively in rustdoc][90183] - [Use computed visibility in rustdoc][88447] Compatibility Notes ------------------- - [Try all stable method candidates first before trying unstable ones][90329]. This change ensures that adding new nightly-only methods to the Rust standard library will not break code invoking methods of the same name from traits outside the standard library. - Windows: [`std::process::Command` will no longer search the current directory for executables.][87704] - [All proc-macro backward-compatibility lints are now deny-by-default.][88041] - [proc_macro: Append .0 to unsuffixed float if it would otherwise become int token][90297] - [Refactor weak symbols in std::sys::unix][90846]. This optimizes accesses to glibc functions, by avoiding the use of dlopen. This does not increase the [minimum expected version of glibc](https://doc.rust-lang.org/nightly/rustc/platform-support.html). However, software distributions that use symbol versions to detect library dependencies, and which take weak symbols into account in that analysis, may detect rust binaries as requiring newer versions of glibc. - [rustdoc now rejects some unexpected semicolons in doctests][91026] Internal Changes ---------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Implement coherence checks for negative trait impls][90104] - [Add rustc lint, warning when iterating over hashmaps][89558] - [Optimize live point computation][90491] - [Enable verification for 1/32nd of queries loaded from disk][90361] - [Implement version of normalize_erasing_regions that allows for normalization failure][91255] [87337]: rust-lang/rust#87337 [87467]: rust-lang/rust#87467 [87704]: rust-lang/rust#87704 [88041]: rust-lang/rust#88041 [88300]: rust-lang/rust#88300 [88447]: rust-lang/rust#88447 [88601]: rust-lang/rust#88601 [88624]: rust-lang/rust#88624 [89062]: rust-lang/rust#89062 [89174]: rust-lang/rust#89174 [89542]: rust-lang/rust#89542 [89551]: rust-lang/rust#89551 [89558]: rust-lang/rust#89558 [89580]: rust-lang/rust#89580 [89652]: rust-lang/rust#89652 [89677]: rust-lang/rust#89677 [89951]: rust-lang/rust#89951 [90041]: rust-lang/rust#90041 [90058]: rust-lang/rust#90058 [90104]: rust-lang/rust#90104 [90117]: rust-lang/rust#90117 [90175]: rust-lang/rust#90175 [90183]: rust-lang/rust#90183 [90297]: rust-lang/rust#90297 [90329]: rust-lang/rust#90329 [90361]: rust-lang/rust#90361 [90417]: rust-lang/rust#90417 [90473]: rust-lang/rust#90473 [90491]: rust-lang/rust#90491 [90733]: rust-lang/rust#90733 [90833]: rust-lang/rust#90833 [90846]: rust-lang/rust#90846 [90896]: rust-lang/rust#90896 [91026]: rust-lang/rust#91026 [91207]: rust-lang/rust#91207 [91255]: rust-lang/rust#91255 [91301]: rust-lang/rust#91301 [cargo/10082]: rust-lang/cargo#10082 [cargo/10107]: rust-lang/cargo#10107 [`Metadata::is_symlink`]: https://doc.rust-lang.org/stable/std/fs/struct.Metadata.html#method.is_symlink [`Path::is_symlink`]: https://doc.rust-lang.org/stable/std/path/struct.Path.html#method.is_symlink [`{integer}::saturating_div`]: https://doc.rust-lang.org/stable/std/primitive.i8.html#method.saturating_div [`Option::unwrap_unchecked`]: https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.unwrap_unchecked [`Result::unwrap_unchecked`]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.unwrap_unchecked [`Result::unwrap_err_unchecked`]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.unwrap_err_unchecked [`NonZero{unsigned}::is_power_of_two`]: https://doc.rust-lang.org/stable/std/num/struct.NonZeroU8.html#method.is_power_of_two [`File::options`]: https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.options [`unix::process::ExitStatusExt::core_dumped`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.core_dumped [`unix::process::ExitStatusExt::stopped_signal`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.stopped_signal [`unix::process::ExitStatusExt::continued`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.continued [`unix::process::ExitStatusExt::into_raw`]: https://doc.rust-lang.org/stable/std/os/unix/process/trait.ExitStatusExt.html#tymethod.into_raw [`Duration::new`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.new [`Duration::checked_add`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_add [`Duration::saturating_add`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.saturating_add [`Duration::checked_sub`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_sub [`Duration::saturating_sub`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.saturating_sub [`Duration::checked_mul`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_mul [`Duration::saturating_mul`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.saturating_mul [`Duration::checked_div`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.checked_div [`Duration::as_secs_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.as_secs_f64 [`Duration::as_secs_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.as_secs_f32 [`Duration::from_secs_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.from_secs_f64 [`Duration::from_secs_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.from_secs_f32 [`Duration::mul_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.mul_f64 [`Duration::mul_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.mul_f32 [`Duration::div_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_f64 [`Duration::div_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_f32 [`Duration::div_duration_f64`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_duration_f64 [`Duration::div_duration_f32`]: https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.div_duration_f32 [`MaybeUninit::as_ptr`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_ptr [`MaybeUninit::as_mut_ptr`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_mut_ptr [`MaybeUninit::assume_init`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init [`MaybeUninit::assume_init_ref`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init_ref
Don't treat negative trait predicates as always knowable We don't need this. It was added in rust-lang#90104 but I don't really know why. It's not sound afaict -- negative trait predicates need the same coherence-ambiguity/orphan check rules as positive ones. r? `@lcnr` cc `@spastorino,` do you remember why?
The main purpose of this PR is to be able to move Error trait to core.
This feature is necessary to handle the following from impl on box.
Without having negative traits affect coherence moving the error trait into
core
and moving thatFrom
impl toalloc
will cause the from impl to no longer compiler because of a potential future incompatibility. The compiler indicates that&str
could introduce anError
impl in the future, and thus prevents theFrom
impl inalloc
that would cause overlap withFrom<E: Error> for Box<dyn Error>
. Addingimpl !Error for &str {}
with the negative trait coherence feature will disable this error by encoding a stability guarantee that&str
will never implementError
, making theFrom
impl compile.We would have this in
alloc
:and this in
core
:r? @nikomatsakis
This PR was built on top of @yaahc PR #85764.
Language team proposal: to rust-lang/lang-team#96