Rust 1.86: what's new, what was stabilized

• The wasm_c_abi future compatibility warning is now a hard error.
• [Remove long-deprecated no-op attributes #![no_start] and #![crate_id].](https://github.com/rust-lang/rust/pull/134300)
• The future incompatibility lint cenum_impl_drop_cast has been made into a hard error. This means it is now an error to cast a field-less enum to an integer if the enum implements Drop.
• SSE2 is now required for "i686" 32-bit x86 hard-float targets; disabling it causes a warning that will become a hard error eventually.

Language

• Stabilize upcasting trait objects to supertraits.
• [Allow safe functions to be marked with the #[target_feature] attribute.](https://github.com/rust-lang/rust/pull/134090)
• The missing_abi lint now warns-by-default.
• Rust now lints about double negations, to catch cases that might have intended to be a prefix decrement operator (--x) as written in other languages. This was previously a clippy lint, clippy::double_neg, and is now available directly in Rust as double_negations.
• More pointers are now detected as definitely not-null based on their alignment in const eval.
• Empty repr() attribute applied to invalid items are now correctly rejected.
• [Inner attributes #![test] and #![rustfmt::skip] are no longer accepted in more places than intended.](https://github.com/rust-lang/rust/pull/134276)

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Compiler

• Debug-assert that raw pointers are non-null on access.
• Change -O to mean -C opt-level=3 instead of -C opt-level=2 to match Cargo's defaults.
• Fix emission of overflowing_literals under certain macro environments.

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Platform Support

• Replace i686-unknown-redox target with i586-unknown-redox.
• Increase baseline CPU of i686-unknown-hurd-gnu to Pentium 4.
• New tier 3 targets:
• {aarch64-unknown,x86_64-pc}-nto-qnx710_iosock. For supporting Neutrino QNX 7.1 with io-socket network stack.
• {aarch64-unknown,x86_64-pc}-nto-qnx800. For supporting Neutrino QNX 8.0 (no_std-only).
• {x86_64,i686}-win7-windows-gnu. Intended for backwards compatibility with Windows 7. {x86_64,i686}-win7-windows-msvc are the Windows MSVC counterparts that already exist as Tier 3 targets.
• amdgcn-amd-amdhsa.
• x86_64-pc-cygwin.
• {mips,mipsel}-mti-none-elf. Initial bare-metal support.
• m68k-unknown-none-elf.
• armv7a-nuttx-{eabi,eabihf}, aarch64-unknown-nuttx, and thumbv7a-nuttx-{eabi,eabihf}.

Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support.

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Libraries

• The type of FromBytesWithNulError in CStr::from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> was changed from an opaque struct to an enum, allowing users to examine why the conversion failed.
• Remove RustcDecodable and RustcEncodable.
• Deprecate libtest's --logfile option.
• On recent versions of Windows, std::fs::remove_file will now remove read-only files.

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Stabilized APIs

• {float}::next_down
• {float}::next_up
• [<[_]>::get_disjoint_mut](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.get_disjoint_mut)
• [<[_]>::get_disjoint_unchecked_mut](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.get_disjoint_unchecked_mut)
• slice::GetDisjointMutError
• HashMap::get_disjoint_mut
• HashMap::get_disjoint_unchecked_mut
• NonZero::count_ones
• Vec::pop_if
• sync::Once::wait
• sync::Once::wait_force
• sync::OnceLock::wait

These APIs are now stable in const contexts:

• hint::black_box
• io::Cursor::get_mut
• io::Cursor::set_position
• str::is_char_boundary
• str::split_at
• str::split_at_checked
• str::split_at_mut
• str::split_at_mut_checked

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Cargo

• When merging, replace rather than combine configuration keys that refer to a program path and its arguments.
• Error if both --package and --workspace are passed but the requested package is missing. This was previously silently ignored, which was considered a bug since missing packages should be reported.
• Deprecate the token argument in cargo login to avoid shell history leaks.
• Simplify the implementation of SourceID comparisons. This may potentially change behavior if the canonicalized URL compares differently in alternative registries.

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Rustdoc

• Add a sans-serif font setting.

<a id="1.86.0-Compatibility-Notes"></a>

Compatibility Notes

• The wasm_c_abi future compatibility warning is now a hard error. Users of wasm-bindgen should upgrade to at least version 0.2.89, otherwise compilation will fail.
• [Remove long-deprecated no-op attributes #![no_start] and #![crate_id].](https://github.com/rust-lang/rust/pull/134300)
• The future incompatibility lint cenum_impl_drop_cast has been made into a hard error. This means it is now an error to cast a field-less enum to an integer if the enum implements Drop.
• SSE2 is now required for "i686" 32-bit x86 hard-float targets; disabling it causes a warning that will become a hard error eventually. To compile for pre-SSE2 32-bit x86, use a "i586" target instead.

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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.

• Build the rustc on AArch64 Linux with ThinLTO + PGO. The ARM 64-bit compiler (AArch64) on Linux is now optimized with ThinLTO and PGO, similar to the optimizations we have already performed for the x86-64 compiler on Linux. This should make it up to 30% faster.