Draft release notes for 1.91

[Mark-Simulacrum]

NOTE: Use the 📝 links to edit those that have a relnotes-tracking-issue,
and they will be updated when we regenerate the notes periodically (manually).

cc @rust-lang/release

Version 1.91.0 (2025-10-30)

Language

• Add target_env = "macabi" and target_env = "sim" cfgs as replacements for the target_abi cfgs with the same values.
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• Remove the #[no_sanitize] attribute in favor of #[sanitize(xyz = "on|off")]
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• Stabilize declaration of c-style variadic functions for sysv64, win64, efiapi, and aapcs ABIs. This brings these ABIs in line with the C ABI: variadic functions can be declared in extern blocks but not defined.
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• Always require coroutine captures to be drop-live
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• Add lint against dangling pointers from local variables
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• Upgrade semicolon_in_expressions_from_macros from warn to deny
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• Stabilize loongarch32 inline asm
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• Add warn-by-default integer_to_ptr_transmutes lint against integer to pointer transmutes
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• Stabilize sse4a and tbm target features
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Compiler

• emit StorageLive and schedule StorageDead for let-else's bindings after matching
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• Port #[link] to the new attribute parsing infrastructure
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• Don't warn on never to any as casts as unreachable
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• Promote aarch64-pc-windows-msvc to Tier 1
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Platform Support

• Promote aarch64-pc-windows-gnullvm and x86_64-pc-windows-gnullvm to tier 2 with host tools. Note: llvm-tools and MSI installers are missing but will be added in the future releases
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Refer to Rust's platform support page
for more information on Rust's tiered platform support.

Libraries

• Tracking Issue for arithmetic and certain bitwise ops on AtomicPtr
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• Print thread ID in panic message
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• Tracking Issue for PanicHookInfo::payload_as_str()
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• Tracking Issue for unsigned_signed_diff
• Fix overly restrictive lifetime in core::panic::Location::file return type
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• Guarantee parameter order for _by() variants of min / max/ minmax in std::cmp
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• lower pattern bindings in the order they're written and base drop order on primary bindings' order
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• Port #[should_panic] to the new attribute parsing infrastructure
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• implement Sum and Product for Saturating(u*)
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• document assumptions about Clone and Eq traits
• Stabilize BTree{Map,Set}::extract_if
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Stabilized APIs

• Path::file_prefix
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• {integer}::strict_add
• {integer}::strict_sub
• {integer}::strict_mul
• {integer}::strict_div
• {integer}::strict_div_euclid
• {integer}::strict_rem
• {integer}::strict_rem_euclid
• {integer}::strict_neg
• {integer}::strict_shl
• {integer}::strict_shr
• {integer}::strict_pow
• {unsigned}::strict_add_unsigned
• {unsigned}::strict_sub_unsigned
• {unsigned}::strict_abs
• {signed}::strict_add_signed
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• core::iter::chain
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• array::repeat
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• Stabilize PathBuf::add_extension and Path::with_added_extension
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• Duration::from_mins
• Duration::from_hours
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• impl PartialEq<{str,String}> for {Path,PathBuf}
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• Ipv4Addr::from_octets
• Ipv6Addr::from_octets
• Ipv6Addr::from_segments
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• impl<T> Default for Pin<Box<T>> where Box<T>: Default, T: ?Sized
• impl<T> Default for Pin<Rc<T>> where Rc<T>: Default, T: ?Sized
• impl<T> Default for Pin<Arc<T>> where Arc<T>: Default, T: ?Sized
  📝
• Cell::as_array_of_cells
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• Partial-stabilize the basics from bigint_helper_methods
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These previously stable APIs are now stable in const contexts:

• [T;N]::each_ref
• [T;N]::each_mut
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• OsString::new
• PathBuf::new
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• TypeId::of
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• ptr::with_exposed_provenance
• ptr::with_exposed_provenance_mut
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Cargo

Rustdoc

• In search results, rank doc aliases lower than non-alias items with the same name
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• Raw pointers now work in type-based search like references. This means you can now search for things like *const u8 ->, and additionally functions that take or return raw pointers will now display their signature properly in search results.
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Compatibility Notes

• Changed the expansion of the assert! macro: The macro now expands to a match expression instead of an if !cond { .. }, which provides better error output when misused, but breaks assert! invocations which rely on a custom Not trait implementation.
  📝
• Apple: Always pass SDK root when linking with cc, and pass it via SDKROOT env var. This should fix linking issues with rustc running inside Xcode. Libraries in /usr/local/lib may no longer be linked automatically, if you develop or use a crate that relies on this, you should explicitly set cargo::rustc-link-search=/usr/local/lib in a build.rs script.
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• Relaxed bounds in associated type bound position like in TraitRef<AssocTy: ?Sized> are now correctly forbidden
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• Mark all deprecation lints in name resolution as deny-by-default and report-in-deps
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• The lint semicolon_in_expressions_from_macros, for macro_rules! macros in expression position that expand to end in a semicolon (;), is now deny-by-default. It was already warn-by-default, and a future compatibility warning (FCW) that warned even in dependencies. This lint will become a hard error in the future.
  📝
• Trait impl modifiers (e.g., unsafe, !, default) in inherent impls are no longer syntactically valid
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• Start reporting future breakage for ILL_FORMED_ATTRIBUTE_INPUT in dependencies
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• fix drop scope for super let bindings within if let
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• Invalid literal suffixes in tuple indexing, tuple struct indexing, and struct field name position are now correctly rejected
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• Closures marked with the keyword static are now syntactically invalid
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• Shebangs inside --cfg and --check-cfg arguments are no longer allowed
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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.

• Update to LLVM 21
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• std: thread: Return error if setting thread stack size fails
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• Rewrite the new attribute argument parser
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Other

[jieyouxu]

@rustbot ping relnotes-interest-group

[rustbot]

Hi relnotes-interest-group, this issue/PR could use some help in reviewing /
adjusting release notes. Could you take a look if available? Thanks <3

cc @alex-semenyuk @jieyouxu @joshtriplett @Kobzol @lcnr @traviscross

[theemathas]

The assert!() change was reverted in #146428