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.
  📝
• 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.
  📝
• Always require coroutine captures to be drop-live
  📝
• Add lint against dangling pointers from local variables
  📝
• Upgrade semicolon_in_expressions_from_macros from warn to deny
  📝
• Stabilize loongarch32 inline asm
  📝
• Add warn-by-default integer_to_ptr_transmutes lint against integer to pointer transmutes
  📝
• Stabilize sse4a and tbm target features
  📝

Compiler

• Error on invalid #[link] attributes
  📝
• Don't warn on never to any as casts as unreachable
  📝

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
  📝
• Promote aarch64-pc-windows-msvc to Tier 1
  📝

Refer to Rust's platform support page
for more information on Rust's tiered platform support.

Libraries

• Print thread ID in panic message
  📝
• Fix overly restrictive lifetime in core::panic::Location::file return type
  📝
• Guarantee parameter order for _by() variants of min / max/ minmax in std::cmp
  📝
• lower pattern bindings in the order they're written and base drop order on primary bindings' order
  📝
• implement Sum and Product for Saturating(u*)
  📝
• document assumptions about Clone and Eq traits

Stabilized APIs

• Path::file_prefix
  📝
• AtomicPtr::fetch_ptr_add
• AtomicPtr::fetch_ptr_sub
• AtomicPtr::fetch_byte_add
• AtomicPtr::fetch_byte_sub
• AtomicPtr::fetch_or
• AtomicPtr::fetch_and
• AtomicPtr::fetch_xor
  📝
• {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
  📝
• PanicHookInfo::payload_as_str
  📝
• core::iter::chain
  📝
• u{N}::unsigned_signed_diff
  📝
• array::repeat
  📝
• PathBuf::add_extension
• PathBuf::with_added_extension
  📝
• Duration::from_mins
• Duration::from_hours
  📝
• impl PartialEq<{str,String}> for {Path,PathBuf}
  📝
• Ipv4Addr::from_octets
• Ipv6Addr::from_octets
• Ipv6Addr::from_segments
  📝
• 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
  📝
• uN::carrying_add
• uN::borrowing_sub
• uN::carrying_mul
• uN::carrying_mul_add
  📝
• BTreeMap::extract_if
• BTreeSet::extract_if
  📝

These previously stable APIs are now stable in const contexts:

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

Cargo

Rustdoc

• In search results, rank doc aliases lower than non-alias items with the same name
  📝
• 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.
  📝

Compatibility Notes

• 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.
  📝
• Relaxed bounds in associated type bound position like in TraitRef<AssocTy: ?Sized> are now correctly forbidden
  📝
• Add unstable #[sanitize(xyz = "on|off")] built-in attribute that shadows procedural macros with the same name
  📝
• Fix the drop checker being more permissive for bindings declared with let-else.
  📝
• Error on invalid #[should_panic] attributes
  📝
• Mark all deprecation lints in name resolution as deny-by-default and report-in-deps
  📝
• 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
  📝
• Start reporting future breakage for ILL_FORMED_ATTRIBUTE_INPUT in dependencies
  📝
• Be more strict when parsing attributes, erroring on many invalid attributes
  📝
• fix drop scope for super let bindings within if let
  📝
• Invalid literal suffixes in tuple indexing, tuple struct indexing, and struct field name position are now correctly rejected
  📝
• Closures marked with the keyword static are now syntactically invalid
  📝
• Shebangs inside --cfg and --check-cfg arguments are no longer allowed
  📝

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
  📝
• std: thread: Return error if setting thread stack size fails
  📝

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

[Noratrieb]

#142681 does not belong into the language list, I think it's intended for compatibility notes? since nothing is stable, but code may break that has a macro with that name

[Noratrieb]

the same applies to #143028 and #143193

[Noratrieb]

platform uspport only includes gnullvm but not aarch windows, I think the section probably shouldn't exist at all?

[Noratrieb]

Libraries seems to be all over the place. many of those don't make sense, like all the tracking issues, and some stuff should probably also be compat notes?

[Noratrieb]

#144689 is once again labelled relnoted because of breaking changes

[Mark-Simulacrum]

The tracking issues indeed should probably move into stabilized APIs. For the other notes, can you update the issues as outlined above? Otherwise I'll try to persist feedback back into issues in the next few days. Thanks!

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).