Draft release notes for 1.99.0

[BoxyUwU]

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).

---

Version 1.99.0 (2026-10-01)

Language

• Add allow-by-default raw_borrows_via_references lint that checks for references that decay immediately into raw borrows
  📝
• Extend unconditional_panic lint to function calls that panics when the chunks/windows size is zero
  📝
• Stabilize C-variadic function definitions
  📝
• Trait method are now resolved on an adjusted never type (producing a FCW)
  📝
• Coerce from inference variables to trait objects if a subtype or supertype of the inference variable is known to be Sized
  📝
• Stabilize #[my_macro] mod foo;. This allows outlined modules (mod foo;) anywhere in the body of a custom attribute or derive macro.
  📝
• Fix the overflowing_literals lint with repeated negation. For instance, it will now now longer lint on --128_i8, which is already detected by the arithmetic_overflow lint.
  📝
• Add POSIX symbols to the invalid_runtime_symbol_definitions and suspicious_runtime_symbol_definitions lints
  📝
• Lint unused #[path] attributes on inline modules
  📝
• Enable unreachable_cfg_select_predicates lint as part of unused lint group
  📝
• Stabilize passing 128-bit integers via vector registers with asm! on x86
  📝
• Macros that expand to a semicolon now produce a warning lint (semicolon_in_expressions_from_non_local_macros) even when the macro comes from another crate. Previously, such warnings only appeared for macros from the same crate, to avoid showing warnings that can't be fixed locally; however, this masked problems, as integration tests from the crate providing the macro get compiled as a separate crate, so tests often wouldn't reveal this issue. If you encounter a lint like this, please make sure to report it to the crate providing the macro so they can fix it; don't just silence it in your own crate.
  • semicolon_in_expressions_from_macros: Lint on non-local macros too
  • Split non-local semicolon_in_expressions_from_macros into a separate lint
    📝
• Explicitly document that some allocations are allowed to grow in-place (but none are allowed to shrink)
  📝
• The contents of an UnsafeCell can now be accessed without going through get
  • The invalid_reference_casting lint was adjusted accordingly
    📝
• Stabilize the ability to use #[unsafe(naked)] functions to define C-variadic functions (#![feature(c_variadic_naked_functions)]).
  📝
• Warn if an invalid doc attribute is used on a macro invocation
  📝
• Infer anonymous lifetimes in the types of associated consts as 'static
  📝

Compiler

• Convert -Ctarget-cpu into a target-modifier for AVR, AMDGCN and NVPTX
  📝
• Enable static_position_independent_executables on all gnu and musl targets
  📝
• When providing a suggestion about a missing method, rustc now prefers an exactly matching name from a doc alias attribute over a similarity search from other method names. If your new users sometimes expect a method under a different name, adding a doc alias will now help them find it via rustc suggestions, in addition to helping them find it via rustdoc search: When suggesting method names, prefer exact doc aliases over similar names
  📝

Platform Support

• Promote riscv64-unknown-linux-musl to Tier 2 with host tools
  📝

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

Libraries

• Relax transmute_copy to accept ?Sized types)
• Update transmute_copy to use a non-unwinding panic
  📝
• Don't escape U+FF9E and U+FF9F in escape_debug_ext
  📝
• Introduce a PinSafePointer trait that generalizes PinCoerceUnsized
  📝

Stabilized APIs

• Implement IntoIterator for [&[mut]] Box<[T; N], A>
  📝
• VecDeque::retain_back
  📝
• Step::forward_overflowing
• Step::backward_overflowing
  📝
• Box::into_non_null
• Box::from_non_null
• Vec::into_parts
• Vec::from_parts
  📝
• core::mem::size_of_val_raw
• core::mem::align_of_val_raw
• core::alloc::Layout::for_value_raw
  📝
• String::from_utf8_lossy_owned
  📝
• FusedIterator for StepBy<I>
  📝
• std::fs::set_times
• std::fs::set_times_nofollow
  📝

These previously stable APIs are now stable in const contexts:

Cargo

• Stabilize build-dir layout v2
• feat(toml): allow overriding inherited default-features in 2024
• feat(profile): Add built-in profile debug

Rustdoc

• Add new unused_footnote_definition rustdoc lint
  📝
• Smarter filtering of trait impls yields performance improvements of 20% on average and up to 40% on some real-world crates
  📝
  

Compatibility Notes

• fully deprecate the legacy integral modules
  📝
• Upgrade no_mangle_generic_items into hard error
  📝
• Do not promote references to extern statics
  📝
• Ensure inferred let pattern types typecheck
  📝
• hermit/fs: Return unsupported() instead of from_raw_os_error(22)
  📝
• Fixed a bug where #[repr(simd)] was accidentally allowed on macro invocations on stable Rust
  📝
• Abort const-eval when there are generics in the type of the value being produced
  📝
• Box::leak: tell people to avoid unleaking
  📝
• PowerPC inline ASM: Fix scalar floats being in the wrong vector lane on little endian
  📝

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.

[BoxyUwU]

cc @rust-lang/release

@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 @lcnr @traviscross @weihanglo

[Mark-Simulacrum]

Stabilize build-dir layout v2

We should check -- this may have gotten pushed out another cycle?

[weihanglo]

Stabilize build-dir layout v2

We should check -- this may have gotten pushed out another cycle?

Yes. That one was reverted and now stabilized ln 1.100.0.

[weihanglo]

From https://doc.rust-lang.org/nightly/cargo/CHANGELOG.html#cargo-199-2026-10-01

- ❗️ Incremental compilation is now disabled by default when running in CI.
  CI is detected via the `CI` environment variable.
  [#17220](https://github.com/rust-lang/cargo/pull/17220)

I am not sure if this is big enough to put as a Cargo's compatibility note.

[weihanglo]

feat(toml): allow overriding inherited default-features in 2024
feat(profile): Add built-in profile debug

For these two cargo items, would you mind copying/tweaking from Cargo's changelog? I am not sure how it fits in the general Rust changelog though.

- 🎉 Add a new built-in profile `debug`.
  This is a preparation for transitioning the `dev` profile away from debugging
  to give a saner default for faster development iterations.
  Currently there is no difference between `dev` and `debug` profiles.
  [docs](https://doc.rust-lang.org/nightly/cargo/reference/profiles.html#debug-1)
  [#17214](https://github.com/rust-lang/cargo/pull/17214)

- ❗️ Workspace members on edition 2024 or later can now override an inherited
  workspace dependency's `default-features` field.
  For example, `serde = { workspace = true, default-features = false }`
  now turns off default features even when the workspace definition enables them.
  On earlier editions, `default-features = false` is ignored with a warning.
  ([RFC 3945](https://github.com/rust-lang/rfcs/pull/3945))
  [#17126](https://github.com/rust-lang/cargo/pull/17126)

Usually with ❗️ it is a cargo compatibility note.

[theemathas]

Should the c_variadic_naked_functions stabilization just be omitted or folded into the C-variadic thing? C-variadic functions were just stabilized this release anyway.