Draft release notes for 1.90

[BoxyUwU]

NOTE: We are trying this issue as a new way to work on the draft release notes.
Use the 📝 links to edit those that have a relnotes-tracking-issue,
and they will be updated when we regenerate the notes periodically.

cc @rust-lang/release

Version 1.90 (2025-09-18)

Language

• Split up the unknown_or_malformed_diagnostic_attributes lint. This lint has been split up into four finer grained lints; whether the diagnostic attribute is unknown, placed on the wrong item, malformed or its format string is invalid. unknown_or_malformed_diagnostic_attributes is now the name of the lint group comprising them
  📝
• Allow constants that evaluate to references to mutable/external memory, but reject such constants as patterns
  📝
• Allow volatile access to non-Rust memory, including address 0
  📝

Compiler

• Use lld by default on x86_64-unknown-linux-gnu.
  📝
• Tier 3 musl targets now link dynamically by default. Affected targets:
  • mips64-unknown-linux-muslabi64
  • powerpc64-unknown-linux-musl
  • powerpc-unknown-linux-musl
  • powerpc-unknown-linux-muslspe
  • riscv32gc-unknown-linux-musl
  • s390x-unknown-linux-musl
  • thumbv7neon-unknown-linux-musleabihf
    📝

Platform Support

• Demote x86_64-apple-darwin to Tier 2 with host tools
  📝

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

Libraries

• Stabilize u*::{checked,overflowing,saturating,wrapping}_sub_signed
  📝
• Allow comparisons between CStr, CString, and Cow<CStr>
  📝
• Remove some unsized tuple impls since unsized tuples can't be constructed
  📝
• Set MSG_NOSIGNAL for UnixStream
  📝
  proc_macro::Ident::new now supports $crate.
  📝
• stabilize const_slice_reverse
• Guarantee the pointer returned from Thread::into_raw has at least 8 bytes of alignment
  📝

Stabilized APIs

• u{n}::checked_sub_signed
• u{n}::overflowing_sub_signed
• u{n}::saturating_sub_signed
• u{n}::wrapping_sub_signed
  📝
• impl Copy for IntErrorKind
• impl Hash for IntErrorKind
  📝
• impl PartialEq<&CStr> for CStr
• impl PartialEq<CString> for CStr
• impl PartialEq<Cow<CStr>> for CStr
• impl PartialEq<&CStr> for CString
• impl PartialEq<CStr> for CString
• impl PartialEq<Cow<CStr>> for CString
• impl PartialEq<&CStr> for Cow<CStr>
• impl PartialEq<CStr> for Cow<CStr>
• impl PartialEq<CString> for Cow<CStr>
  📝

These previously stable APIs are now stable in const contexts:

• <[T]>::reverse
  📝
• f32::floor
• f32::ceil
• f32::trunc
• f32::fract
• f32::round
• f32::round_ties_even
• f64::floor
• f64::ceil
• f64::trunc
• f64::fract
• f64::round
• f64::round_ties_even
  📝

Cargo

• Add http.proxy-cainfo config for proxy certs
• Use gix for cargo package
• feat(publish): Stabilize multi-package publishing
• Add [hints] table in Cargo.toml, and a hints.mostly-unused hint

Rustdoc

• Add ways to collapse all impl blocks. Previously the "Summary" button and "-" keyboard shortcut would never collapse impl blocks, now they do when shift is held
  📝
• Display unsafe attributes with unsafe() wrappers
  📝

Compatibility Notes

• Make core::iter::Fuse's Default impl construct I::default() internally as promised in the docs instead of always being empty
  📝
• On Unix std::env::home_dir will use the fallback if the HOME environment variable is empty
  📝
• We now reject unsupported extern "{abi}"s consistently in all positions. This primarily affects the use of implementing traits on an extern "{abi}" function pointer, like extern "stdcall" fn(), on a platform that doesn't support that, like aarch64-unknown-linux-gnu. Direct usage of these unsupported ABI strings by declaring or defining functions was already rejected, so this is only a change for consistency.
  📝
• const-eval: error when initializing a static writes to that static
  📝
• Check that the proc_macro_derive macro has correct arguments when applied to the crate root
  📝

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.

Other

• Add windows-gnullvm hosts to the manifest

[BoxyUwU]

@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

[BoxyUwU]

The "Other" entry should fix itself upon re-generation. The milestone for its relnotes tracking issue wasn't updated after a beta backport of the original PR

[lqd]

Use lld by default on x86_64-unknown-linux-gnu 📝

For this one, the current hope/plan (I still need to finish the call for testing blogpost) is to have lld remain on beta for at least another cycle and not ride to stable in 1.90 -- to have more time for people to test on beta and report issues: between the summer vacation for the northern half of the world, not many people testing on nightly and beta, it feels a bit safer.

I could just change the milestone in #145730 and it won’t appear in the relnotes in the OP, right?

[jieyouxu]

The

stabilize const_slice_reverse

is actually already #144136 and should be part of "Const Stabilized APIs".

[BoxyUwU]

how bizarre that that's there without an associated tracking issue :/

[mati865]

Add windows-gnullvm hosts to the manifest

This one was pushed back by one release (by reverting the change on beta branch).

[ehuss]

Add [hints] table in Cargo.toml, and a hints.mostly-unused hint

This is a nightly feature, so it probably should not be here.