Rust 1.90: what's new, what was stabilized

• Use lld by default on x86_64-unknown-linux-gnu.
• Make core::iter::Fuse's Default impl construct I::default() internally as promised in the docs instead of always being empty
• Set MSG_NOSIGNAL for UnixStream
• 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

Language

• Split up the unknown_or_malformed_diagnostic_attributes lint. This lint has been split up into four finer-grained lints, with unknown_or_malformed_diagnostic_attributes now being the lint group that contains these lints: 1. unknown_diagnostic_attributes: unknown to the current compiler 2. misplaced_diagnostic_attributes: placed on the wrong item 3. malformed_diagnostic_attributes: malformed attribute syntax or options 4. malformed_diagnostic_format_literals: malformed format string literal
• Allow constants whose final value has references to mutable/external memory, but reject such constants as patterns
• Allow volatile access to non-Rust memory, including address 0

<a id="1.90-Compiler"></a>

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

<a id="1.90-Platform-Support"></a>

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.

<a id="1.90-Libraries"></a>

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.
• Guarantee the pointer returned from Thread::into_raw has at least 8 bytes of alignment

<a id="1.90-Stabilized-APIs"></a>

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](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.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

<a id="1.90-Cargo"></a>

Cargo

• Add http.proxy-cainfo config for proxy certs
• Use gix for cargo package
• feat(publish): Stabilize multi-package publishing

<a id="1.90-Rustdoc"></a>

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

<a id="1.90-Compatibility-Notes"></a>

Compatibility Notes

• Use lld by default on x86_64-unknown-linux-gnu. See also <https://blog.rust-lang.org/2025/09/01/rust-lld-on-1.90.0-stable/>.
• Make core::iter::Fuse's Default impl construct I::default() internally as promised in the docs instead of always being empty
• Set MSG_NOSIGNAL for UnixStream This may change program behavior but results in the same behavior as other primitives (e.g., stdout, network sockets). Programs relying on signals to terminate them should update handling of sockets to handle errors on write by exiting.
• 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