Rust 1.78: what's new, what was stabilized

• Many unsafe precondition checks now run for user code with debug assertions enabled
• riscv only supports split_debuginfo=off for now
• Consistently check bounds on hidden types of impl Trait
• Change equality of higher ranked types to not rely on subtyping
• When called, additionally check bounds on normalized function return type
• Expand coverage for arithmetic_overflow lint
• Fix detection of potential interior mutability in const initializers

Language

• [Stabilize #[cfg(target_abi = ...)]](https://github.com/rust-lang/rust/pull/119590/)
• [Stabilize the #[diagnostic] namespace and #[diagnostic::on_unimplemented] attribute](https://github.com/rust-lang/rust/pull/119888/)
• Make async-fn-in-trait implementable with concrete signatures
• Make matching on NaN a hard error, and remove the rest of illegal_floating_point_literal_pattern
• static mut: allow mutable reference to arbitrary types, not just slices and arrays
• Extend invalid_reference_casting to include references casting to bigger memory layout
• Add non_contiguous_range_endpoints lint for singleton gaps after exclusive ranges
• Add wasm_c_abi lint for use of older wasm-bindgen versions This lint currently only works when using Cargo.
• Update indirect_structural_match and pointer_structural_match lints to match RFC
• Make non-PartialEq-typed consts as patterns a hard error
• Split refining_impl_trait lint into _reachable, _internal variants
• Remove unnecessary type inference when using associated types inside of higher ranked where-bounds
• Weaken eager detection of cyclic types during type inference
• trait Trait: Auto {}: allow upcasting from dyn Trait to dyn Trait + Auto

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Compiler

• Made INVALID_DOC_ATTRIBUTES lint deny by default
• Increase accuracy of redundant use checking
• Suggest moving definition if non-found macro_rules! is defined later
• Lower transmutes from int to pointer type as gep on null

Target changes:

• Windows tier 1 targets now require at least Windows 10
• Enable CMPXCHG16B, SSE3, SAHF/LAHF and 128-bit Atomics in tier 1 Windows
• Add wasm32-wasip1 tier 2 (without host tools) target
• Add wasm32-wasip2 tier 3 target
• Rename wasm32-wasi-preview1-threads to wasm32-wasip1-threads
• Add arm64ec-pc-windows-msvc tier 3 target
• Add armv8r-none-eabihf tier 3 target for the Cortex-R52
• Add loongarch64-unknown-linux-musl tier 3 target

Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support.

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Libraries

• Bump Unicode to version 15.1.0, regenerate tables
• Make align_offset, align_to well-behaved in all cases
• PartialEq, PartialOrd: document expectations for transitive chains
• Optimize away poison guards when std is built with panic=abort
• Replace pthread RwLock with custom implementation
• Implement unwind safety for Condvar on all platforms
• Add ASCII fast-path for char::is_grapheme_extended

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Stabilized APIs

• impl Read for &Stdin
• Accept non 'static lifetimes for several std::error::Error related implementations
• Make impl<Fd: AsFd> impl take ?Sized
• impl From<TryReserveError> for io::Error

These APIs are now stable in const contexts:

• Barrier::new()

<a id="1.78.0-Cargo"></a>

Cargo

• Stabilize lockfile v4
• Respect rust-version when generating lockfile
• Control --charset via auto-detecting config value
• Support target.<triple>.rustdocflags officially
• Stabilize global cache data tracking

<a id="1.78.0-Compatibility-Notes"></a>

Compatibility Notes

• Many unsafe precondition checks now run for user code with debug assertions enabled This change helps users catch undefined behavior in their code, though the details of how much is checked are generally not stable.
• riscv only supports split_debuginfo=off for now
• Consistently check bounds on hidden types of impl Trait
• Change equality of higher ranked types to not rely on subtyping
• When called, additionally check bounds on normalized function return type
• Expand coverage for arithmetic_overflow lint
• Fix detection of potential interior mutability in const initializers This code was accidentally accepted. The fix can break generic code that borrows a value of unknown type, as there is currently no way to declare "this type has no interior mutability". In the future, stabilizing the Freeze trait will allow proper support for such code.

<a id="1.78.0-Internal-Changes"></a>

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 18
• Build rustc with 1CGU on x86_64-pc-windows-msvc
• Build rustc with 1CGU on x86_64-apple-darwin
• Introduce run-make V2 infrastructure, a run_make_support library and port over 2 tests as example
• Windows: Implement condvar, mutex and rwlock using futex