gccrs: Implement 'unsize' lang item

The 'unsize' language item is now implemented in the GCC Rust front-end.

By Enes Cevik July 17, 2026 committed

This patch introduces the ‘unsize’ language item to the GCC Rust compiler. This item is crucial for handling unsized types, enabling features like array-to-slice coercions by providing a way for types to express their relationship with unsized counterparts.

In Details

The Rust language item system in gccrs is extended with the UNSIZE kind, and the BaseType::unsize_to function is added to rust-tyty.cc. This function will be used by coercion rules to determine if a type can be coerced to another, specifically targeting unsized types. This lays the groundwork for implementing various unsized coercion scenarios.

For Context

lang item: A special item in the Rust compiler that represents fundamental language concepts or traits, such as the Sized or Copy traits, or operations like unsize.
unsized type: A type whose size cannot be determined at compile time, such as slices ([T]) and trait objects (dyn Trait). These types must be handled via pointers or references.

Filed Under: gccrsrustlanguage featuretype system

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