Lib.rs

ABI handling for rustc

What is an "ABI"?

Literally, "application binary interface", which means it is everything about how code interacts, at the machine level, with other code. This means it technically covers all of the following:

• object binary format for e.g. relocations or offset tables
• in-memory layout of types
• procedure calling conventions

When we discuss "ABI" in the context of rustc, we are probably discussing calling conventions. To describe those rustc_abi also covers type layout, as it must for values passed on the stack. Despite rustc_abi being about calling conventions, it is good to remember these usages exist. You will encounter all of them and more if you study target-specific codegen enough! Even in general conversation, when someone says "the Rust ABI is unstable", it may allude to either or both of

• repr(Rust) types have a mostly-unspecified layout
• extern "Rust" fn(A) -> R has an unspecified calling convention

Crate Goal

ABI is a foundational concept, so the rustc_abi crate serves as an equally foundational crate. It cannot carry all details relevant to an ABI: those permeate code generation and linkage. Instead, rustc_abi is intended to provide the interface for reasoning about the binary interface. It should contain traits and types that other crates then use in their implementation. For example, a platform's extern "C" fn calling convention will be implemented in rustc_target but rustc_abi contains the types for calculating layout and describing register-passing. This makes it easier to describe things in the same way across targets, codegen backends, and even other Rust compilers, such as rust-analyzer!