Crate abi_stable

source ·
Expand description

For Rust-to-Rust ffi, with a focus on creating libraries loaded at program startup, and with load-time type-checking.

This library allows defining Rust libraries that can be loaded at runtime, even if they were built with a different Rust version than the crate that depends on it.

These are some usecases for this library:

  • Converting a Rust dependency tree from compiling statically into a single binary, into one binary (and potentially) many dynamic libraries, allowing separate re-compilation on changes.

  • Creating a plugin system (without support for unloading).


Currently this library has these features:

  • Features the sabi_trait attribute macro, for creating ffi-safe trait objects.

  • Ffi-safe equivalent of some trait objects with DynTrait.

  • Provides ffi-safe alternatives/wrappers for many standard library types, in the std_types module.

  • Provides ffi-safe wrappers for some types defined in external crates, in the external_types module.

  • Provides the StableAbi trait for asserting that types are ffi-safe.

  • The prefix types feature for building extensible modules and vtables, without breaking ABI compatibility.

  • Supports ffi-safe nonexhaustive enums, wrapped in NonExhaustive.

  • Checking at load-time that the types in the dynamic library have the expected layout, allowing for semver compatible changes while checking the layout of types.

  • Provides the StableAbi derive macro to both assert that the type is ffi compatible, and to get the layout of the type at load-time to check that it is still compatible.


For examples of using abi_stable you can look at the readme example, or for the crates in the examples directory in the repository for this crate. This crate also has examples in the docs for most features.

To run the example crates you’ll generally have to build the *_impl crate, then run the *_user crate (all *_user crates should have a help message and a

Minimum Rust version

This crate support Rust back to 1.61.0

You can manually enable support for Rust past 1.61.0 with the rust_*_* cargo features.

Crate Features

These are default cargo features that enable optional crates :

  • “channels”: Depends on crossbeam-channel, wrapping channels from it for ffi in abi_stable::external_types::crossbeam_channel .

  • “serde_json”: Depends on serde_json, providing ffi-safe equivalents of &serde_json::value::RawValue and Box<serde_json::value::RawValue>, in abi_stable::external_types::serde_json .

To disable the default features use:

version = "<current_version>"
default-features = false
features = [  ]

enabling the features you need in the features array.

Manually enabled

These are crate features to manually enable support for newer language features:

  • “rust_1_64”: Turns many functions for converting types to slices into const fns.

  • “rust_latest_stable”: Enables the “rust_1_*” features for all the stable releases.


interface crate: the crate that declares the public functions, types, and traits that are necessary to load a library at runtime.

ìmplementation crate: A crate that implements all the functions in a interface crate.

user crate: A crate that depends on an interface crate and loads 1 or more ìmplementation crates for it.

module: refers to a struct of function pointers and other static values. The root module of a library implements the RootModule trait. These are declared in the interface crate,exported in the implementation crate, and loaded in the user crate.

Rust-to-Rust FFI types.

Types must implement StableAbi to be safely passed through the FFI boundary, which can be done using the StableAbi derive macro.

For how to evolve dynamically loaded libraries you can look at the library_evolution module.

These are the kinds of types passed through FFI:

  • Value kind:
    This is the default kind when deriving StableAbi. The layout of these types must not change in a minor versions.

  • Nonexhaustive enums :
    Enums wrapped inside NonExhaustive, which can add variants in minor versions of the library.

  • Trait objects :
    Trait object-like types generated using the sabi_trait attribute macro, which erase the type of the value they wrap,implements the methods of the trait, and can only be unwrapped back to the original type in the dynamic library/binary that created it.

  • Opaque kind:
    Types wrapped in DynTrait, whose layout can change in any version of the library, and can only be unwrapped back to the original type in the dynamic library/binary that created it.

  • Prefix types :
    Types only accessible through some custom pointer types, most commonly vtables and modules, which can be extended in minor versions while staying ABI compatible, by adding fields at the end.

Extra documentation

Macros (derive and attribute)

  • sabi_trait attribute macro:
    For generating ffi-safe trait objects.

  • StableAbi derive :
    For asserting abi-stability of a type, and obtaining the layout of the type at runtime.

  • Nonexhaustive enums :
    Details for how to declare nonexhaustive enums.

  • Prefix types (using the StableAbi derive macro):
    The way that vtables and modules are implemented, allowing extending them in minor versions of a library.


  • pub use abi_stable::sabi_types::RMut;
  • pub use abi_stable::sabi_types::RRef;


  • types and traits related to abi stability.
  • Utilities for const contexts.
  • Additional docs for macros, and guides.
  • Types and traits related to type erasure.
  • Ffi wrapper for types defined outside the standard library.
  • Types used in documentation examples.
  • Contains the InlineStorage trait,and related items.
  • Traits and types related to loading an abi_stable dynamic library, as well as functions/modules within.
  • Zero-sized types .
  • Contains types and traits for nonexhaustive enums.
  • Traits for pointers.
  • Types,traits,and functions used by prefix-types.
  • Miscelaneous items re-exported from core_extensions.
  • Types and submodules for doing runtime reflection.
  • Contains items related to the #[sabi_trait] attribute.
  • ffi-safe types that aren’t wrappers for other types.
  • Contains many ffi-safe equivalents of standard library types. The vast majority of them can be converted to and from std equivalents.
  • Where miscellaneous traits reside.
  • Types for modeling the layout of a datatype
  • Types used to represent values at compile-time, eg: True/False.
  • Utility functions.



  • DynTrait implements ffi-safe trait objects, for a selection of traits.


  • The header used to identify the version number of abi_stable that a dynamic libraries uses.


Attribute Macros

  • This attribute is used for functions which export a module in an implementation crate.
  • The sabi_extern_fn attribute macro allows defining extern "C" function that abort on unwind instead of causing undefined behavior.
  • This attribute generates an ffi-safe trait object on the trait it’s applied to.

Derive Macros