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Ion beta readiness contract

Ion is currently a 0.x language and toolchain. This document defines what the project means by a future beta release: a narrow, documented compatibility surface rather than a promise that every experimental feature is complete.

Beta compatibility window

For a beta tag, Ion should keep programs written against the stable beta subset compiling for the rest of the beta line unless a release note explicitly calls out a breaking change. Breaking changes require at least one of:

  • a new minor version while Ion remains 0.x;
  • a migration note in CHANGELOG.md;
  • an unstable-feature escape hatch documented in this file or ION_SPEC.md.

Stable beta subset

The stable beta subset is the part of Ion that examples, CI, and the standard library may rely on without an unstable marker:

  • ownership, moves, drops, no-escape &T / &mut T, and borrow conflicts;
  • functions, structs, enums, type aliases, modules, imports, and visibility;
  • primitive numeric and boolean types, arrays, slices, String, Vec<T>, Box<T>, and Result<T, E>;
  • if, while, for, loop, break, continue, match, guards, and defer;
  • FFI through extern "C" and unsafe;
  • channels and spawn with structural Send;
  • ion-build projects with ion.toml fields documented in ION_SPEC.md.

Unstable or constrained in beta

The following features may change shape before 1.0:

  • tuples beyond flat two-field values;
  • capture-free function literals and function-pointer coercions;
  • generic ergonomics with optional built-in trait bounds (Copy, Eq, Send);
  • byte-oriented string iteration;
  • conservative move analysis in while and for loops;
  • generated C layout details not covered by docs/ABI.md.

Compatibility policy

  • The compiler, bundled standard library, runtime C ABI, and ion-build manifest format are versioned together.
  • Public stdlib import paths such as stdlib/io.ion, stdlib/fmt.ion, stdlib/fs.ion, and stdlib/result.ion should not be removed or renamed in a patch release.
  • Additions are allowed in patch releases. Breaking language, stdlib, runtime, or manifest changes require a minor version while Ion is 0.x.
  • Unsupported behavior inside unsafe remains outside the compatibility guarantee.

Platform support

Beta support is limited to platforms exercised in CI:

  • Linux with GCC or Clang-compatible C tooling.
  • Windows with MinGW GCC through Git Bash.

macOS and MSVC are best-effort until CI covers them.