II.3 Summary of built-in attributes

Attribute	Applies to	Section
`@derive(I, …)`	data, enum, distinct	§4.6
`@sealed`	data	§4.3
`@explicit`	interface	§4.6
`@open`	enum	§4.4
`@error`	data, enum	§6.5
`@reflect`	type	§15.2
`@v(n)`, `@since(n)`, `@removed(n)`, `@tag(n)`, `@renamed(from)`	type, field	§9.2
`@repr(W)`, `@codec(…)`, `@zigzag`, `@preserve_unknown`	type, field	§9.3–9.5
`@grpc_status(code)`	error type	§10.1
`@table`, `@pk`, `@auto`, `@index`, `@shard`	data, field	§12.8
`@isolation(level)`	function with `atomic(db)`	§11.2
`@owner`, `@visible`, `@encrypted`, `@audit`	table, field, rpc	§13
`@public`, `@allow(…)`	rpc	§13.4
`@timeout`, `@retry`, `@idempotent`, `@readonly`, `@cache`, `@ratelimit`, `@resumable`, `@window`, `@trace`	rpc	§10.2
`@retain`	topic	§10.5
`@parallel(n)`	pipeline	§10.6
`@idle(d)`, `@transient`	actor, actor field	§12.5
`@near(expr)`, `@pinned`, `@nonblocking`	function, actor, extern function	§8.1, §12.6
`@tag(…)`, `@skip`, `@flaky`, `@test_only`	test, declaration	§18
`@deprecated(msg)`, `@inline`, `@cold`	any declaration	II.3

@tag(n) with an integer argument on a field is the field number; @tag(labels…) on a test is selection labels.

Also: @internal (rpc, inaccessible to external principals, §13.4), @c_layout (data for FFI, §16.1), @flaky(retries: n) and @skip(reason) (test, §18.1), @must_close(by: method?) (type, §4.3).

Implementation-defined. Optimiser hints (D-8063; D-7690, D-7691). @inline and @cold on a fn and the functions std.opt.likely(c), unlikely(c) are hints: they do not change the results of a program, and an implementation MAY ignore them. This implementation: @inline sets the LLVM attribute alwaysinline, @cold sets cold (attribute arguments are ignored without a diagnostic); likely(c) and unlikely(c) return c and pass the expectation to the code generator (llvm.expect). @arena (a request arena, §7.4) is not implemented: placement in an arena remains an optimisation that must not be relied upon.

Source: docs/spec/oxl-standard.en.md