Types

Primitives:

Plus any struct / enum name (scoped: pkg::module::Type), arrays (Type[] or fixed Type[10]), and unions (union(TypeA TypeB)).

Text: string

• string is the text type: UTF-8, any length.
• str was an accepted spelling until it was removed. Writing it is now a build error that tells you to use string.
• String (capitalised) is not Comline syntax. It is the Rust type that string generates into, so you see it in generated Rust, never in a .ids file. Writing it gets the same hint.

Type aliases

type NAME = TYPE gives a type a second name:

type UserId = u64
type Handle = string

struct User {
    id: UserId
    handle: Handle
}

An alias is transparent: UserId is a u64 everywhere, in the wire format and in generated code (pub type UserId = u64;, export type UserId = number;). An alias can name another alias.

An alias can carry @validators, which then check every value of it, and @deprecated, which warns wherever it's used.

In generated code

TypeScript's number holds integers exactly only up to 2^53, so an s64 or u64 above that loses precision there.

Fixed arrays

A fixed array keeps its length: in Rust it's always [T; N]. TypeScript has no fixed-length array short of a tuple, so there it's T[], and the length isn't checked. Fixed and dynamic arrays encode the same way on the wire.

Rust's serde derive only handles arrays up to 32 elements. Longer ones go through serde-big-array:

• As a struct or error field's own type (hash: u8[64]), the field gets #[serde(with = "serde_big_array::BigArray")]. In lib mode the generated Cargo.toml already has serde-big-array = "0.5". In code mode the file starts with a comment naming that line for you to add.
• Anywhere else, the Rust generator refuses it with an error naming the spot: an optional field, an element of another array, a function argument or return, a type alias target. Wrap the array in a struct field instead.

Changing a fixed array's length (u8[4] → u8[8]) is a breaking change, the same as changing f32 to f64.

Unions

TypeScript renders a union as A | B. The Rust generator doesn't render unions yet: it stops with an error naming the field, argument or return that uses one, rather than generating code that won't compile (comline-rust#51).