Built-in types

The following built-in types can be passed as arguments/returned by Rust methods:

Rust type	UDL type	Notes
`bool`	`boolean`
`u8/i8..u64/i64`	`u8/i8..u64/i64`
`f32`	`float`
`f64`	`double`
`String`	`string`
`Vec<u8>`	`bytes`	Different from `sequence<u8>` only in foreign type mappings
`SystemTime`	`timestamp`	Precision may be lost when converting to Python and Swift types
`Duration`	`duration`	Precision may be lost when converting to Python and Swift types
`&T`	`[ByRef] T`	This works for `&str` and `&[T]`
`Option<T>`	`T?`
`Vec<T>`	`sequence<T>`
`HashMap<String, T>`	`record<DOMString, T>`	Only string keys are supported
`()`	`void`	Empty return
`Result<T, E>`	N/A	See Errors section

And of course you can use your own types, which is covered in the following sections.

The UniFFI user guide