Optional Features
By default, Rhai includes all the standard functionalities in a small, tight package.
Most features are here to opt-out of certain functionalities that are not needed. Notice that this deviates from Rust norm where features are additive.
Excluding unneeded functionalities can result in smaller, faster builds as well as more control over what a script can (or cannot) do.
Features that Enable Special Functionalities
| Feature | Additive? | Description |
|---|---|---|
sync | no | restricts all values types to those that are Send + Sync; under this feature, all Rhai types, including Engine, Scope and AST, are all Send + Sync |
decimal | no | enables the Decimal number type |
unicode-xid-ident | no | allows Unicode Standard Annex #31 as identifiers |
serde | yes | enables serialization/deserialization via serde (pulls in the serde crate) |
metadata | yes | enables exporting functions metadata; additionally pulls in serde_json |
internals | yes | exposes internal data structures (e.g. AST nodes); beware that Rhai internals are volatile and may change from version to version |
Features that Disable Certain Language Features
| Feature | Additive? | Description |
|---|---|---|
no_float | no | disables floating-point numbers and math |
no_index | no | disables arrays and indexing features |
no_object | no | disables support for custom types and object maps |
no_function | no | disables script-defined functions; implies no_closure |
no_module | no | disables loading external modules |
no_closure | no | disables capturing external variables in anonymous functions to simulate closures, or capturing the calling scope in function calls |
Features that Disable Certain Engine Features
| Feature | Additive? | Description |
|---|---|---|
unchecked | no | disables arithmetic checking (such as over-flows and division by zero), call stack depth limit, operations count limit, modules loading limit and data size limit. Beware that a bad script may panic the entire system! |
no_optimize | no | disables script optimization |
no_position | no | disables position tracking during parsing |
Features that Configure the Engine
| Feature | Additive? | Description |
|---|---|---|
f32_float | no | sets the system floating-point type (FLOAT) to f32 instead of f64; no effect under no_float |
only_i32 | no | sets the system integer type (INT) to i32 and disable all other integer types |
only_i64 | no | sets the system integer type (INT) to i64 and disable all other integer types |
Features for no-std Builds
The following features are provided exclusively for no-std targets.
Do not use them when not compiling for no-std.
| Feature | Additive? | Description |
|---|---|---|
no_std | no | builds for no-std; notice that additional dependencies will be pulled in to replace std features |
Features for WebAssembly (WASM) Builds
The following features are provided exclusively for WASM targets. Do not use them for non-WASM targets.
| Feature | Additive? | Description |
|---|---|---|
wasm-bindgen | no | uses wasm-bindgen to compile for WASM |
stdweb | no | uses stdweb to compile for WASM |
Example
The Cargo.toml configuration below turns on these six features:
sync(everythingSend + Sync)unchecked(disable all checking – should not be used with untrusted user scripts)only_i32(only 32-bit signed integers)no_float(no floating point numbers)no_module(no loading external modules)no_function(no defining functions)
[dependencies]
rhai = { version = "{{version}}", features = [ "sync", "unchecked", "only_i32", "no_float", "no_module", "no_function" ] }
The resulting scripting engine supports only the i32 integer numeral type (and no others like u32, i16 or i64),
no floating-point, is Send + Sync (so it can be safely used across threads), and does not support defining functions
nor loading external modules.
This configuration is perfect for an expression parser in a 32-bit embedded system without floating-point hardware.
Caveat – Features Are Not Additive
Most Rhai features are not strictly additive – i.e. they do not only add optional functionalities.
In fact, most features are subtractive – i.e. they remove functionalities.
There is a reason for this design, because the lack of a language feature by itself is a feature.
See here for more details.