rhai/doc/src/start/features.md

Optional Features

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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.

Feature	Additive?	Description
unchecked	no	disables arithmetic checking (such as over-flows and division by zero), call stack depth limit, operations count limit and modules loading limit. Beware that a bad script may panic the entire system!
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
no_optimize	no	disables [script optimization]
no_float	no	disables floating-point numbers and math
f32_float	no	sets the system floating-point type to f32 instead of f64
only_i32	no	sets the system integer type to i32 and disable all other integer types. INT is set to i32
only_i64	no	sets the system integer type to i64 and disable all other integer types. INT is set to i64
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]
no_module	no	disables loading external [modules]
no_closure	no	disables [capturing][automatic currying] external variables in [anonymous functions] to simulate closures, or capturing the calling scope in function calls
no_std	no	builds for no-std (implies no_closure). Notice that additional dependencies will be pulled in to replace std features
serde	yes	enables serialization/deserialization via serde. Notice that the serde crate will be pulled in together with its dependencies
unicode-xid-ident	no	allows Unicode Standard Annex #31 as identifiers
internals	yes	exposes internal data structures (e.g. [AST] nodes). Beware that Rhai internals are volatile and may change from version to version

Example

The Cargo.toml configuration below turns on these six features:

• sync (everything Send + 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.