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	Description
unchecked	Disable 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	Restrict 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	Disable [script optimization].
no_float	Disable floating-point numbers and math.
only_i32	Set the system integer type to i32 and disable all other integer types. INT is set to i32.
only_i64	Set the system integer type to i64 and disable all other integer types. INT is set to i64.
no_index	Disable [arrays] and indexing features.
no_object	Disable support for [custom types] and [object maps].
no_function	Disable script-defined [functions].
no_module	Disable loading external [modules].
no_closure	Disable [capturing][automatic currying] external variables in [anonymous functions] to simulate closures, or capturing the calling scope in function calls.
no_std	Build for no-std (implies no_closure). Notice that additional dependencies will be pulled in to replace std features.
serde	Enable serialization/deserialization via serde. Notice that the serde crate will be pulled in together with its dependencies.
internals	Expose internal data structures (e.g. [AST] nodes). Beware that Rhai internals are volatile and may change from version to version.
unicode-xid-ident	Allow Unicode Standard Annex #31 as identifiers.

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

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.