Features of Rhai

Easy

Simple language similar to JavaScript+Rust with dynamic typing.
Tight integration with native Rust functions and types including getters/setters, methods and indexers.
Freely pass Rust values into a script as variables/constants via an external Scope – all clonable Rust types are supported seamlessly without the need to implement any special trait.
Easily call a script-defined function from Rust.
Very few additional dependencies – right now only smallvec, thin-vec, num-traits, ahash, bitflags and smartstring; for no-std and WASM builds, a number of additional dependencies are pulled in to provide for missing functionalities.
Plugins system powered by procedural macros simplifies custom API development.

Fast

Fairly efficient evaluation – 1 million iterations in 0.14 sec on a single-core, 2.6 GHz Linux VM (see benchmarks).
Compile once to AST for repeated evaluations.
Scripts are optimized – useful for template-based machine-generated scripts.

Dynamic

Function overloading.
Operator overloading.
Organize code base with dynamically-loadable modules, optionally overriding the resolution process.
Dynamic dispatch via function pointers with additional support for currying.
Closures that can capture shared variables.
Some support for object-oriented programming (OOP).
Hook into variables access via a variable resolver, or control definition of variables via a variable definition filter.

Safe

Relatively little unsafe code – yes there are some for performance reasons.
Sand-boxed – the scripting Engine, if declared immutable, cannot mutate the containing environment unless explicitly permitted.
Passes Miri.

Rugged

Don’t Panic guarantee – Any panic is a bug. It never panics the host system.
Protected against malicious attacks – such as stack-overflow, over-sized data, and runaway scripts etc. – that may come from untrusted third-party user-land scripts.
Track script evaluation progress and manually terminate a script run.

Flexible

Re-entrant scripting Engine can be made Send + Sync (via the sync feature).
Support for Decimal numbers.
Serialization/deserialization support via serde.
Support for minimal builds by excluding unneeded language features.
Supports most build targets including no-std and WASM.
Surgically disable keywords and operators to restrict the language.
Use as a DSL by defining custom operators and/or extending the language with custom syntax.
A debugging interface provides powerful debugging support.

Rhai - Embedded Scripting for Rust