Minimal Build


In order to compile a minimal build – i.e. a build optimized for size – perhaps for no-std embedded targets or for compiling to WASM, it is essential that the correct linker flags are used in Cargo.toml:

lto = "fat"         # turn on Link-Time Optimizations
codegen-units = 1   # trade compile time with maximum optimization
opt-level = "z"     # optimize for size

Use i32 Only

For embedded systems that must optimize for code size, the architecture is commonly 32-bit. Use only_i32 to prune away large sections of code implementing functions for other numeric types (including i64).

If, for some reason, 64-bit long integers must be supported, use only_i64 instead of only_i32.

Opt-Out of Features

Opt out of as many features as possible, if they are not needed, to reduce code size because, remember, by default all code is compiled into the final binary since what a script requires cannot be predicted. If a language feature will never be needed, omitting it is a prudent strategy to optimize the build for size.

Removing the script optimizer (no_optimize) yields a sizable code saving, at the expense of a less efficient script.

Omitting arrays (no_index) yields the most code-size savings, followed by floating-point support (no_float), safety checks (unchecked) and finally object maps and custom types (no_object).

Where the usage scenario does not call for loading externally-defined modules, use no_module to save some bytes. Disable script-defined functions (no_function) and possibly closures (no_closure) when the features are not needed. Both of these have some code size savings but not much.

For embedded scripts that are not expected to cause errors, the no_position feature can be used to disable position tracking during parsing. No line number/character position information is kept for error reporting purposes. This may result in a slightly smaller build due to elimination of code related to position tracking.

Use a Raw Engine

Engine::new_raw creates a raw engine. A raw engine supports, out of the box, only a very restricted set of basic arithmetic and logical operators.

Selectively include other necessary functionalities by picking specific packages to minimize the footprint.

Packages are shared (even across threads via the sync feature), so they only have to be created once.

In addition, a Engine::new_raw disables the strings interner, which might actually increase memory usage if many strings are created in scripts. Therefore, selectively turn on the strings interner via Engine::set_max_strings_interned.