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#1128 in Magic Beans

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Wasm builder is a utility for building a project as a Wasm binary

The Wasm builder is a tool that integrates the process of building the WASM binary of your project into the main cargo build process.

Project setup

A project that should be compiled as a Wasm binary needs to:

  1. Add a build.rs file.
  2. Add wasm-builder as dependency into build-dependencies.

The build.rs file needs to contain the following code:

use substrate_wasm_builder::WasmBuilder;

fn main() {
    // Builds the WASM binary using the recommended defaults.
    // If you need more control, you can call `new` or `init_with_defaults`.

As the final step, you need to add the following to your project:

include!(concat!(env!("OUT_DIR"), "/wasm_binary.rs"));

This will include the generated Wasm binary as two constants WASM_BINARY and WASM_BINARY_BLOATY. The former is a compact Wasm binary and the latter is the Wasm binary as being generated by the compiler. Both variables have Option<&'static [u8]> as type.


Wasm builder supports to enable cargo features while building the Wasm binary. By default it will enable all features in the wasm build that are enabled for the native build except the default and std features. Besides that, wasm builder supports the special runtime-wasm feature. This runtime-wasm feature will be enabled by the wasm builder when it compiles the Wasm binary. If this feature is not present, it will not be enabled.

Environment variables

By using environment variables, you can configure which Wasm binaries are built and how:

  • SKIP_WASM_BUILD - Skips building any Wasm binary. This is useful when only native should be recompiled. If this is the first run and there doesn't exist a Wasm binary, this will set both variables to None.
  • WASM_BUILD_TYPE - Sets the build type for building Wasm binaries. Supported values are release or debug. By default the build type is equal to the build type used by the main build.
  • FORCE_WASM_BUILD - Can be set to force a Wasm build. On subsequent calls the value of the variable needs to change. As wasm-builder instructs cargo to watch for file changes this environment variable should only be required in certain circumstances.
  • WASM_BUILD_RUSTFLAGS - Extend RUSTFLAGS given to cargo build while building the wasm binary.
  • WASM_BUILD_NO_COLOR - Disable color output of the wasm build.
  • WASM_TARGET_DIRECTORY - Will copy any build Wasm binary to the given directory. The path needs to be absolute.
  • WASM_BUILD_TOOLCHAIN - The toolchain that should be used to build the Wasm binaries. The format needs to be the same as used by cargo, e.g. nightly-2020-02-20.
  • WASM_BUILD_WORKSPACE_HINT - Hint the workspace that is being built. This is normally not required as we walk up from the target directory until we find a Cargo.toml. If the target directory is changed for the build, this environment variable can be used to point to the actual workspace.
  • WASM_BUILD_STD - Sets whether the Rust's standard library crates will also be built. This is necessary to make sure the standard library crates only use the exact WASM feature set that our executor supports. Enabled by default.
  • CARGO_NET_OFFLINE - If true, --offline will be passed to all processes launched to prevent network access. Useful in offline environments.

Each project can be skipped individually by using the environment variable SKIP_PROJECT_NAME_WASM_BUILD. Where PROJECT_NAME needs to be replaced by the name of the cargo project, e.g. kitchensink-runtime will be NODE_RUNTIME.


Wasm builder requires the following prerequisites for building the Wasm binary:

  • rust nightly + wasm32-unknown-unknown toolchain


  • rust stable and version at least 1.68.0 + wasm32-unknown-unknown toolchain

If a specific rust is installed with rustup, it is important that the wasm target is installed as well. For example if installing the rust from 20.02.2020 using rustup install nightly-2020-02-20, the wasm target needs to be installed as well rustup target add wasm32-unknown-unknown --toolchain nightly-2020-02-20.


~463K SLoC