#webassembly #component #wit #encode


Tooling for working with *.wit and component files together

40 releases (17 breaking)

new 0.19.0 Nov 29, 2023
0.17.0 Oct 30, 2023
0.13.1 Jul 26, 2023
0.7.4 Mar 10, 2023
0.0.0 May 10, 2022

#216 in WebAssembly

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Used in 42 crates (32 directly)

Apache-2.0 WITH LLVM-exception

45K SLoC


wit-component is a crate for creating and interacting with WebAssembly components based on the component model proposal.

CLI usage

The wit-component crate is available through the wasm-tools CLI suite under two subcommands:

$ wasm-tools component new core.wasm -o component.wasm

$ wasm-tools component wit component.wasm


  • Creates WebAssembly component binaries from input core WebAssembly modules. Input modules communicate with the canonical ABI to imported and exported interfaces described with *.wit files. The wit interface is either embedded directly in the core wasm binary.

  • Supports "adapters" which can be used to bridge legacy core WebAssembly imported functions into component model functions. Adapters are themselves core wasm binaries which will be embedded into the final component. An adapter's exports can be imported by the main core wasm binary and the adapter can then call component model imports.

  • A *.wit interface can be extracted from an existing component to see the interface that it exports and intends to import.


Note that this crate is intended to be a low-level detail of tooling for components. Developers will not necessarily interact with this tooling day-to-day, instead using wrappers such as cargo-component which will automatically execute wit-component to produce component binaries.

First wit-component supports the wasm-based encoding of a WIT package:

$ cat demo.wit
package my:demo;

interface host {
  hello: func();

world demo {
  import host;

$ wasm-tools component wit demo.wit -o demo.wasm --wasm

# The output `demo.wasm` is a valid component binary
$ wasm-tools validate --features component-model demo.wasm
$ wasm-tools print demo.wasm

# The `*.wit` file can be recovered from the `demo.wasm` as well
$ wasm-tools component wit demo.wasm

Toolchain authors can use wit-component to embed this component types section into a core wasm binary. For a small demo here a raw *.wat wasm text file will be used where the demo.wit argument is specified manually, however.

$ cat demo.core.wat
  (import "my:demo/host" "hello" (func))

$ wasm-tools component embed demo.wit --world demo demo.core.wat -o demo.wasm

# See that there's a new `component-type` custom section
$ wasm-tools objdump demo.wasm

# Convert the core wasm into a component now
$ wasm-tools component new demo.wasm -o demo.component.wasm

# Like before the output `demo.wasm` is a valid component binary
$ wasm-tools validate --features component-model demo.component.wasm
$ wasm-tools print demo.component.wasm

# Additionally like before the `*.wit` interface can still be extracted
$ wasm-tools component wit demo.component.wasm

Here the demo.component.wasm can now be shipped to a component runtime or embedded into hosts.


~62K SLoC