Cargo.toml is the manifest file for Rust's package manager, cargo. This file contains metadata such as name, version, and dependencies for packages, which are call "crates" in Rust.

There's a bunch of metadata that the template gives us, but there are three key parts to discuss:

1. crate-type
2. wasm-bindgen dependency
3. [features] and wee_alloc, console_error_panic_hook dependencies

[lib]
crate-type = ["cdylib", "rlib"]

A Rust-wasm crate is a bit different from a normal crate, and as a result, we need to note this in our Cargo.toml.

This [lib] annotation is typically not needed in Cargo projects, and if you're familiar with other Rust crates you'll remember that the most common crate types are rlib (the default) or bin for binaries (which don't need a crate-type annotation).

Here though crate-type = ["cdylib"] typically signifies that you'd like the compiler to create a dynamic system library, but for WebAssembly target it simply means "create a *.wasm file without a start function". On other platforms this output type will create *.so file on Linux, *.dylib on macOS, and *.dll Windows.

We also specify crate-type = ["rlib"] to ensure that our library can be unit tested with wasm-pack test (which we'll see later). Without this we wouldn't be able to test our library because the cdylib crate type is incompatible with wasm-pack's style of unit tests.

You can read more about linking and crate types, here.

wasm-bindgen is our most important dependency. This package allows us to use the #[wasm-bindgen] attribute to tag code that represents the interface we want between our JavaScript and Rust-generated wasm. We can import JS and export Rust by using this attribute.

wasm-bindgen = "0.2"

We'll see more about how to use this library when we discuss what has been generated in lib.rs.

⚠️ If you are coming from JavaScript, you might note that when we add the dependency there is no ^ or ~ symbol- it looks like we're locking to the 0.2 version. However, that's not the case! In Rust, the ^ is implied. You can read more about this in the cargo documentation on specifying dependencies.

3. [features] and wee_alloc, console_error_panic_hook dependencies

As part of our effort to design a template that helps people discover useful crates for their particular use case, this template includes two dependencies that can be very useful for folks developing Rust-wasm crates: console_error_panic_hook and wee_alloc.

Because these dependencies are useful primarily in a specific portion of the Rust-wasm crate development workflow, we've also set up a bit of glue code that allows us to include them both as dependencies, but also allows them to be optionally included.

[features]
default = ["console_error_panic_hook"]

[dependencies]
wasm-bindgen = "0.2"

# The `console_error_panic_hook` crate provides better debugging of panics by
# logging them with `console.error`. This is great for development, but requires
# all the `std::fmt` and `std::panicking` infrastructure, so isn't great for
# code size when deploying.
console_error_panic_hook = { version = "0.1.1", optional = true }

# `wee_alloc` is a tiny allocator for wasm that is only ~1K in code size
# compared to the default allocator's ~10K. It is slower than the default
# allocator, however.
#
# Unfortunately, `wee_alloc` requires nightly Rust when targeting wasm for now.
wee_alloc = { version = "0.4.2", optional = true }

In our code, we'll mark certain parts of code as running only if certain [features] are enabled, specifically, console_error_panic_hook and wee_alloc. By default, only console_error_panic_hook is enabled. To disable or enable either feature, by default, we can edit the default vector under [features].

To learn more about these features, we discuss them in-depth in the src/lib.rs and src/utils.rs sections.

Briefly, they include:

• console_error_panic_hook for logging panic messages to the developer console.
• wee_alloc, an allocator optimized for small code size.