"Error loading target specification" when building a WebAssembly module with Cargo - rust

Question

I've constructed a simple Rust app as follows:
cargo new hello_world --bin
I can compile to WebAssembly using the Rust compiler as follows:
rustc +nightly --target wasm32-unknown-unknown -O src/main.rs
I'd like to use Cargo to manage external dependencies, but when I build for the same target using cargo:
cargo build --release --target wasm32-unknown-unknown
It fails as follows:
$ cargo build --release --target wasm32-unknown-unknown
error: failed to run `rustc` to learn about target-specific information
Caused by:
process didn't exit successfully: `rustc - --crate-name ___ --print=file-names --target wasm32-unknown-unknown --crate-type bin --crate-type rlib` (exit code: 101)
--- stderr
error: Error loading target specification: Could not find specification for target "wasm32-unknown-unknown"
|
= help: Use `--print target-list` for a list of built-in targets

Answer 1

As the wasm32-unknown-unknown target has only recently been added to Rust, it's only available in the nightly toolchain. You have to specify that you wish to use the nightly toolchain:
cargo +nightly build --release --target wasm32-unknown-unknown
# ^^^^^^^^
You were already doing this when calling rustc directly:
rustc +nightly --target wasm32-unknown-unknown -O src/main.rs
# ^^^^^^^^
It's probably easier if you:
Use a toolchain override in your project directory:
$ rustup override set nightly
Set a default target in your project's .cargo/config:
[build]
target = "wasm32-unknown-unknown"
Then you can just cargo build --release. (It's recommended to avoid debug mode for now).