How to use rust-toolchain to pin rust versions in a workspace?

In a workspace, I have and do the following:

$ cat rust-toolchain
channel = "nightly-2021-01-24"
components = [ "rustfmt", "rust-std", "clippy" ]
targets = [ "wasm32-unknown-unknown", "x86_64-unknown-linux-gnu" ]

$ cargo fmt
error: invalid channel name '[toolchain]' in '.../rust-toolchain'
error: caused by: invalid toolchain name: '[toolchain]'

I'm not sure what I missed.

The TOML format for rust-toolchain was added in a recent version of rustup. Try this command to make sure you have the latest version:

rustup self update
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Thanks, that was the issue - I just reverted to using the simpler format for now due to being on Nix where running rustup self update is either not possible or not easy.