rust-embedded Discovery `05-led-roulette` fails to build

Question

Background

I am following this guide to build the projects in the corresponding repo . I successfully compiled, ran and verified project 03 .

Problem

However, following the instructions to build project 05 produces "error[E0463]: can't find crate for core ":

$ cargo build --features v2 --target thumbv7em-none-eabihf
info: syncing channel updates for 'nightly-2021-08-18-aarch64-apple-darwin'
info: latest update on 2021-08-18, rust version 1.56.0-nightly (30a0a9b69 2021-08-17)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-src'
info: downloading component 'rust-std'
info: downloading component 'rust-std' for 'riscv32imac-unknown-none-elf'
info: downloading component 'rust-std' for 'riscv32imc-unknown-none-elf'
info: downloading component 'rust-std' for 'thumbv6m-none-eabi'
info: downloading component 'rust-std' for 'thumbv7em-none-eabi'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-src'
info: installing component 'rust-std'
info: installing component 'rust-std' for 'riscv32imac-unknown-none-elf'
info: installing component 'rust-std' for 'riscv32imc-unknown-none-elf'
info: installing component 'rust-std' for 'thumbv6m-none-eabi'
info: installing component 'rust-std' for 'thumbv7em-none-eabi'
info: installing component 'rustc'
info: installing component 'rustfmt'
info: syncing channel updates for 'stable-aarch64-apple-darwin'
info: latest update on 2022-01-20, rust version 1.58.1 (db9d1b20b 2022-01-20)
info: downloading component 'rust-src'
info: installing component 'rust-src'
   Compiling critical-section v0.2.5
error[E0463]: can't find crate for `core`
  |
  = note: the `thumbv7em-none-eabihf` target may not be installed
  = help: consider downloading the target with `rustup target add thumbv7em-none-eabihf`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `critical-section` due to previous error

This seems like an unexpected error, since it suggests that I'm missing the thumbv7em-none-eabihf target toolchain, and yet project 03 would not have compiled correctly without this toolchain installed.

Question

Can I fix something about my system to correctly build project 05? Or is there a bug in the project?

Related system data:

If anyone would like more information, please lmk in the comments and I can update this section with more outputs from my machine.

$ rustup component list --installed
cargo-aarch64-apple-darwin
clippy-aarch64-apple-darwin
rust-src
rust-std-aarch64-apple-darwin
rust-std-thumbv7em-none-eabihf
rustc-aarch64-apple-darwin
rustfmt-aarch64-apple-darwin

$ rustup show
Default host: aarch64-apple-darwin
rustup home:  /Users/___/.rustup

installed toolchains
--------------------

stable-aarch64-apple-darwin (default)
nightly-2021-08-18-aarch64-apple-darwin

installed targets for active toolchain
--------------------------------------

aarch64-apple-darwin
thumbv7em-none-eabihf

active toolchain
----------------

stable-aarch64-apple-darwin (default)
rustc 1.58.1 (db9d1b20b 2022-01-20)

Answer 1

Turns out my rust toolchain was broken; I originally installed everything with nix, but replacing some of the nix installs with rustup installs (ie, cargo) fixed the issues.