This is a demonstration of what I think is a bug in cargo. In the a/ directory there's a .cargo/config file with bullshit contents. However, cargo doesn't read this config file so no error is raised (this is done to demonstrate that cargo doesn't read this config).

However, moving the same .cargo/config directory to the root:

cp -r a/.cargo .

does raise an error: the config file is not a toml file. I expected the .cargo/config in the subdirectory to apply to the building of the a/ crate.