cargo build fails when ~/.cargo/credentials is unreadable

[RalfJung]

Problem

When running cargo inside a sandbox that rejects access to ~/.cargo/credentials, cargo build fails. I prefer to run VScode in a sandbox and not give it access to confidential files such as ~/.cargo/credentials. cargo build really shouldn't need access to my credentials, so this seems like it should be avoidable.

Steps

1. Change permissions such that ~/.cargo/credentials is unreadable
2. Do some cargo action that does not require credentials
3. You get: error: Permission denied (os error 13)

The error message is quite bad as it doesn't even say which file is affected! I had to use strace to determine that (which was hard because strace is blocked in the sandbox...)

EDIT: Error message fixed via #7649.

Possible Solution(s)

Only read credentials when needed, i.e., in cargo publish.

Notes

Output of cargo version:

cargo 1.41.0-nightly (8280633db 2019-11-11)

[est31]

The error message is quite bad as it doesn't even say which file is affected

Unfortunately, the Display impl of std errors doesn't output the file name but this opaque error msg. Not sure if there is an upstream discussion about it.