Rust analyzer with VSCode "does not have the feature 'foo'"

I have three rust projects in separate directories. One of them uses a feature "foo" that the others do not. I get "rust-analyzer failed to load workspace" complaining about that feature. My solution has been add that feature in all my Cargo.toml files, but there must be a better way.

I suspected the problem has something to do with my VSCode settings, so I started a brand new hello-rust project. I'm getting the same error without a settings.json. (I have a settings.json in my home folder, but it only has entries for lldb.)