We can't really do much here, the language server protocol has nothing that allows us to tell the client that the file is readonly. And on the VSCode side readonly files are configured by a setting, I don't like rust-analyzer modifying the settings of a user too much, especially if we do it automatically.

We could offer a command maybe / put a button the landing page that adds the appropriate entry to the setting.

For context, the following config will make the standard libraries of all toolchains as well as all your crates-io libraries readonly

"files.readonlyInclude": {
    "**/.cargo/registry/src/**/*.rs": true,
    "**/lib/rustlib/src/rust/library/**/*.rs": true,
},

I did not know those two settings existed. I agree that they prevent most accidental edits.

I think it would be a really nice service if rust-analyzer somehow made the users aware of this setting/helped them enable it, though I don't have an opinion on how to do this.

I guess we could also show a notification here: "you're editing a file under ~/.cargo, you really don't want that".