Compilation error messages could be better

[ADD-SP]

For struct like

struct Unnamed(String);

The derive macro EncodeLabelSet emits

error: proc-macro derive panicked
 --> tests/build/friendly-compilation-error-msg.rs:7:49
  |
7 |     #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
  |                                                 ^^^^^^^^^^^^^^
  |
  = help: message: Can not derive Encode for struct with unnamed fields.

I think proc-macro derive panicked is a little bit confuse, and the end developer needs to continue reading down the error message to find the cause.

An usual practice is emit compile_error!, which show the causes more directly.