scylla-macros: report errors as compile errors, not panics

[piodul]

Our current procedural macros handle errors by panicking. This leads to compilation errors which inform the user about that fact first, and then explain the real issue in the "help" part:

error: proc-macro derive panicked
 --> examples/user-defined-type.rs:6:10
  |
6 | #[derive(FromUserType)]
  |          ^^^^^^^^^^^^
  |
  = help: message: derive(FromUserType) works only on structs!

This commit gets rid of .expect() and panic() calls. Instead, errors are now propagated via Result<T, syn::Error> and properly converted to a compilation error at the end. This approach results in nicer error messages and allows to attach the message to a particular part of the original code that could cause the problem.

After the changes, error messages look like this:

error: derive(FromUserType) works only for structs with named fields
 --> examples/user-defined-type.rs:7:1
  |
7 | enum Foo {
  | ^^^^

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[piodul]

@Lorak-mmk @avelanarius review ping

[Lorak-mmk]

Just one question: why is .into() in e.into_compile_error().into() required? Doesn't .into_compile_error() already return TokenStream?

[piodul]

Just one question: why is .into() in e.into_compile_error().into() required? Doesn't .into_compile_error() already return TokenStream?

That's because into_compile_error() returns proc_macro2::TokenStream, whereas entry points to procedural macros must return proc_macro::TokenStream.

Ref: https://docs.rs/proc-macro2/latest/proc_macro2/