Use parse_quote! to make build time development easier

[tclem]

Makes better use of parse_quote! and specific syn AST types so that development of twirp-build is a bit easier. This gives much more specific error messages with line numbers that are helpful for debugging the code gen code. There is no change to generated code.

🎩 to @jorendorff for showing me how this works!

@jorendorff there was one place I didn't know how to handle: method chaining with syn::ExprMethodCall.

[Copilot]

Pull Request Overview

This PR refactors the code generation in twirp-build to use parse_quote! instead of quote! for better development experience. It adds type annotations to AST structures and leverages specific syn AST types, which provides more detailed error messages with line numbers for debugging code generation.

• Replaces quote! with parse_quote! for AST node generation
• Adds specific syn type annotations to AST collections
• Improves error reporting during code generation development

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[Copilot]

The handler_impl variable is missing a closing implementation block. The parse_quote! block at line 200 should include the entire implementation, but it's cut off and continued in handler_direct_impl. Consider combining these into a single implementation block or properly separating them.

[tclem]

@jorendorff there was one place I didn't know how to handle: method chaining with syn::ExprMethodCall.

I sort of figured this out... Let me know what you think.