Diagnostic-based compiler errors for proc_macro2

Use Diagnostic compiler messages from proc_macro2 code with Result-like syntax.

Provides a DiagnosticResult which stores a Diagnostic based upon the API of [proc_macro::Diagnostic] and allows ? usage to return early from proc_macro2 code.

Note

This crate is a little opinionated in an attempt to make it simpler to create good compiler errors:

• Top-level diagnostics must be either an Error or a Warning
• (Only) Help (& Notes -> still to do) can be added to a diagnostic
• Errors always span the original call site - add a Help or Note to add information related to other spans
• Warnings will always finish with a Note detailing the original call site
• Multi-level nesting is not possible

Usage

#![feature(never_type)]
#![feature(try_trait_v2)]

# extern crate proc_macro;

use proc_macro2_diagnostic::{DiagnosticResult,DiagnosticStream};
use quote::quote;

fn zst(name: &str) -> DiagnosticStream {
    match name {
        "fail" => DiagnosticResult::error("failed")?,
        _ => DiagnosticResult::Ok(quote!{struct #name;}),
    }
}