Diagnostics for stable and nightly proc-macros!

1. Depend on the library in your proc-macro.

[dependencies]
proc-macro2-diagnostics = "0.10"

2. Import SpanDiagnosticExt and use its methods on a proc_macro2::Span to create Diagnostics:

use syn::spanned::Spanned;
use proc_macro2::TokenStream;
use proc_macro2_diagnostics::{SpanDiagnosticExt, Diagnostic};

fn my_macro(input: TokenStream) -> Result<TokenStream, Diagnostic> {
    Err(input.span().error("there's a problem here..."))
}

3. If there's an error, emit the diagnostic as tokens:

extern crate proc_macro;

pub fn real_macro(tokens: proc_macro::TokenStream) -> proc_macro::TokenStream {
    match my_macro(tokens.into()) {
        Ok(tokens) => tokens.into(),
        Err(diag) => diag.emit_as_expr_tokens().into()
    }
}

This does the right thing on nightly or stable.

On stable, due to limitations, any top-level, non-error diagnostics are emitted as errors. This will abort compilation. To avoid this, you may want to cfg-gate emitting non-error diagnostics to nightly.

By default, error messages are colored on stable. To disable, disable default features:

[dependencies]
proc-macro2-diagnostics = { version = "0.10", default-features = false }

The compiler always colors diagnostics on nightly.

Licensed under either of the following, at your option:

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)