Proc_macro error handling

1

I am trying to provide better compile errors using quote_spanned and compile_error macro however compiler is giving a lot of nonsensical help messages. Is there a way to prevent this?
The only named struct fields are supported part is my actual error but compiler adds whole bunch of extra stuff

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2

Please post the code you are using to generate the output. It's overwhelmingly likely that the compiler isn't "adding extra stuff", but your proc-macro indeed produces incorrect code.

3 
pub type Result<T> = std::result::Result<T, TokenStream>;
pub fn fail(span: Span, txt: impl AsRef<str>) -> Result<Infallible>
{
	let txt = txt.as_ref();
	Err(quote_spanned! { span => compile_error!(#txt) }.into())
}

this is my method to fail with a span.

pub fn parse_root(i: &DeriveInput) -> crate::Result<(&DataStruct, &FieldsNamed)>
//{{{
{
	let _attrs = parse_attrs(i, &["name"])?; // top level
	match i.data {
		syn::Data::Struct(ref d) => match d.fields {
			syn::Fields::Named(ref f) => {
				if f.named.is_empty() {
					fail(i.ident.span(), "struct must have at least one named field")?;
					unreachable!()
				}
				Ok((d, f))
			}
			_ => {
				fail(i.ident.span(), "Only named struct fields are supported")?;
				unreachable!()
			}
		},
		_ => panic!("#[derive(Tbl)] can only be used on structs"),
	}
}```
and this is how I use it
4

For macros that use syn, syn::Error is the canonical way to do compile errors correctly. In your case maybe something like: syn::Error::new(span, txt).to_compile_error()

2 Likes
5

Oh I didnt even know such a method existed quote_spanned is all over the internet for some reason and the long waited Diagnostics API people talk about but syn::Error for some reason was not mentioned anywhere I checked

6

That fixed it all thanks :smiley:

7

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