Macro rustc_errors::diagnostic_builder::forward

macro_rules! forward {
    (
        ($f:ident, $with_f:ident)($($name:ident: $ty:ty),* $(,)?)
    ) => { ... };
}

DiagnosticBuilder impls DerefMut, which allows access to the fields and methods of the embedded Diagnostic. However, that doesn’t allow method chaining at the DiagnosticBuilder level. Each use of this macro defines two builder methods at that level, both of which wrap the equivalent method in Diagnostic.

• A &mut self -> &mut Self method, with the same name as the underlying Diagnostic method. It is mostly to modify existing diagnostics, either in a standalone fashion, e.g. err.code(code), or in a chained fashion to make multiple modifications, e.g. err.code(code).span(span).
• A self -> Self method, which has a with_ prefix added. It is mostly used in a chained fashion when producing a new diagnostic, e.g. let err = struct_err(msg).with_code(code), or when emitting a new diagnostic , e.g. struct_err(msg).with_code(code).emit().

Although the latter method can be used to modify an existing diagnostic, e.g. err = err.with_code(code), this should be avoided because the former method gives shorter code, e.g. err.code(code).