fn ban_async_in_2015(&self, async_span: Span) {
        if async_span.rust_2015() {
            struct_span_err!(
                self.diagnostic(),
                async_span,
                E0670,
                "`async fn` is not permitted in the 2015 edition",
            )
            .emit();
        }
    }
/// We are parsing `async fn`. If we are on Rust 2015, emit an error.
We are parsing `async fn`. If we are on Rust 2015, emit an error.
We are parsing ` async fn ` . If we are on Rust 2015 emit an error .
fn ban_async_in_2015(&self, async_span: Span) {
        if async_span.rust_2015() {
            struct_span_err!(
                self.diagnostic(),
                async_span,
                E0670,
                "`async fn` is not permitted in the 2015 edition",
            )
            .emit();
        }
    }
fn ban_async_in_2015 ( & self , async_span : Span ) { if async_span . rust_2015 ( ) { struct_span_err ! ( self diagnostic ( ) async_span E0670 " " ) . emit ( ) ; } }
We are parsing `async fn`.
We are parsing ` async fn ` .
&self
async_span
Span