attributes: remove ErrorModes, improve wording on doc

tracing-attributes/src/lib.rs

@@ -192,9 +192,9 @@ use syn::{
 /// }
 /// ```
 /// 
-/// The above example will be emitting error events using the `std::fmt::Display` implementation.
-/// If `E` implements `std::fmt::Debug`, you can also make it use that implementation with
-/// `err(Debug)`:
+/// By default, error values will be recorded using their `std::fmt::Display` implementations.
+/// If an error implements `std::fmt::Debug`, it can be recorded using its `Debug` implementation
+/// instead, by writing `err(Debug)`:
 /// 
 /// ```
 /// # use tracing_attributes::instrument;
@@ -744,7 +744,8 @@ impl Parse for InstrumentArgs {
                 }
                 args.fields = Some(input.parse()?);
             } else if lookahead.peek(kw::err) {
-                let ErrorModes(mode) = input.parse()?;
+                let _ = input.parse::<kw::err>();
+                let mode = ErrorMode::parse(input)?;
                 args.err_mode = Some(mode);
             } else if lookahead.peek(Token![,]) {
                 let _ = input.parse::<Token![,]>()?;
@@ -837,17 +838,6 @@ impl Parse for ErrorMode {
     }
 }
 
-struct ErrorModes(ErrorMode);
-
-impl Parse for ErrorModes {
-
-    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
-        let _ = input.parse::<kw::err>();
-        let mode = ErrorMode::parse(input)?;
-        Ok(Self(mode))
-    }
-}
-
 #[derive(Debug)]
 struct Fields(Punctuated<Field, Token![,]>);