elide defaulted fields in debug output

Author: nikomatsakis

crates/formality-macros/src/debug.rs

@@ -242,14 +242,24 @@ fn debug_variant_with_attr(
 
 fn debug_field_with_mode(name: &Ident, mode: &FieldMode) -> TokenStream {
     match mode {
-        FieldMode::Single | FieldMode::Optional => {
+        FieldMode::Single => {
             quote_spanned! { name.span() =>
                 write!(fmt, "{}", sep)?;
                 write!(fmt, "{:?}", #name)?;
                 sep = " ";
             }
         }
 
+        FieldMode::Optional => {
+            quote_spanned! { name.span() =>
+                if !::formality_core::util::is_default(#name) {
+                    write!(fmt, "{}", sep)?;
+                    write!(fmt, "{:?}", #name)?;
+                    sep = " ";
+                }
+            }
+        }
+
         FieldMode::Many => {
             quote_spanned! { name.span() =>
                 for e in #name {