Less horrible logic for missing fields that unconditionally return error

serde_derive/src/de.rs

@@ -2539,13 +2539,9 @@ fn deserialize_map_in_place(
         .map(|&(field, ref name)| {
             let missing_expr = expr_is_missing(field, cattrs);
             // If missing_expr unconditionally returns an error, don't try
-            // to assign its value to self.place. Maybe this could be handled
-            // more elegantly.
-            if missing_expr
-                .as_ref()
-                .into_tokens()
-                .to_string()
-                .starts_with("return ")
+            // to assign its value to self.place.
+            if field.attrs.default().is_none() && cattrs.default().is_none()
+                && field.attrs.deserialize_with().is_some()
             {
                 let missing_expr = Stmts(missing_expr);
                 quote! {

serde_derive/src/internals/attr.rs

@@ -738,6 +738,16 @@ pub enum Default {
     Path(syn::ExprPath),
 }
 
+impl Default {
+    #[cfg(feature = "deserialize_in_place")]
+    pub fn is_none(&self) -> bool {
+        match *self {
+            Default::None => true,
+            Default::Default | Default::Path(_) => false,
+        }
+    }
+}
+
 impl Field {
     /// Extract out the `#[serde(...)]` attributes from a struct field.
     pub fn from_ast(