Construct skipped fields only after reading all non-skipped fields

This should increase performance if construction of default object is expensive

Author: Mingun

serde_derive/src/de.rs

@@ -771,19 +771,26 @@ fn read_fields_in_order(
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let mut index_in_seq = 0_usize;
-    let let_values = vars.clone().zip(fields).map(|(var, field)| {
-        if field.attrs.skip_deserializing() {
-            let default = Expr(expr_is_missing(field, cattrs));
-            quote! {
-                let #var = #default;
-            }
-        } else {
+    let let_values = vars.clone().zip(fields).filter_map(|(var, field)| {
+        if !field.attrs.skip_deserializing() {
             let read = read_field(params, index_in_seq, field, cattrs, expecting);
             index_in_seq += 1;
-            quote! {
+
+            return Some(quote! {
                 let #var = #read;
-            }
+            });
+        }
+        None
+    });
+    let let_skipped = vars.clone().zip(fields).filter_map(|(var, field)| {
+        if field.attrs.skip_deserializing() {
+            let default = Expr(expr_is_missing(field, cattrs));
+
+            return Some(quote! {
+                let #var = #default;
+            });
         }
+        None
     });
 
     let mut result = if is_struct {
@@ -822,6 +829,7 @@ fn read_fields_in_order(
     quote_block! {
         #let_default
         #(#let_values)*
+        #(#let_skipped)*
         _serde::__private::Ok(#result)
     }
 }
@@ -883,15 +891,9 @@ fn read_fields_in_order_in_place(
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let mut index_in_seq = 0usize;
-    let write_values = fields.iter().map(|field| {
-        let member = &field.member;
-
-        if field.attrs.skip_deserializing() {
-            let default = Expr(expr_is_missing(field, cattrs));
-            quote! {
-                self.place.#member = #default;
-            }
-        } else {
+    let write_values = fields.iter().filter_map(|field| {
+        if !field.attrs.skip_deserializing() {
+            let member = &field.member;
             let value_if_none = expr_is_missing_seq(Some(quote!(self.place.#member = )), index_in_seq, field, cattrs, expecting);
             let write = match field.attrs.deserialize_with() {
                 None => {
@@ -919,8 +921,20 @@ fn read_fields_in_order_in_place(
                 }
             };
             index_in_seq += 1;
-            write
+            return Some(write);
         }
+        None
+    });
+    let write_skipped = fields.iter().filter_map(|field| {
+        if field.attrs.skip_deserializing() {
+            let member = &field.member;
+            let default = Expr(expr_is_missing(field, cattrs));
+
+            return Some(quote! {
+                self.place.#member = #default;
+            });
+        }
+        None
     });
 
     let this_type = &params.this_type;
@@ -942,6 +956,7 @@ fn read_fields_in_order_in_place(
     quote_block! {
         #let_default
         #(#write_values)*
+        #(#write_skipped)*
         _serde::__private::Ok(())
     }
 }