Convert empty slices and maps to nil

Author: sublee

internal/convgen/assign/index.go

@@ -45,12 +45,16 @@ func (fac *factory) tryIndex(x, y Object) (*indexAssigner, error) {
 // writeAssignCode writes code that assigns x to y by converting each element.
 func (a indexAssigner) writeAssignCode(w *codefmt.Writer, varX, varY, varErr string) {
 	if a.isSliceY {
+		w.Printf("if len(%s) != 0 {\n", varX)
+		defer w.Printf("}\n")
+
 		w.Printf("%s = make([]%t, len(%s))\n", varY, a.elemY, varX)
 	}
 
 	varI := w.Name("i")
 	varV := w.Name("v")
 	w.Printf("for %s, %s := range %s {\n", varI, varV, varX)
+	defer w.Printf("}\n")
 
 	a.assigner.writeAssignCode(w, varV, varY+"["+varI+"]", varErr)
 	if a.requiresErr() {
@@ -61,6 +65,4 @@ func (a indexAssigner) writeAssignCode(w *codefmt.Writer, varX, varY, varErr str
 		w.Printf("break\n")
 		w.Printf("}\n")
 	}
-
-	w.Printf("}\n")
 }

internal/convgen/assign/key.go

@@ -75,11 +75,15 @@ func (fac *factory) tryKey(x, y Object) (*keyAssigner, error) {
 // writeAssignCode writes code that assigns x to y by converting each key and
 // element.
 func (a keyAssigner) writeAssignCode(w *codefmt.Writer, varX, varY, varErr string) {
+	w.Printf("if len(%s) != 0 {\n", varX)
+	defer w.Printf("}\n")
+
 	w.Printf("%s = make(map[%t]%t, len(%s))\n", varY, a.keyY, a.elemY, varX)
 
 	varKeyX := w.Name("k")
 	varValX := w.Name("v")
 	w.Printf("for %s, %s := range %s {\n", varKeyX, varValX, varX)
+	defer w.Printf("}\n")
 
 	varKeyY := w.Name("k")
 	w.Printf("var %s %t\n", varKeyY, a.keyY)
@@ -98,6 +102,4 @@ func (a keyAssigner) writeAssignCode(w *codefmt.Writer, varX, varY, varErr strin
 		w.Printf("break\n")
 		w.Printf("}\n")
 	}
-
-	w.Printf("}\n")
 }

testdata/program/NilAsEmpty/main/main.go

@@ -0,0 +1,30 @@
+//go:build convgen
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/sublee/convgen"
+)
+
+type X struct {
+	Slice []int
+	Map   map[int]int
+}
+
+type Y X
+
+var XtoY = convgen.Struct[X, Y](nil)
+
+func main() {
+	// Output: true true
+	xNil := X{}
+	yNil := XtoY(xNil)
+	fmt.Println(yNil.Slice == nil, yNil.Map == nil)
+
+	// Output: true true
+	xLen0 := X{Slice: []int{}, Map: map[int]int{}}
+	yLen0 := XtoY(xLen0)
+	fmt.Println(yLen0.Slice == nil, yLen0.Map == nil)
+}

testdata/program/NilAsEmpty/want/program_output.txt

@@ -0,0 +1,2 @@
+true true
+true true