compiler: do not emit code for unnamed unused variables

If variable is unnamed and does not contain function call then it's
treated as unused and code generation may be omitted for it
initialization/declaration.

pkg/compiler/codegen.go

@@ -596,15 +596,32 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 						}
 					}
 				}
-				for i := range t.Names {
-					if len(t.Values) != 0 {
-						if i == 0 || !multiRet {
-							ast.Walk(c, t.Values[i])
+				for i, id := range t.Names {
+					if id.Name != "_" {
+						if len(t.Values) != 0 {
+							if i == 0 || !multiRet {
+								ast.Walk(c, t.Values[i])
+							}
+						} else {
+							c.emitDefault(c.typeOf(t.Type))
 						}
-					} else {
-						c.emitDefault(c.typeOf(t.Type))
+						c.emitStoreVar("", t.Names[i].Name)
+						continue
+					}
+					// If var decl contains call then the code should be emitted for it, otherwise - do not evaluate.
+					if len(t.Values) == 0 {
+						continue
+					}
+					var hasCall bool
+					if i == 0 || !multiRet {
+						hasCall = containsCall(t.Values[i])
+					}
+					if hasCall {
+						ast.Walk(c, t.Values[i])
+					}
+					if hasCall || i != 0 && multiRet {
+						c.emitStoreVar("", "_") // drop unused after walk
 					}
-					c.emitStoreVar("", t.Names[i].Name)
 				}
 			}
 		}