jsgen: cannot assign unsafe nil values (fix #24407, #24436) (#24458)

Author: gchumillas

vlib/v/gen/js/builtin_types.v

@@ -329,6 +329,20 @@ fn (mut g JsGen) gen_builtin_prototype(c BuiltinPrototypeConfig) {
 	g.writeln('function ${c.typ_name}__eq(self,other) { return ${c.eq}; } ')
 }
 
+fn (mut g JsGen) gen_nil_const() {
+	g.writeln('const nil__ = new \$ref(new nil());')
+	g.gen_builtin_prototype(
+		typ_name:      'nil'
+		val_name:      'str'
+		default_value: 'new String("&nil")'
+		constructor:   'this.str = str.toString(); this.len = this.str.length'
+		value_of:      'null'
+		to_string:     '"&nil"'
+		eq:            'new bool(self.valueOf() === other.valueOf())'
+		to_jsval:      'null'
+	)
+}
+
 // generate builtin type definitions, used for casting and methods.
 fn (mut g JsGen) gen_builtin_type_defs() {
 	g.inc_indent()

vlib/v/gen/js/js.v

@@ -143,6 +143,7 @@ pub fn gen(files []&ast.File, mut table ast.Table, pref_ &pref.Preferences) stri
 		graph.add(g.file.mod.name, imports)
 		// builtin types
 		if g.file.mod.name == 'builtin' && !g.generated_builtin {
+			g.gen_nil_const()
 			g.gen_builtin_type_defs()
 			g.writeln('Object.defineProperty(array.prototype,"len", { get: function() {return new int(this.arr.arr.length);}, set: function(l) { this.arr.arr.length = l.valueOf(); } }); ')
 			g.writeln('Object.defineProperty(map.prototype,"len", { get: function() {return new int(this.length);}, set: function(l) { } }); ')
@@ -965,7 +966,7 @@ fn (mut g JsGen) expr(node_ ast.Expr) {
 			g.gen_lock_expr(node)
 		}
 		ast.Nil {
-			g.write('null')
+			g.write('nil__')
 		}
 		ast.NodeError {}
 		ast.None {
@@ -1345,7 +1346,7 @@ fn (mut g JsGen) gen_assign_stmt(stmt ast.AssignStmt, semicolon bool) {
 				g.expr(left)
 			}
 
-			is_ptr := stmt.op == .assign && stmt.left_types[i].is_ptr() && !array_set
+			is_ptr := stmt.op == .assign && stmt.right_types[i].is_ptr() && !array_set
 			if is_ptr {
 				g.write('.val')
 			}

vlib/v/gen/js/tests/unsafe.v

@@ -0,0 +1,74 @@
+struct Bar {
+	value int
+}
+
+struct Foo1 {
+mut:
+	name &string
+}
+
+struct Foo2 {
+mut:
+	name ?&string
+}
+
+struct Foo3 {
+mut:
+	bar &Bar
+}
+
+fn main() {
+	// Basic test of assigning nil to a string pointer
+	mut str := 'hi!'
+	mut str_ptr := &str
+	unsafe {
+		str_ptr = nil
+	}
+	println(str_ptr) // should print '&nil'
+	assert str_ptr == unsafe { nil }
+
+	// Test initializing a pointer field with nil
+	f1 := Foo1{
+		name: unsafe { nil }
+	}
+	assert f1.name == unsafe { nil }
+
+	// Test assigning nil to an optional pointer field
+	mut f2 := Foo2{}
+	unsafe {
+		f2.name = nil
+	}
+	if f2.name != none {
+		assert f2.name == unsafe { nil }
+	} else {
+		assert false
+	}
+
+	// Test assigning nil to a struct pointer
+	mut f3 := &Foo2{}
+	unsafe {
+		f3 = nil
+	}
+	assert f3 == unsafe { nil }
+
+	// Test with custom struct fields
+	mut f4 := Foo3{
+		bar: &Bar{42}
+	}
+	unsafe {
+		f4.bar = nil
+	}
+	assert f4.bar == unsafe { nil }
+
+	// Test with nil pointers in arrays
+	mut ptrs := []&string{len: 3, init: unsafe { nil }}
+	p0, p1 := 'hello', 'world'
+	ptrs[0] = &p0
+	ptrs[1] = &p1
+	unsafe {
+		ptrs[2] = nil
+	}
+	assert ptrs[0] != unsafe { nil }
+	assert ptrs[1] != unsafe { nil }
+	assert ptrs[2] == unsafe { nil }
+}