checker: disallow assigning nil to struct fields (#18725)

vlib/v/checker/struct.v

@@ -596,6 +596,13 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 						inited_fields << struct_field.name
 					}
 				}
+				expected_type_sym := c.table.final_sym(init_field.expected_type)
+				if expected_type_sym.kind in [.string, .array, .map, .array_fixed, .chan, .struct_]
+					&& init_field.expr.is_nil() && !init_field.expected_type.is_ptr()
+					&& mut init_field.expr is ast.UnsafeExpr {
+					c.error('cannot assign `nil` to struct field `${init_field.name}` with type `${expected_type_sym.name}`',
+						init_field.expr.pos.extend(init_field.expr.expr.pos()))
+				}
 			}
 			// Check uninitialized refs/sum types
 			// The variable `fields` contains two parts, the first part is the same as info.fields,

vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.out

@@ -0,0 +1,42 @@
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:15:8: error: cannot assign `nil` to struct field `name` with type `string`
+   13 |
+   14 | a := &Foo{
+   15 |     name: unsafe { nil }
+      |           ~~~~~~~~~~~~
+   16 |     bar: unsafe { nil }
+   17 |     name2: unsafe { nil }
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:16:7: error: cannot assign `nil` to struct field `bar` with type `Bar`
+   14 | a := &Foo{
+   15 |     name: unsafe { nil }
+   16 |     bar: unsafe { nil }
+      |          ~~~~~~~~~~~~
+   17 |     name2: unsafe { nil }
+   18 |     arr: unsafe { nil }
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:17:9: error: cannot assign `nil` to struct field `name2` with type `string`
+   15 |     name: unsafe { nil }
+   16 |     bar: unsafe { nil }
+   17 |     name2: unsafe { nil }
+      |            ~~~~~~~~~~~~
+   18 |     arr: unsafe { nil }
+   19 |     mp: unsafe { nil }
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:18:7: error: cannot assign `nil` to struct field `arr` with type `[]int`
+   16 |     bar: unsafe { nil }
+   17 |     name2: unsafe { nil }
+   18 |     arr: unsafe { nil }
+      |          ~~~~~~~~~~~~
+   19 |     mp: unsafe { nil }
+   20 |     arr_fix: unsafe { nil }
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:19:6: error: cannot assign `nil` to struct field `mp` with type `map[string]string`
+   17 |     name2: unsafe { nil }
+   18 |     arr: unsafe { nil }
+   19 |     mp: unsafe { nil }
+      |         ~~~~~~~~~~~~
+   20 |     arr_fix: unsafe { nil }
+   21 | }
+vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv:20:11: error: cannot assign `nil` to struct field `arr_fix` with type `[3]int`
+   18 |     arr: unsafe { nil }
+   19 |     mp: unsafe { nil }
+   20 |     arr_fix: unsafe { nil }
+      |              ~~~~~~~~~~~~
+   21 | }
+   22 | println(a)

vlib/v/checker/tests/struct_field_assign_internal_types_nil_err.vv

@@ -0,0 +1,22 @@
+type String = string
+
+struct Bar {}
+
+struct Foo {
+	name    string
+	bar 	Bar
+	name2   String
+	arr     []int
+	mp      map[string]string
+	arr_fix [3]int
+}
+
+a := &Foo{
+	name: unsafe { nil }
+	bar: unsafe { nil }
+	name2: unsafe { nil }
+	arr: unsafe { nil }
+	mp: unsafe { nil }
+	arr_fix: unsafe { nil }
+}
+println(a)