checker: disallow casting strings to pointers outside unsafe (#19977)

Author: Delta456

vlib/v/checker/checker.v

@@ -3174,6 +3174,10 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		snexpr := node.expr.str()
 		tt := c.table.type_to_str(to_type)
 		c.error('cannot cast string to `${tt}`, use `${snexpr}[index]` instead.', node.pos)
+	} else if final_from_sym.kind == .string && to_type.is_pointer() && !c.inside_unsafe {
+		tt := c.table.type_to_str(to_type)
+		c.error('cannot cast string to `${tt}` outside `unsafe`, use ${tt}(s.str) instead',
+			node.pos)
 	} else if final_from_sym.kind == .array && !from_type.is_ptr() && to_type != ast.string_type
 		&& !(to_type.has_flag(.option) && from_type.idx() == to_type.idx()) {
 		ft := c.table.type_to_str(from_type)

vlib/v/checker/tests/invalid_string_cast_to_pointers_err.out

@@ -0,0 +1,17 @@
+vlib/v/checker/tests/invalid_string_cast_to_pointers_err.vv:2:9: error: cannot cast string to `voidptr` outside `unsafe`, use voidptr(s.str) instead
+    1 | test := 'test'
+    2 | println(voidptr(test))
+      |         ~~~~~~~~~~~~~
+    3 | println(byteptr(test))
+    4 | println(charptr(test))
+vlib/v/checker/tests/invalid_string_cast_to_pointers_err.vv:3:9: error: cannot cast string to `byteptr` outside `unsafe`, use byteptr(s.str) instead
+    1 | test := 'test'
+    2 | println(voidptr(test))
+    3 | println(byteptr(test))
+      |         ~~~~~~~~~~~~~
+    4 | println(charptr(test))
+vlib/v/checker/tests/invalid_string_cast_to_pointers_err.vv:4:9: error: cannot cast string to `charptr` outside `unsafe`, use charptr(s.str) instead
+    2 | println(voidptr(test))
+    3 | println(byteptr(test))
+    4 | println(charptr(test))
+      |         ~~~~~~~~~~~~~

vlib/v/checker/tests/invalid_string_cast_to_pointers_err.vv

@@ -0,0 +1,4 @@
+test := 'test'
+println(voidptr(test))
+println(byteptr(test))
+println(charptr(test))