cgen: implement IError.free() and use it for the builtin `none` const

vlib/builtin/option.v

@@ -62,3 +62,27 @@ fn opt_ok(data voidptr, mut option Option, size int) {
 		C.memcpy(byteptr(&option.err) + sizeof(IError), data, size)
 	}
 }
+
+[unsafe]
+pub fn (e &Error) free() {
+	unsafe { e.msg.free() }
+}
+
+[unsafe]
+pub fn (n &None__) free() {
+	unsafe { n.msg.free() }
+}
+
+[typedef]
+struct C.IError {
+	_object voidptr
+}
+
+[unsafe]
+pub fn (ie &IError) free() {
+	unsafe {
+		ie.msg.free()
+		cie := &C.IError(ie)
+		C.free(cie._object)
+	}
+}

vlib/v/gen/c/cgen.v

@@ -975,8 +975,9 @@ fn (mut g Gen) stmts_with_tmp_var(stmts []ast.Stmt, tmp_var string) {
 					stmt_pos = stmt.expr.position()
 				}
 				if stmt_pos.pos == 0 {
-					print('autofree: first stmt pos = 0. ')
-					println(stmt.type_name())
+					$if trace_autofree ? {
+						println('autofree: first stmt pos = 0. $stmt.type_name()')
+					}
 					return
 				}
 			}
@@ -4849,6 +4850,8 @@ fn (mut g Gen) const_decl_init_later(mod string, name string, val string, typ ta
 			g.cleanups[mod].writeln('\tstring_free(&$cname);')
 		} else if sym.kind == .map {
 			g.cleanups[mod].writeln('\tmap_free(&$cname);')
+		} else if styp == 'IError' {
+			g.cleanups[mod].writeln('\tIError_free(&$cname);')
 		}
 	}
 }