The error message when a module is not found is not very helpful:

error: Module not found
  --> bindings\build.rs:3:25
   |
3  |           windows::mynamespace::{
   |  _________________________^
4  | |             Foo,
5  | |             Bar,
6  | |             Baz,
...  |
40 | |             Qux
41 | |         }
   | |_________^

The issue is that the windows::mynamespace module itself is missing. The error span should point to that path instead of all the types being imported.