The error message when trying to generate a type that does not exist is less than ideal:

error: proc macro panicked
  --> bindings\build.rs:2:5
   |
2  | /     winrt::build!(
3  | |         windows::foo::{
4  | |             NoExist,
5  | |             Bar,
...  |
41 | |         }
42 | |     );
   | |______^
   |
   = help: message: Could not find type `Windows.Foo.NoExist"

Note that in a long list of types, if the non-existing type is not one of the first few, it won't even show in the error message span.