declare.rs

com::declare!(

    // A COM interface declared here
    [uuid(...)]
    interface IOldSchool {...}

    // A COM class declared here (not defined here)
    [uuid(...)]
    class ExternalClass : IOldSchool;

    // A COM class defined here
    [uuid(...)]
    class ExportedClass : IOldSchool {...}

    // A local COM class defined here (no uuid and not exported)
    class LocalClass : IOldSchool {...}
);

impl ExportedClass {
    fn new() -> Result<ExportedClass> {
        Self {...}
    }
}

impl LocalClass {
    fn new() -> Result<ExportedClass> {
        Self {...}
    }
}

winrt::declare!(

    // A WinRT struct defined here
    struct StructType { x: i32, y: i32 };

    // A WinRT interface defined here (uuid is inferred)
    interface IInterfaceType {
        fn method(&self) -> Result<i32>;
    }

    // A WinRT class defined here (implements both WinRT and COM interfaces)
    class ClassyType : windows::foundation::IStringable, IInterfaceType, IOldSchool {
        value : String

        // constructors and statics declared here so we can wire up class factory
        fn new() -> Result<ClassyType>;
        fn something_static() -> Result<i32>;
    }
);

impl ClassyType {
    fn new() -> Result<ClassyType> {
        Ok(Self { value: "value".to_owned() })
    }
    fn something_static() -> Result<i32> {
        Ok(123)
    }
}

impl windows::foundation::IStringable for ClassyType {
    fn to_string(&self) -> Result<winrt::HString> {
        ...
    }
}