I wish I could make my own keywords

Think for a moment about the compiler checking const correctness adds.

Then, imagine a c++ with a syntax where we could create our own 'const'
like qualifier keywords.

In a multi threaded development environment one could imagine wanting to
declare a 'threadsafe' qualifier.
declare_qualifier threadsafe;

threadsafe would be a qualifier for methods - methods qualified as
threadsafe would cause compiler errors if they attempted to call non
threadsafe qualified methods.

Or, in the context of processing data from the user

   declare_qualifier unsanitized;

Now, with some 'rules', we could create a keyword that would be used to
guard data directly entered by the user, and functions that work on that
data. Functions not expecting unsanitzed data would be disallowed from
reading or writing to unsanitized variables.
Conversion functions would use casting, and actual data parsing, to move
the data from a unsanitized into a 'sanitized' (or non-unsanitized)
buffer at which point it could be passed safely to SQL or other layers
in the app that are not so careful about what they process.

Im sure these are not the only two uses possible for custom qualifiers,
and the rules for each one, to work in their role successfully, would
have to have some way of being specified. But its time I think that
compiers tried harder to allow programmers to write 'correct' software
using tools more versatile than just 'const'.