Declare (global) variables that must be set before execution?

Is it possible to declare variables that don't have default values assigned?

The expected behavior would be that all recipes (that use the variable?) will fail to run without setting them at the command line.

This would replicate the behavior with recipe parameters, but for global variables.

Start by comparing the requested global-variable behavior with the existing recipe-parameter behavior described in the issue. Determine how variables are parsed and validated before execution, then define the affected recipes and command-line failure behavior; the issue does not name files or tests to run.

Written by the indexing model from the issue text.