Macro __mx_condition_field_name 

#[doc(hidden)]
macro_rules! __mx_condition_field_name {
    (lit $name:literal) => { ... };
    (ident $name:ident) => { ... };
    (expr $name:expr) => { ... };
}

Internal: one condition-data field name, validated against the reserved slots. Not part of the public API.

• lit "name" / ident name: the name is a literal, so the reserved-name check runs at compile time (const assertion) and the field costs one to_string() at runtime.
• expr <e>: a computed name; checked at runtime by crate::condition::check_condition_field_name.