Complex constraint expressions are not currently supported

[Nicceboy]

In general, ASN.1 supports multiple ranges in a constraint or combination of ranges and single value constraints.

However, rasn constraints support currently only a single constraint value per constraint type (excluding PermittedAlphabetConstraint).

This applies for constraint types that can have a multitude of single value and/or range value constraints applying into them at the same time, where the effective constraint value is not actually continuous and should be described as union.

For example:

#[derive(AsnType, Debug, Clone, Decode, PartialEq, PartialOrd, Eq, Ord, Hash)]
#[rasn(delegate, size("3"), size("7"), size("9..15"))]
pub struct RestrictedString(pub OctetString);

The previous sample currently only notes the last constraint ("9..15"). Combining the values to the single size() call is not permitted.
Usually this is described with union ∪ or | syntax, so that all values are permitted from these.
If there are layers of types with different constraints, the effective constraint is intersection of all these constraints. Described with ∩ or ^ elsewhere as direct notation.

For most use-cases, the current functionality can be enough, but this should be noted some day.

It seems to be a non-trivial problem, if we want to do it mostly in compile-time.

PermittedAlphabetConstraint makes union currently on macro level, since it is easier for alphabets.
However, any additional intersections are not currently possible on compile-time as the constraint value is passed as static reference. Also proper runtime intersection is missing for this type.

Extensible constraint additions should be also noted for intersection/union support.