How to Pass Extended Context to Nested Structs?

[DidingasLushis]

I'm trying to create a nested struct hierarchy where a child struct needs additional context parameters beyond what its parent struct receives. Specifically, I want to pass both the inherited endian context AND a reference to a sibling field (field_a2: &u8) to fields (structs).

#[derive(DekuRead, DekuWrite, Debug, PartialEq, Clone)]
#[deku(endian = "endian", ctx = "endian: deku::ctx::Endian")]
pub struct DataA {
    field_a1: u16,
    field_a2: u8,
    
    #[deku(ctx="(endian, field_a2)")]
    special_field: HandlesData,
}

#[derive(DekuRead, DekuWrite, Debug, PartialEq, Clone)]
#[deku(endian = "endian", ctx = "endian: deku::ctx::Endian, field_a2: &u8")]
pub struct HandlesData {
    num_handles: u8,
    
    #[deku(count = "num_handles + *field_a2")]
    handles: Vec<Handles>,
}

Error: Type mismatch - error[E0308]: mismatched types
--> src/frame/data_a.rs:13:18
|
13 | #[deku(ctx = "(endian , field_a2)")]
| ^^^^^^^^^^^^^^^^^^^^^ expected &u8, found (Endian, &u8)

Questions

1. Is there a way to extend/augment the context when passing it to nested fields?

   • Parent receives: (endian)
   • Child needs: (endian, field_a2: &u8)

2. What's the correct syntax for passing references to sibling fields as context to nested structs?

3. Should I restructure my data model to avoid this pattern, or is there a Deku idiom I'm missing?

Desired Behavior

When reading from bytes:

1. Read field_a1 and field_a2 in DataA
2. Pass both endian AND a reference to the just-read field_a2 value to HandlesData
3. HandlesData uses field_a2 in its dynamic count calculation: count = "num_handles + *field_a2"

Environment

• deku version: 0.19.1
• Rust version: (stable/nightly)
• Use case: Binary protocol parsing with conditional features and cross-field dependencies

Additional Context

This pattern is needed because:

• The protocol has backwards compatibility via feature flags (#[cfg(feature = "feature_handles")])
• Different protocol versions have different field layouts
• Later fields depend on values from earlier fields (dynamic sizing based on field_a2)
• The container struct may be used in multiple contexts, but the nested struct needs additional information

Is there a recommended pattern for this in Deku?

[DidingasLushis]

I think my confusion may be that enums with id = "header.id()" do not inherit context?