pub fn create_or_merge(source: Source, attrs: Attrs)Expand description
Insert or update a configuration layer for the given Source.
If a layer with the same Source already exists, its attributes
are updated in place: all keys present in attrs are absorbed
into the existing layer, overwriting any previous values for those
keys while leaving all other keys in that layer unchanged.
If no layer for source exists yet, this behaves like set: a
new layer is created with the provided attrs.
This is useful for incremental / additive updates (for example, runtime configuration driven by a Python API), where callers want to change a subset of keys without discarding previously installed values in the same layer.
By contrast, set replaces the entire layer for source with
attrs, discarding any existing values in that layer.