Function build_subtree_proof 

pub fn build_subtree_proof(
    tree: &MerkleTree,
    nonce: &[u8; 32],
    challenged_peer_id: &[u8; 32],
    bytes_for: impl Fn(&XorName) -> Option<Vec<u8>>,
) -> Result<SubtreeProof, BuildProofError>

Build the single-contiguous-subtree proof for (nonce, tree) (responder).

bytes_for(&key) returns the chunk bytes the responder holds for a key, or None if it cannot read them. Walks the same nonce-selected path the auditor will re-derive, reads the unselected sibling cut-hashes directly from the committed tree (so they are provably consistent with the gossiped root), and builds each selected leaf’s plain and nonced hashes from the real bytes.

Errors

See BuildProofError. MissingBytes is the one the caller penalises; the others indicate an internal inconsistency.