Constant DEFAULT_RECEIVE_LIMITS 

pub const DEFAULT_RECEIVE_LIMITS: ReceiveLimits;

Generous-but-finite defaults: every real message passes (genuine nesting is almost always < 10 deep); only the pathological / adversarial ones are refused. max_depth sits BELOW [MAX_ENCODE_DEPTH] on purpose — the recursive DECODER’s stack frame is heavier than the encoder’s (one giant match), so the depth that is safe to recurse on a small (wasm ~1 MiB) stack is lower than what we allow ourselves to encode. A deployment tightens these per peer through the handshake.