Struct depths provide statistics on how many variables in the random program are structs. If a variable is not struct type, the depth is 0; if it is “plain” struct type(none of the fields is struct), the depth is 1; if it is nested struct type, the depth is 2 or more.
_expression_ depth is a way we track the complexity of expressions, i.e., number of sub-expressions. Unfortunately those numbers are unreliable due to a bug in Csmith. We’ll fix this problem in a later release.