Function ty_contains_mapping 

fn ty_contains_mapping<'hir>(
    gcx: Gcx<'hir>,
    hir: &'hir Hir<'hir>,
    ty: Ty<'hir>,
    seen: &mut Vec<StructId>,
) -> bool

Returns true if ty is, or transitively contains, a mapping.

delete zeroes each member of a storage value, but it cannot enumerate a mapping’s keys, so any mapping reachable from ty keeps its entries. Recurses through structs and arrays; seen guards against recursive struct definitions (only reachable through mapping/array members, which Solidity permits).