Module key 

ExprKey — a hashable, owning representation of an Expr node.

Expr is already Hash + Eq (derived), so ExprKey is simply a newtype wrapper that makes the intent explicit: it identifies a node’s structural content, not its arena slot.

Using a newtype keeps the cache HashMap key type distinct from Expr so callers cannot accidentally conflate “a key for lookup” with “a live expression value”.

Structs

ExprKey

Structural key derived from the content of one Expr node.