HIR

Typed tree lowering, binding ids, desugaring, closure captures, and why HIR exists.

Purpose

HIR removes reader-friendly surface syntax before the compiler starts doing backend work.

HIR is still a tree, but it is a typed tree with stable binding identities and less syntactic sugar. That makes it the right layer for semantic normalization before MIR turns control flow into blocks.

Every expression carries a type.
Local uses point to binding ids instead of names.
Surface sugar is converted into smaller core forms.
Closure captures are explicit.

Binding ids

Names are resolved once, then later passes use ids instead of redoing scope lookup.

Each parameter, let binding, closure parameter, and pattern binding receives an id.
Shadowing becomes harmless because two bindings with the same spelling still have different ids.
Global names and builtins can stay name-based where they are not local bindings.

Desugarings

HIR makes convenient syntax explicit before MIR sees it.

F-strings become format primitives and concatenation.
?? and ?. become matches over absence.
? becomes a match with early return.
Compound assignment becomes an ordinary assignment.
while let becomes a loop containing a match.

Closure captures

Closures list what they capture and whether each capture is by reference or by value.

Capture analysis is id-based, so nested scopes and shadowing do not require later passes to reason about names. A non-move closure captures by reference; a move closure captures owned values by value.