HIR - Lowering (AST → HIR)

Code in src/hir/lower.rs, lower_expr.rs, and lower_stmt.rs.

Entry points: lower_func / lower_program

lower_program iterates over the top-level AST statements and calls lower_func for each StmtKind::FunctionDef / StmtKind::FiberDef, saving the result under the function index from func_indices (the same index register previously populated by globals::register_globals_recursive - see compiler/compiler/compiler_core.md). It returns a map of HirFunc indexed by function number, which is later consumed by run_inliner_pass and compile_hir_to_chunk.

lower_func creates a HirFuncBuilder, registers parameters as locals (define_local), and then lowers the body statement-by-statement via lower_stmt.

HirFuncBuilder

The equivalent of FunctionCompiler from the classic AST→bytecode compiler, but operating at the lowering stage. scopes is a stack of name → HirLocal maps, and next_local allocates subsequent indices. define_local registers the declaration in locals (later consumed by codegen for register allocation) and binds the name in the current scope; lookup_local searches scopes starting from the innermost one.

Local name resolution to indices is therefore fully handled at the lowering stage - subsequent stages (inlining, codegen) operate exclusively on numerical HirLocal values, not on names.

lower_stmt / lower_expr

lower_stmt maps each variant of AST StmtKind to its corresponding HirStmtKind, flattening constructs like MultiVarDecl (multiple variable declaration) into a sequence of single VarDecl statements. Before lowering subexpressions, it builds a flat snapshot of currently visible variables (resolved_locals) from all active scopes and passes it to lower_expr along with func_indices and globals. In this way, an expression referencing a local variable compiles directly to its HirLocal, whereas a reference to an unresolved name falls back to HirExprKind::Global.

lower_expr (lower_expr.rs) performs a similar mapping for expressions, including resolving the resulting type saved in HirExpr::ty - which is required by the codegen phase to generate correct opcodes without re-analyzing types (see hir_codegen.md).