HIR Lowering

Status: Complete (1D) Crate: compiler/hir-lowering Entry Point: pub fn lower_program(items: &[Item]) -> Result<HirProgram, LoweringError>

Overview

The HIR lowering slice turns a type-checked surface AST into the typed High-Level IR (neuro-hir), the backend-agnostic contract every backend lowers from. Its defining job is to attach a fully resolved type to every expression: the frontend type checker validates types but does not expose them, so the lowerer re-derives each expression's type while walking the AST.

neurc runs lowering immediately after semantic_analysis::type_check in both check and compile. The output feeds the LLVM backend (and the MLIR backend under the mlir feature).

Architecture

• Dependencies: ast-types (read-only AST traversal), neuro-hir (output node set), shared-types, thiserror (LoweringError). syntax-parsing is a dev-dependency only (tests build ASTs through the parser), never a production cross-slice dependency.
• Public API: single lower_program entry point + LoweringError.
• No semantic coupling: lowering re-derives types rather than importing semantic_analysis::Type, importing it would couple two feature slices, which VSA forbids (duplicate over couple).

Behavior

Lowering assumes well-typedness, it computes types, it does not validate them. A shape the checker should have rejected surfaces as a LoweringError, never a panic.

A registration pre-pass mirrors the checker's: struct field tables (plus @derive(Copy/Clone) intent), impl method signatures under mangled Struct__method keys, free-function signatures, and module constants. Bodies then lower under a lexical scope stack and a loop-context stack.

Two type derivations are contextual, faithfully mirroring the checker:

• Literals take a suffix type, else the expected type when it fits the literal's family, else the default i32 / f64.
• A function/method body's trailing expression is an implicit return, typed against the declared return type; nested block / if-arm tails are typed with no hint.

Three nodes carry a deliberately-chosen type the source has no first-class form for:

• a loop value-expression takes its break v type (or void);
• a method-name callee (FieldAccess) carries the call's result type (there is no method value);
• a Range carries void (valid only as a string.slice / string.char_slice argument, whose lowering reads its bounds directly).

Divergent panic-family calls (panic / assert / unreachable) adopt their context's expected type, or void in statement position. The AST's Expr::Paren grouping node is dropped, tree structure already encodes grouping.

Testing

Slice unit tests cover the lowering rules; neurc/tests/hir_lowering.rs provides end-to-end coverage. The workspace architecture test enforces the slice's infrastructure-only dependencies.

Resources

• neuro-hir CONTEXT, the HIR node set
• hir-lowering CONTEXT, slice contract