Design Rationale
The IR design follows patterns from the Rust compiler (HIR / THIR / MIR): the AST is preserved for source-level tooling, and a separate, type- resolved, ID-keyed representation is used for code generation.
Why Separate from AST?
- Clean separation: AST preserves source fidelity, IR optimises for codegen.
- No syntax noise: IR omits comments, parentheses and grouping.
It keeps the spans for debug information, and it keeps the names
of the
useimports only asIrModule.imports. - Different consumers: Linters and LSPs use AST, code generators use IR.
Why ID-Based References?
- Copyable: IDs are
Copy, no lifetime complexity. - Cheap: O(1)
Veclookup by index. - Type-safe:
StructIdcannot be used where aTraitIdis expected. - Stable: an ID does not change when a definition is appended. A pass that removes definitions renumbers the IDs after them, and rewrites each reference to them.
Why Type on Every Expression?
- No re-inference: Code generators don't need to re-derive types.
- Single source of truth: Type is computed once during lowering.
- Simpler codegen: Just read
expr.ty()and emit appropriate code.
Why Visitor Pattern?
- Selective processing: Implement only the methods you need.
- Controlled traversal: Producer decides traversal order.
- Extensible: New node types don't break existing visitors.
See Also
- AST Reference: for syntax analysis and source-level tooling
- Architecture Overview: overall compiler design
- Built-in Passes:
MonomorphisePass,ResolveReferencesPass,ClosureConversionPass,DefunctionalisePass,DeadCodeEliminationPass,ConstantFoldingPass