ID Types

The IR uses typed IDs for referencing definitions. IDs are simple newtypes wrapping u32, making them copyable and cheap to pass around.

#![allow(unused)]
fn main() {
#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
pub struct StructId(pub u32);

#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
pub struct TraitId(pub u32);

#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
pub struct EnumId(pub u32);
}

IDs index into the corresponding Vec in IrModule. The prelude definitions come first, so the first user struct is not StructId(0). Look a definition up by name:

#![allow(unused)]
fn main() {
// Use helper method (returns Option)
if let Some(struct_def) = module.get_struct(id) {
    // use struct_def
}

// Lookup by name
if let Some(id) = module.struct_id("User") {
    if let Some(struct_def) = module.get_struct(id) {
        // use struct_def
    }
}

// Direct indexing (when ID is known valid)
let struct_def = &module.structs[id.0 as usize];
}

ID Type Safety

IDs are type-safe: you cannot accidentally use a StructId where a TraitId is expected. This prevents a common class of bugs:

#![allow(unused)]
fn main() {
let struct_id = StructId(0);
let trait_id = TraitId(0);

// Compile error: types don't match
// module.get_struct(trait_id);
}

Other typed IDs

Beyond the three definition-level IDs above, these typed IDs flow through the IR. The lowering sets FunctionId on a call and ImplId in a DispatchKind::Static. ResolveReferencesPass sets the others:

• BindingId: function-local let bindings, parameters, loop variables
• FieldIdx: index into the matching struct/enum variant's fields
• VariantIdx: index into the matching enum's variants
• MethodIdx: index into the matching impl's functions or trait's methods
• LetId: module-level let bindings
• ImplId: impl blocks
• FunctionId: index into IrModule.functions

A pass that removes or reorders definitions changes the ids of the definitions after them. Each built-in pass rewrites every id that it changes, so the ids in its result are consistent.

These appear on IrExpr, IrMatchArm, and IrBlockStatement.