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camc314
6be314f226
## Summary
- Removes `VariableDeclarator::kind`.
- Makes `VariableDeclaration::kind` the single source of truth for every declarator in its declaration list.
- Makes contradictory parent and child declaration kinds unrepresentable.
## Breaking Change
The Rust AST changes from:
```rust
pub struct VariableDeclaration<'a> {
pub node_id: Cell<NodeId>,
pub span: Span,
pub kind: VariableDeclarationKind,
pub declarations: Vec<'a, VariableDeclarator<'a>>,
#[ts]
pub declare: bool,
}
pub struct VariableDeclarator<'a> {
pub node_id: Cell<NodeId>,
pub span: Span,
#[estree(skip)]
pub kind: VariableDeclarationKind,
#[estree(via = VariableDeclaratorId)]
pub id: BindingPattern<'a>,
#[ts]
#[estree(skip)]
pub type_annotation: Option<Box<'a, TSTypeAnnotation<'a>>>,
pub init: Option<Expression<'a>>,
#[ts]
pub definite: bool,
}
```
to:
```rust
pub struct VariableDeclaration<'a> {
pub node_id: Cell<NodeId>,
pub span: Span,
pub kind: VariableDeclarationKind,
pub declarations: Vec<'a, VariableDeclarator<'a>>,
#[ts]
pub declare: bool,
}
pub struct VariableDeclarator<'a> {
pub node_id: Cell<NodeId>,
pub span: Span,
#[estree(via = VariableDeclaratorId)]
pub id: BindingPattern<'a>,
#[ts]
#[estree(skip)]
pub type_annotation: Option<Box<'a, TSTypeAnnotation<'a>>>,
pub init: Option<Expression<'a>>,
#[ts]
pub definite: bool,
}
```
## Why?
A variable declaration has one declaration kind shared by every declarator in its declaration list:
```js
const first = 1, second = 2;
```
Previously, the AST stored that kind on both the parent declaration and every child declarator:
```text
VariableDeclaration {
kind: Const,
declarations: [
VariableDeclarator { kind: Const, ... },
VariableDeclarator { kind: Const, ... },
],
}
```
This allowed contradictory ASTs to be constructed:
```text
VariableDeclaration {
kind: Const,
declarations: [
VariableDeclarator { kind: Var, ... },
],
}
```
Different consumers read different copies:
- Code generation used `VariableDeclaration::kind`.
- Semantic binding used `VariableDeclarator::kind`.
- Lint rules, transforms, and minifier passes used a mixture of both.
As a result, one AST could be emitted as `const` while its binding was treated as function-scoped `var`.
Transforms also had to keep the copies synchronized manually. For example, const-to-let normalization updated the parent and then looped through every declarator to update each child. Explicit resource management performed similar paired mutations for `using` declarations.
Keeping the kind only on `VariableDeclaration` removes this synchronization requirement and prevents consumers from observing conflicting declaration semantics.
## Declaration forms
The declaration kind now belongs exclusively to the node that owns the declaration list:
```js
let value;
```
```text
VariableDeclaration {
kind: Let,
declarations: [
VariableDeclarator { id: value, ... },
],
}
```
Multiple declarators share the same parent kind:
```js
const first = 1, second = 2;
```
```text
VariableDeclaration {
kind: Const,
declarations: [
VariableDeclarator { id: first, ... },
VariableDeclarator { id: second, ... },
],
}
```
## How to migrate
Code that previously read the kind from a declarator:
```rust
if declarator.kind.is_const() {
// ...
}
```
should now read it from the containing declaration:
```rust
if declaration.kind.is_const() {
// ...
}
```
Consumers operating through an AST node store should retrieve the parent `VariableDeclaration`:
```rust
let AstKind::VariableDeclaration(declaration) =
nodes.parent_kind(declarator.node_id())
else {
unreachable!();
};
let kind = declaration.kind;
```
Common migrations:
| Previous representation | New representation |
| --- | --- |
| `declarator.kind` | Containing `declaration.kind` |
| Match `VariableDeclarator::kind` during semantic binding | Read the parent `VariableDeclaration` from ancestry |
| Pass `kind` to `VariableDeclarator::new` | Remove the `kind` argument |
| Update parent and every child kind | Update only `VariableDeclaration::kind` |
| Recover kind after detaching a declarator | Capture or pass the parent kind before detaching it |
| Inspect a symbol’s declarator kind in lint rules | Resolve its parent declaration through the node store |
When constructing declarators, use the generated kindless `VariableDeclarator` builders and place them inside a `VariableDeclaration` carrying the required kind.
fixes https://github.com/oxc-project/backlog/issues/237