# swc architecture

This document gives a high level overview of swc internals. You may find it useful if you want to contribute to swc or if you are interested in the inner workings of swc.

## Macros

See [blog post about swc macros](https://swc.rs/blog/2020/01/04/pmutil#macros-built-with-pmutil).

swc uses proc macro extensively to reduce work. Please see links below to know what each macro do.

- [enum_kind][]
- [string_enum][]
- [ast_node][]

And some adhoc-macros are used.

- [parser_macros][]
- [codegen_macros][]

These macro breaks macro hygiene.

## Structure

### `/atoms`

Handle string interning for the swc project. The crate depends on [string_cache](https://github.com/servo/string-cache) from servo.

### `/common`

Contains code related to span, hygiene and error reporting.

Also, it contains / re-exports codes for visitor pattern. `Visit<T>` is non-mutating visitor, while `Fold<T>` is a mutating visitor.

### `/ecmascript/ast`

Contains ast nodes for javascript and script.

### `/ecmascript/codegen`

Converts javascript ast into javascript code.

### `/ecmascript/parser`

Parses javascript and typescript

### `/ecmascript/transforms`

Theres are three core transforms named `resolver`, `hygiene`, `fixer`. Other transforms depends on them.

#### `/ecmascript/transforms/src/resolver`

This pass resolves and marks all identifiers in the file.

e.g.

```js
let a = 1;
{
  let a = 1;
}
```

becomes

```js
let a#0 = 1;
{
  let a#1 = 1;
}
```

where number after `#` denotes the hygiene id. If two identifiers have same symbol but different hygiene id, it's different.

#### `/ecmascript/transforms/src/hygiene`

Hygiene pass actually changes symbol of identifiers with same symbol but different hygiene id.

```js
let a#0 = 1;
{
  let a#1 = 2;
}
```

becomes

```js
let a = 1;
{
  let a1 = 2;
}
```

#### `/ecmascript/transforms/src/fixer`

Fixes borken ast. This allow us to simply fold types like `BinExpr` without caring about operator precedence.

It means,

```rust
let v = BinExpr {
    left: "1 + 2",
    op: "*",
    right: "3",
};
```

(other passes generates ast like this)

is converted into

```rust
let v = BinExpr {
    left: "(1 + 2)",
    op: "*",
    right: "3",
};
```

and printed as

```js
(1 + 2) * 3;
```

#### `/ecmascript/transforms/src/compat`

Contains codes related to converting new generation javascript codes for old browsers.

#### `/ecmascript/transforms/src/modules`

Contains code related to transforming es6 modules to other modules.

#### `/ecmascript/transforms/src/optimization`

Contains code related to making code faster on runtime. Currently only small set of optimization is implemented.

## Tests

swc uses [official ecmascript conformance test suite called test262][test262] for testing.

Parser tests ensures that parsed result of test262/pass is identical with test262/pass-explicit.

Codegen tests ensures that generated code is equivalent to goldened reference files located at [tests/references](ecmascript/codegen/tests/references).

[enum_kind]: https://swc.rs/rustdoc/enum_kind/derive.Kind.html
[string_enum]: https://swc.rs/rustdoc/string_enum/derive.StringEnum.html
[ast_node]: https://swc.rs/rustdoc/ast_node/index.html
[parser_macros]: https://swc.rs/rustdoc/swc_ecma_parser_macros/index.html
[codegen_macros]: https://swc.rs/rustdoc/swc_ecma_codegen_macros/index.html
[test262]: https://github.com/tc39/test262