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# 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
<!-- TODO: fix link -->
<!-- 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.
- [string_enum][]
- [ast_node][]
And some adhoc-macros are used.
- [parser_macros][]
- [codegen_macros][]
These macro breaks macro hygiene.
## Structure
### [`/crates/swc_atoms`](crates/swc_atoms)
Handle string interning for the SWC project. The crate depends on [hstr](https://github.com/dudykr/ddbase).
### [`/crates/swc_common`](crates/swc_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.
### [`/crates/swc_ecma_ast`](crates/swc_ecma_ast)
Contains AST nodes for javascript and typescript.
### [`/crates/swc_ecma_codegen`](crates/swc_ecma_codegen)
Converts javascript AST into javascript code.
### [`/crates/swc_ecma_parser`](crates/swc_ecma_parser)
Parses javascript and typescript
### [`/crates/swc_ecma_transforms_base`](crates/swc_ecma_transforms_base)
There are three core transforms named `resolver`, `hygiene`, `fixer`. Other transforms depend on them.
#### [`/crates/swc_ecma_transforms_base/src/resolver`](crates/swc_ecma_transforms_base/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 the number after the hash (`#`) denotes the hygiene id. If two identifiers have the same symbol but different hygiene ids, they are considered different.
#### [`/crates/swc_ecma_transforms_base/src/hygiene`](crates/swc_ecma_transforms_base/src/hygiene)
The hygiene pass actually changes symbols of identifiers with the same symbol but different hygiene ids to different symbols.
```js
let a#0 = 1;
{
let a#1 = 2;
}
```
becomes
```js
let a = 1;
{
let a1 = 2;
}
```
#### [`/crates/swc_ecma_transforms_base/src/fixer`](crates/swc_ecma_transforms_base/src/fixer)
Fixes broken 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 generate AST like this)
is converted into
```rust
let v = BinExpr {
left: "(1 + 2)",
op: "*",
right: "3",
};
```
and printed as
```js
(1 + 2) * 3;
```
<!-- TODO: add correct references to files -->
<!-- #### `/ecmascript/transforms/src/compat`
Contains code related to converting new generation javascript code into code understood by 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 a small set of optimizations is implemented. -->
## Tests
SWC uses the [official ecmascript conformance test suite called test262][test262] for testing.
Parser tests ensure that the parsed results of `test262/pass` are identical with `test262/pass-explicit`.
Codegen tests ensure that the generated code is equivalent to the golden fixture files located at [tests/references](crates/swc_ecma_codegen/tests).
[string_enum]: https://rustdoc.swc.rs/string_enum/derive.StringEnum.html
[ast_node]: https://rustdoc.swc.rs/ast_node/index.html
[parser_macros]: https://rustdoc.swc.rs/swc_ecma_parser_macros/index.html
[codegen_macros]: https://rustdoc.swc.rs/swc_ecma_codegen_macros/index.html
[test262]: https://github.com/tc39/test262
|
