ARCHITECTURE.md

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.

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 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.

let a = 1;
{
  let a = 1;
}

becomes

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.

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

becomes

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,

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

(other passes generates ast like this)

is converted into

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

and printed as

(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 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.