Mofei Zhang
241a615732
identifiers
---
A few fixes for finding context identifiers:
Previously, we counted every babel identifier as a reference. This is
problematic because babel counts every string symbol as an identifier.
```js
print(x); // x is an identifier as expected
obj.x // x is.. also an identifier here
{x: 2} // x is also an identifier here
```
This PR adds a check for `isReferencedIdentifier`. Note that only non-lval
references pass this check
```js
print(x); // isReferencedIdentifier(x) -> true
obj.x // isReferencedIdentifier(x) -> false
{x: 2} // isReferencedIdentifier(x) -> false
x = 2 // isReferencedIdentifier(x) -> false
```
Which brings us to change #2.
Previously, we counted assignments as references due to the identifier visiting
+ checking logic. The logic was roughly the following (from #1691)
```js
contextVars = intersection(reassigned, referencedByInnerFn);
```
Now that assignments (lvals) and references (rvals) are tracked separately, the
equivalent logic is this. Note that assignment to a context variable does not
need to be modeled as a read (`console.log(x = 5)` always will evaluates and
prints 5, regardless of the previous value of x).
```
contextVars = union(reassignedByInnerFn, intersection(reassigned,
referencedByInnerFn))
```
---
Note that variables that are never read do not need to be modeled as context
variables, but this is unlikely to be a common pattern.
```js
function fn() {
let x = 2;
const inner = () => {
x = 3;
}
}
```
React Forget
React Forget is an experimental Babel plugin to automatically memoize React Hooks and Components.
Development
# tsc --watch
$ yarn dev
# in another terminal window
$ yarn test --watch
Notes
An overview of the implementation can be found in the Architecture Overview.
This transform
- needs plugin-syntax-jsx as a dependency to inherit the syntax from.
- should be run before plugin-transform-react-jsx
- assume the enforcement of rules of hooks, i.e.
- only call hooks from React functions
- only call hooks at the top level
- https://www.npmjs.com/package/eslint-plugin-react-hooks
Scaffolding
- https://github.com/facebook/flow/tree/master/packages/babel-plugin-transform-flow-enums
- https://github.com/babel/babel/blob/main/packages/babel-plugin-transform-react-jsx/src/create-plugin.ts
Reference
Rust Development
First-Time Setup
- Install Rust using
rustup. See the guide at https://www.rust-lang.org/tools/install. - Install Visual Studio Code from https://code.visualstudio.com/. Note to Meta employees: install the stock version from that website, not the pre-installed version.
- Install the Rust Analyzer VSCode extension through the VSCode marketplace. See instructions at https://rust-analyzer.github.io/manual.html#vs-code.
- Install
cargo editwhich extends cargo with commands to manage dependencies. See https://github.com/killercup/cargo-edit#installation - Install
cargo instawhich extens cargo with a command to manage snapshots. See https://insta.rs/docs/cli/
Workspace Hygiene
Adding Dependencies
To add a dependency, add it to the top-level Cargo.toml
// Cargo.toml
[workspace.dependencies]
...
new_dep = { version = "x.y.z" }
...
Then reference it from your crate as follows:
// crates/forget_foo/Cargo.toml
[dependencies]
...
new_dep = { workspace = true }
...
Adding new crates
Rust's compilation strategy is largely based on parallelizing at the granularity of crates, so builds can be faster when projects have more but smaller crates. Where possible it helps to structure crates to minimize dependencies. For example, our various compiler passes depend on each other in the sense that they often must run in a certain order. However, they often don't need to call each other, so they can generally be split into crates of similar types of passes, so that those crates can compile in parallel.
As a rule of thumb, add crates at roughly the granularity of our existing top-level folds. If you have some one-off utility code that
doesn't fit neatly in a crate, add it to forget_utils rather than add a one-off crate for it.
Running Tests
Run all tests with the following from the root directory:
cargo test
The majority of our tests will (should) live in the forget_fixtures crate, which is a test-only crate that runs compilation end-to-end with snapshot
tests. To run just these tests use:
# quiet version
cargo test -p forget_fixtures
# without suppressing stdout/stderr output
cargo test -p forget_fixtures -- --nocapture
Another hint is that VSCode will show a "Run test" option if you hover over a test in the source code, this lets you run a single test easily. The command line will also give you the CLI command to run just that one test.
Updating Snapshots
The above tests make frequent use of snapshot tests. If snapshots do not match the tests will fail with a diff, if the new output is correct you can accept the changes with:
cargo insta accept
If this command fails, see the note in "first-time setup" about installing cargo insta.
CI Configuration
GitHub CI is configured in .github/workflows/rust.yml.