From e723a208f494b7fc40411da2d0e510f7282e1997 Mon Sep 17 00:00:00 2001
From: Joe Savona <joesavona@fb.com>
Date: Thu, 5 Jun 2025 22:12:23 -0700
Subject: [PATCH] Update base for Update on "[compiler] Infer alias effects for
 function expressions"
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This is a stab at addressing a pattern that mofeiz and I have both stumbled across. Today, FunctionExpression's context list describes values from the outer context that are accessed in the function, and with what effect they were accessed. This allows us to describe the fact that a value from the outer context is known to be mutated inside a function expression, or is known to be captured (aliased) into some other value in the function expression. However, the basic `Effect` kind is insufficient to describe the full semantics. Notably, it doesn't let us describe more complex aliasing relationships.

From an example mofeiz added:

```js
const x = {};
const y = {};
const f = () => {
  const a = [y];
  const b = x;
  // this sets y.x = x
  a[0].x = b;
}
f();
mutate(y.x);  // which means this mutates x!
```

Here, the Effect on the context operands are `[mutate y, read x]`. The `mutate y` is bc of the array push. But the `read x` is surprising — `x` is captured into `y`, but there is no subsequent mutation of y or x, so we consider this a read. But as the comments indicate, the final line mutates x! We need to reflect the fact that even though x isn't mutated inside the function, it is aliased into y, such that if y is subsequently mutated that this should count as a mutation of x too.

The idea of this PR is to extend the FunctionEffect type with a CaptureEffect variant which lists out the aliasing groups that occur inside the function expression. This allows us to bubble up the results of alias analysis from inside a function. The idea is to:

* Return the alias sets from InferMutableRanges
* Augment them with capturing of the form above, handling cases such as the `a[0].x = b`
* For each alias group, record a CaptureEffect for any group that contains 2+ context operands
* Extend the alias sets in the _outer_ function with the CaptureEffect sets from FunctionExpression/ObjectMethod instructions.

More details as comments on code

[ghstack-poisoned]