Allow context values to be mutated in callbacks

Adds a new type for representing context values, which is transitive. So 
`useContext(a).b.c` also gets inferred as a context type. This allows us to 
refine our inference, and allow passing callbacks that modify context where a 
"frozen" lambda is exepcted.

compiler/packages/babel-plugin-react-forget/src/HIR/Globals.ts

@@ -9,6 +9,7 @@ import { Effect, ValueKind } from "./HIR";
 import {
   BUILTIN_SHAPES,
   BuiltInArrayId,
+  BuiltInContextId,
   BuiltInUseRefId,
   BuiltInUseStateId,
   ShapeRegistry,
@@ -239,7 +240,7 @@ const BUILTIN_HOOKS: Array<[string, FunctionType]> = [
     addHook(DEFAULT_SHAPES, [], {
       positionalParams: [],
       restParam: Effect.Read,
-      returnType: { kind: "Poly" },
+      returnType: { kind: "Object", shapeId: BuiltInContextId },
       calleeEffect: Effect.Read,
       hookKind: "useContext",
       returnValueKind: ValueKind.Mutable,

compiler/packages/babel-plugin-react-forget/src/HIR/HIR.ts

@@ -1111,6 +1111,10 @@ export function isSetStateType(id: Identifier): boolean {
   return id.type.kind === "Function" && id.type.shapeId === "BuiltInSetState";
 }
 
+export function isContextType(id: Identifier): boolean {
+  return id.type.kind === "Object" && id.type.shapeId === "BuiltInContext";
+}
+
 export function getHookKind(env: Environment, id: Identifier): HookKind | null {
   const idType = id.type;
   if (idType.kind === "Function") {

compiler/packages/babel-plugin-react-forget/src/HIR/ObjectShape.ts

@@ -170,6 +170,7 @@ export const BuiltInUseStateId = "BuiltInUseState";
 export const BuiltInSetStateId = "BuiltInSetState";
 export const BuiltInUseRefId = "BuiltInUseRefId";
 export const BuiltInRefValueId = "BuiltInRefValue";
+export const BuiltInContextId = "BuiltInContext";
 export const BuiltInMixedReadonlyId = "BuiltInMixedReadonly";
 
 /**
@@ -293,6 +294,10 @@ addObject(BUILTIN_SHAPES, BuiltInUseRefId, [
   ["current", { kind: "Object", shapeId: BuiltInRefValueId }],
 ]);
 
+addObject(BUILTIN_SHAPES, BuiltInContextId, [
+  ["*", { kind: "Object", shapeId: BuiltInContextId }],
+]);
+
 addObject(BUILTIN_SHAPES, BuiltInRefValueId, []);
 
 addObject(BUILTIN_SHAPES, BuiltInMixedReadonlyId, [

compiler/packages/babel-plugin-react-forget/src/Inference/AnalyseFunctions.ts

@@ -10,6 +10,7 @@ import {
   Effect,
   HIRFunction,
   Identifier,
+  isContextType,
   isRefValueType,
   isSetStateType,
   isUseRefType,
@@ -142,7 +143,8 @@ function infer(
     if (
       isUseRefType(dep.identifier) ||
       isRefValueType(dep.identifier) ||
-      isSetStateType(dep.identifier)
+      isSetStateType(dep.identifier) ||
+      isContextType(dep.identifier)
     ) {
       // TODO: this is a hack to ensure we treat functions which reference refs
       // as having a capture and therefore being considered mutable. this ensures

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/useContext-mutable-value.expect.md

@@ -0,0 +1,48 @@
+
+## Input
+
+```javascript
+// @debug
+function Component(props) {
+  const FooContext = useContext(Foo);
+  const onClick = () => {
+    FooContext.current = true;
+  };
+  return <div onClick={onClick} />;
+}
+
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react"; // @debug
+function Component(props) {
+  const $ = useMemoCache(4);
+  const FooContext = useContext(Foo);
+  const c_0 = $[0] !== FooContext.current;
+  let t0;
+  if (c_0) {
+    t0 = () => {
+      FooContext.current = true;
+    };
+    $[0] = FooContext.current;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const onClick = t0;
+  const c_2 = $[2] !== onClick;
+  let t1;
+  if (c_2) {
+    t1 = <div onClick={onClick} />;
+    $[2] = onClick;
+    $[3] = t1;
+  } else {
+    t1 = $[3];
+  }
+  return t1;
+}
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/useContext-mutable-value.js

@@ -0,0 +1,8 @@
+// @debug
+function Component(props) {
+  const FooContext = useContext(Foo);
+  const onClick = () => {
+    FooContext.current = true;
+  };
+  return <div onClick={onClick} />;
+}

compiler/packages/sprout/src/SproutTodoFilter.ts

@@ -460,6 +460,7 @@ const skipFilter = new Set([
   "fbtparam-text-must-use-expression-container",
   "fbtparam-with-jsx-fragment-value",
   "fbt-preserve-jsxtext",
+  "useContext-mutable-value",
 ]);
 
 export default skipFilter;