[compiler][bugfix] Bail out when a memo block declares hoisted fns #32765
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josephsavona
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Apr 30, 2025
mofeiZ
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Apr 30, 2025
Fixes an edge case in React Compiler's effects inference model. Returned values should only be typed as 'frozen' if they are (1) local and (2) not a function expression which may capture and mutate this function's outer context. See test fixtures for details --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33047). * #32765 * #32747 * __->__ #33047
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Fixes an edge case in React Compiler's effects inference model. Returned values should only be typed as 'frozen' if they are (1) local and (2) not a function expression which may capture and mutate this function's outer context. See test fixtures for details --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33047). * #32765 * #32747 * __->__ #33047 DiffTrain build for [12f4cb8](12f4cb8)
Note that bailing out adds false positives for hoisted functions whose only references are within other functions. For example, this rewrite would be safe.
```js
// source program
function foo() {
return bar();
}
function bar() {
return 42;
}
// compiler output
let bar;
if (/* deps changed */) {
function foo() {
return bar();
}
bar = function bar() {
return 42;
}
}
```
These false positives are difficult to detect because any maybe-call of foo before the definition of bar would be invalid.
Instead of bailing out, we should rewrite hoisted function declarations to the following form.
```js
let bar$0;
if (/* deps changed */) {
// All references within the declaring memo block
// or before the function declaration should use
// the original identifier `bar`
function foo() {
return bar();
}
function bar() {
return 42;
}
bar$0 = bar;
}
// All references after the declaring memo block
// or after the function declaration should use
// the rewritten declaration `bar$0`
```
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…32765) Note that bailing out adds false positives for hoisted functions whose only references are within other functions. For example, this rewrite would be safe. ```js // source program function foo() { return bar(); } function bar() { return 42; } // compiler output let bar; if (/* deps changed */) { function foo() { return bar(); } bar = function bar() { return 42; } } ``` These false positives are difficult to detect because any maybe-call of foo before the definition of bar would be invalid. Instead of bailing out, we should rewrite hoisted function declarations to the following form. ```js let bar$0; if (/* deps changed */) { // All references within the declaring memo block // or before the function declaration should use // the original identifier `bar` function foo() { return bar(); } function bar() { return 42; } bar$0 = bar; } // All references after the declaring memo block // or after the function declaration should use // the rewritten declaration `bar$0` ``` DiffTrain build for [0c1575c](0c1575c)
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…32765) Note that bailing out adds false positives for hoisted functions whose only references are within other functions. For example, this rewrite would be safe. ```js // source program function foo() { return bar(); } function bar() { return 42; } // compiler output let bar; if (/* deps changed */) { function foo() { return bar(); } bar = function bar() { return 42; } } ``` These false positives are difficult to detect because any maybe-call of foo before the definition of bar would be invalid. Instead of bailing out, we should rewrite hoisted function declarations to the following form. ```js let bar$0; if (/* deps changed */) { // All references within the declaring memo block // or before the function declaration should use // the original identifier `bar` function foo() { return bar(); } function bar() { return 42; } bar$0 = bar; } // All references after the declaring memo block // or after the function declaration should use // the rewritten declaration `bar$0` ``` DiffTrain build for [0c1575c](0c1575c)
This was referenced May 5, 2025
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Note that bailing out adds false positives for hoisted functions whose only references are within other functions. For example, this rewrite would be safe.
These false positives are difficult to detect because any maybe-call of foo before the definition of bar would be invalid.
Instead of bailing out, we should rewrite hoisted function declarations to the following form.