`$effect.pre` isn't re-run after mutating state in a nested `$effect.pre`

[wjainek]

Describe the bug

When nesting two $effect.pres, where the outer one depends on a value the inner one mutates, the outer effect isn't re-run. This is inconsistent with the simpler case where mutation happens directly in the outer effect, or when using $effect instead of $effect.pre.

Reproduction
The following code demonstrates the issue:

let value = $state(0);

$effect.pre(() => {
    console.log("Outer");
    value; // outer effect is made dependent on value

    $effect.pre(() => {
        console.log("Inner");
        value = 10; // inner effect mutates value
    });
});

// Expected output: Outer, Inner, Outer, Inner
// Actual output:   Outer, Inner

Expected behavior
Accessing the value in the outer effect establishes a dependency. After mutating the value in the inner effect, the outer effect should re-run. (And hence, a new inner effect should be created). The expected output is: Outer, Inner, Outer, Inner.

Actual behavior
The outer effect is not re-run after the value changes. Hence we get the output Outer, Inner.

Comparison case 1
When mutating the value directly in the outer effect, it is correctly re-run:

let value = $state(0);

$effect.pre(() => {
    console.log("Outer");
    value;      // outer effect is made dependent on value
    value = 10; // outer effect mutates value
});

// Expected output: Outer, Outer
// Actual output:   Outer, Outer

Comparison case 2
When replacing the inner $effect.pre with $effect, the outer effect is correctly re-run:

let value = $state(0);

$effect.pre(() => {
    console.log("Outer");
    value; // outer effect is made dependent on value

    $effect(() => {
        console.log("Inner");
        value = 10; // inner effect mutates value
    });
});

// Expected output: Outer, Inner, Outer, Inner
// Actual output:   Outer, Inner, Outer, Inner

Reproduction

https://svelte.dev/playground/4fb0fdc4fad240b99a06501f6c80df24?version=5.23.0

Logs

System Info

System:
    OS: macOS 15.3.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 15.04 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.9.0 - /opt/homebrew/bin/node
    npm: 11.2.0 - /opt/homebrew/bin/npm
  Browsers:
    Chrome: 134.0.6998.88
    Safari: 18.3
  npmPackages:
    svelte: ^5.15.0 => 5.23.0

Severity

annoyance