Svelte 5: `$derived` state mutation in function expressions does not error at compile time

[navorite]

Describe the bug

Currently, state mutation inside derived runes is not detected at compile time if a function expression is used e.g. $dervied.by or $dervied with an IIFE. Instead, if you increase the count to 5, you'll see the error at runtime. This is not ideal.

Reproduction

REPL

Logs

No response

System Info

N/A

Severity

annoyance

[dummdidumm]

I don't think it's worth it tackling this. We will only be able to catch the simplest of cases and the additional code to detect these cases will bloat the compiler code.

[trueadm]

Yeah we definitely won't be doing this statically. It will give people a false hope that extracting logic into another function will fix their problem.