Svelte 5: regression, read-only props make optimistic updates impossible

[Thiagolino8]

Describe the bug

Mutating the prop data has always been the way to make optimistic updates with sveltekit, being the way recommended by Rich himself
But because props have become read-only this is no longer possible whether using the v3/4 or V5 syntax, which will certainly break many existing apps

Reproduction

V4 compiler
V5 compiler

Logs

Error: Non-bound props cannot be mutated — to make the `todos` settable, ensure the object it is used within is bound as a prop `bind:<prop>={...}`. Fallback values can never be mutated.

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.18.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 9.4.2 - /usr/local/bin/npm
    pnpm: 8.10.5 - /usr/local/bin/pnpm
  npmPackages:
    svelte: next => 5.0.0-next.28

Severity

blocking an upgrade

[elliott-with-the-longest-name-on-github]

This is probably more of a Kit issue than a Svelte 5 issue; I bet Kit could just add bind:data= instead of plain data= to fix the issue.