$state looses reactivity with destructuring

[adamjkb]

Describe the bug

Using destructuring assignment when $state rune is returned in a getter.

- const counter = createCounter();
+ let { count, increment } = createCounter();

The above result in different behaviour in template and $effect as well.

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE32R0U7DMAxFf8WKkLaJqoPXsiGh8c4HMB5G5o6M1qkSF4Gq_DtOunaMTXuKYvueOPd2qjQVelW8doo2NapCPTWNyhT_NPHiv7BilLu3rdOxsvDamYYf17RmUzfWMXSgHW4YV7YlRgcBSmdrmORz3VfyHpPv_eRBdKLUljzDoQ3LU8B0Fqe4woSOtQwMyUiNxEK_PL6mxfy4Gz2jZ9dqbh1uC5Dme8tsCSzpyujPZTcSQ_rLKlY9dOm9EGG9IMEWH07OF2d2hjZVcYE2_PQaVdpndLG2tltTGtyqQhbGkI1JnLp3DGXv_waC3ymEsiXNRlb65w10g5cJJ-bdeJaB6d3skMWoHHcfVDGnqLldwn3yOPQKh2IrDTO7gX2UjSOp3sfDIevP8ZlElGY4N-Et_ALR2HogmgIAAA==

Logs

No response

System Info

System:
    OS: macOS 14.5
    CPU: (12) arm64 Apple M3 Pro
    Memory: 127.70 MB / 18.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.12.1 - ~/.nvm/versions/node/v20.12.1/bin/node
    npm: 10.5.0 - ~/.nvm/versions/node/v20.12.1/bin/npm
    pnpm: 9.1.2 - /opt/homebrew/bin/pnpm
  Browsers:
    Brave Browser: 125.1.66.118
    Safari: 17.5

Severity

annoyance

[brunnerh]

This is expected, you need derived to make the variables reactive:

let { count, increment } = $derived(createCounter());

[timootten]

Hey, why is this expected?

It seems kind of strange to me that it doesn't work straight away:
let { count, increment } = createCounter();

[brunnerh]

count is a primitive value, without any further added logic, the destructuring will cause it to be read (and thus copied) once and never change.

There is no good way for Svelte to know that it needs to add that extra logic here to keep updating the value.
Treating everything as potentially changing could cause other issues and adds overhead in computational cost and bundle sizes.