`$derived` and `$derived.by` in `{@const}` tags results in obscure error message

[theonlypwner]

Describe the bug

There is an error when using the $derived rune within a snippet.

Reproduction

https://svelte.dev/playground/hello-world?version=5.18.0#H4sIAAAAAAAACm2OTQ6CMBBGr1JHEyAhErYECe68g7gAOoYmdWjaETVN726qsnP7vp88D9TfECo4odazeMxWS5GiVIwygxyuSqOD6uyBXyb2IoB8XR2N2bsFNUc29A7_8XEmRmIHFfitI2UMsnBpn4WOOvbtOJNjMYiD2Em0akG5ZvVUNl8xP4RNXUxl05Evfh-xQr61SBKtcGnysU-yADkwPhkqtncMl_AGCs9xquUAAAA=

{#snippet s(a)}
	{@const b = $derived(a)}
	<h1>Hello {b}!</h1>
{/snippet}

{@render s('world')}

https://svelte.dev/playground/hello-world?version=5.18.0#H4sIAAAAAAAACm2PwQrCMBBEf2VdBVMoLV6LlnrzH6yHtlmxELchWasl5N-lWm9eH2-GmYDc3AkLPJExAzwHZzQo0r2QTjDFa2_IY3EOKJOdvRlg-ksdrc38SEZm1jae_vFuYCEWjwWGtefeWhLwqklizbWEqhvYC7RwgI0m14-ks3ZSKoFDCYu0v-3K78LQxtU-v-3KmkO-lM0Kh8oRa3Lg1fZzY5tETFHoJViIe1C8xDciFl9H7gAAAA==

{#snippet s(a)}
	{@const b = $derived.by(() => a)}
	<h1>Hello {b}!</h1>
{/snippet}

{@render s('world')}

Logs

rune_outside_svelte The `$derived` rune is only available inside `.svelte` and `.svelte.js/ts` files https://svelte.dev/e/rune_outside_svelte

System Info

Tested in Svelte REPL at `2025-01-17T02:55Z`

Severity

annoyance

[theonlypwner]

It seems to be broken even if the @const isn't in a #snippet block.

https://svelte.dev/playground/hello-world?version=5.18.0#H4sIAAAAAAAACm2OQQrCMBAAvxJXoS0US6-hFr35B-OhNlsaSDelWasS8neJ4s3rMAMTgLoJQcIZrXXi4RarRY7aMOoCShiMRQ_yEoBfc_ISgPJXneZ571e0nNit8_iP944YiT1ICFsziDoqCsfekWeRfHEQO42LWVHn2ecgK6KiZqzb71VIVtw01Vi3ikJlhgglMD4ZJC93jNf4BmSc1THIAAAA

{#if 1}
{@const name = $derived('world')}
<h1>Hello {name}!</h1>
{/if}

https://svelte.dev/playground/hello-world?version=5.18.0#H4sIAAAAAAAACm2O0QqCMBRAf-V2C1QQxddhUm_9Q_ag7oqDtYm7WTL277Git14P58DxaLo7ocALaW3haRctISWpmGSGOY5Kk0Nx9cjbHL0IMP9V53ku3EqaI-s7R__4YA2TYYcC_V6NUIXW-NNgjWOIPhzhIGlRK8mi39I0g2MDyWclyUJr6qlqvns-6mFXl1PVtMaXagyYI9OLUfDyoHALb87xtfjRAAAA

{#if 1}
{@const name = $derived.by(() => 'world')}
<h1>Hello {name}!</h1>
{/if}

[theonlypwner]

I found a workaround: remove $derived and replace $derived.by with an immediately invoked function expression.

I think the error message could be improved, since the code is actually in a .svelte file.

[brunnerh]

You don't need $derived in the template at all, @const already is a derived value.

E.g. this:

{@const b = a * 2}

Compiles to:

const b = $.derived_safe_equal(() => a() * 2);

[dummdidumm]

We're not even transforming the $derived in that position, hence the runtime error; this should become a compiler error to make it clear you cannot use it there