fix: evaluate dynamic image sources once - #16900
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Install the latest version of pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/4059e215e76868fcd216b1640e6e3836ef08c168Open in |
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teemingc
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Aug 22, 2026
teemingc
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Aug 22, 2026
| /** | ||
| * @param {Expression | Super} expression | ||
| */ | ||
| function is_reference(expression) { |
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Not sure if this needs to account for more expression types such as https://github.com/sveltejs/dts-buddy/blob/c1c53c6b93e5db189243540dcb193bf8cd2b0bd1/src/utils.js#L550
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
![vercel[bot]](https://avatars.githubusercontent.com/in/8329?s=60&v=4){kind=small}
…nst ...}` declaration tag, which throws a hard compile error ("Declaration tags cannot be used in legacy mode") in any legacy-mode component, and is also non-reactive in the cases where it does compile.
This commit fixes the issue reported at packages/enhanced-img/src/vite-plugin.js:419
## The bug
To avoid evaluating a computed `src` expression multiple times (once per template
position), `dynamic_img_to_picture` in `packages/enhanced-img/src/vite-plugin.js`
(line ~419) caches it in a Svelte **declaration tag**:
```js
`${src_expression ? `{const ${src_var_name} = ${src_expression}}\n` : ''}...`
```
Note the bare `{const}`, not `{@const}`. This has two problems, one of them fatal.
### 1. Hard compile error in legacy-mode components (primary failure)
Declaration tags (`{const ...}`) were only added in Svelte 5.56 and are **not
allowed in legacy-mode components** — the compiler throws:
> `Declaration tags cannot be used in legacy mode`
Verified against `svelte@5.56.10`, a component's mode determines the outcome:
| Component contents | Result with `{const}` |
| --- | --- |
| `export let` (legacy) | **THROW** |
| `$:` reactive statement (legacy) | **THROW** |
| `<svelte:options runes={false} />` | **THROW** |
| plain `let` only (ambiguous) | OK |
| runes (`$props`, `$state`) | OK |
So **any legacy-mode Svelte component** containing
`<enhanced:img src={dynamicExpression}>` will **fail to compile**. This is a
regression introduced by the "evaluate dynamic sources once" change (commit
`1d1bd60`), which also bumped the `svelte` peer dep from `^5.0.0` to `^5.56.0`.
Why the existing test suite missed it: the fixture `test/Input.svelte` only uses
`let foo` (ambiguous mode, no `export`/no runes), which is one of the few
configurations where `{const}` happens to compile — masking the bug.
### 2. Non-reactive even when it compiles (secondary)
In the ambiguous/runes cases where `{const}` does compile, it emits a plain
once-evaluated binding at the top of the component function. Compiling
`{const v = large ? big : small}` (where `large` is `$state`) yields:
```js
const v = $.get(large) ? big : small; // read ONCE, outside any effect
$.if(node, ($$render) => { if (typeof v === 'string') ... }); // static v
$.template_effect(() => $.set_attribute(img, 'src', v)); // static v
```
The picture never updates when the source expression depends on reactive state,
unlike the previous inline behavior.
## The fix
Emit a **reactive** `{@const}` instead. Because `{@const}` is only valid as an
immediate child of a block/component, the whole picture is wrapped in an
always-true `{#if true}` block:
```js
if (src_expression) {
return `{#if true}{@const ${src_var_name} = ${src_expression}}\n${picture}\n{/if}`;
}
return picture;
```
`{@const}` compiles to a `$.derived(...)` (reactive, memoized once-per-update) and
every downstream use becomes `$.get(v)` — restoring reactivity while still
evaluating the expression only once. Crucially, `{@const}` is valid in **both**
legacy and runes mode and has existed since **Svelte 5.0**, so it fixes the
compile error too.
Simple references (`image`, `object.image`) are still inlined
(`should_declare === false`) and are unaffected.
## Bonus: revert the peer-dep bump
Because `{@const}` works on Svelte 5.0, the `svelte` peer-dependency bump to
`^5.56.0` in `packages/enhanced-img/package.json` is no longer needed. The patch
reverts it back to `^5.0.0` (the value prior to commit `1d1bd60`).
## Tests
Updated `test/markup-plugin.spec.js` and the `test/Output.svelte` snapshot to
expect the reactive `{#if true}{@const ...}` wrapping. The `get_image(i)`
single-evaluation assertions still hold.
Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: teemingc <chewteeming01@gmail.com>
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Fixes: #12231
This PR tries to add a Svelte declaration tag if the image source is an expression that should only be computed once.
It requires a newer version of Svelte to do so, which is a a breaking change.uses the old declaration tag by wrapping it in an if block that is always true. This works in both legacy and runes modesvelte-markup-parserwith the official Svelte parser so that we can check for colliding declaration references.Please don't delete this checklist! Before submitting the PR, please make sure you do the following:
Tests
pnpm testand lint the project withpnpm lintandpnpm checkTargeted validation: enhanced-img formatting and type checks, unit tests, integration tests, and diff checks.
Changesets
pnpm changesetand following the prompts. Changesets that add features should beminorand those that fix bugs should bepatch. Please prefix changeset messages withfeat:,fix:, orchore:.Added a patch changeset for
@sveltejs/enhanced-img.Edits