[v6] deprecate import.meta.env.ASSETS_PREFIX (#12468)

Co-authored-by: Sarah Rainsberger <5098874+sarah11918@users.noreply.github.com>

Author: florian-lefebvre

src/content/docs/en/guides/environment-variables.mdx

@@ -58,7 +58,6 @@ Astro includes a few environment variables out of the box:
 - `import.meta.env.DEV`: `true` if your site is running in development; `false` otherwise. Always the opposite of `import.meta.env.PROD`.
 - `import.meta.env.BASE_URL`: The base URL your site is being served from. This is determined by the [`base` config option](/en/reference/configuration-reference/#base).
 - `import.meta.env.SITE`: This is set to [the `site` option](/en/reference/configuration-reference/#site) specified in your project's `astro.config`.
-- `import.meta.env.ASSETS_PREFIX`: The prefix for Astro-generated asset links if the [`build.assetsPrefix` config option](/en/reference/configuration-reference/#buildassetsprefix) is set. This can be used to create asset links not handled by Astro.
 
 Use them like any other environment variable.
 

src/content/docs/en/guides/upgrade-to/v6.mdx

@@ -105,7 +105,7 @@ The following deprecated features are no longer supported and are no longer docu
 
 Some deprecated features may temporarily continue to function until they are completely removed. Others may silently have no effect, or throw an error prompting you to update your code.
 
-### `Astro` in `getStaticPaths()`
+### Deprecated: `Astro` in `getStaticPaths()`
 
 <SourcePR number="14432" title="feat: deprecate Astro in getStaticPaths"/>
 
@@ -131,6 +131,28 @@ export async function getStaticPaths() {
 
 <ReadMore>Read more about [built-in environment variables such as `import.meta.env.SITE`](/en/guides/environment-variables/#default-environment-variables) that are accessible when [using `getStaticPaths()` to dynamically generate static routes](/en/guides/routing/#static-ssg-mode).</ReadMore>
 
+### Deprecated: `import.meta.env.ASSETS_PREFIX`
+
+<SourcePR number="14461" title="feat: deprecate import.meta.env.ASSETS_PREFIX"/>
+
+In Astro 5.x, it was possible to access `build.assetsPrefix` in your Astro config via the built-in environment variable `import.meta.env.ASSETS_PREFIX`. However, Astro v5.7.0 introduced the `astro:config` virtual model to expose a non-exhaustive, serializable, type-safe version of the Astro configuration which included access to `build.assetsPrefix` directly. This became the preferred way to access the prefix for Astro-generated asset links when set, although the environment variable still existed.
+
+Astro 6.0 deprecates this variable in favor of `build.assetsPrefix` from the `astro:config/server` module.
+
+#### What should I do?
+
+Replace any occurances of `import.meta.env.ASSETS_PREFIX` with the `build.assetsPrefix` import from `astro:config/server`. This is a drop-in replacement to provide the existing value, and no other changes to your code should be necessary:
+
+```ts del={4} ins={2,5}
+import { someLogic } from "./utils"
+import { build } from "astro:config/server"
+
+someLogic(import.meta.env.ASSETS_PREFIX)
+someLogic(build.assetsPrefix)
+```
+
+<ReadMore>Read more about the [`astro:config` virtual modules](/en/reference/modules/astro-config/).</ReadMore>
+
 ## Removed
 
 The following features have now been entirely removed from the code base and can no longer be used. Some of these features may have continued to work in your project even after deprecation. Others may have silently had no effect.