feat: deprecate import.meta.env.ASSETS_PREFIX (#14461)

Co-authored-by: Matt Kane <m@mk.gg>

Author: florian-lefebvre

.changeset/green-garlics-heal.md

@@ -0,0 +1,21 @@
+---
+'astro': major
+---
+
+Deprecates `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:
+
+```diff
+import { someLogic } from "./utils"
++import { build } from "astro:config/server"
+
+-someLogic(import.meta.env.ASSETS_PREFIX)
++someLogic(build.assetsPrefix)
+```

packages/astro/client.d.ts

@@ -5,8 +5,10 @@
 /// <reference path="./types/fonts.d.ts" />
 
 interface ImportMetaEnv {
+	// TODO: remove in Astro 7
 	/**
 	 * The prefix for Astro-generated asset links if the build.assetsPrefix config option is set. This can be used to create asset links not handled by Astro.
+	 * @deprecated This will be removed in a future major version of Astro. Use `build.assetsPrefix` from `astro:config/server` instead.
 	 */
 	readonly ASSETS_PREFIX: string | Record<string, string>;
 	/**