nextjs-lightningcss-custom-media/solution/gold.patch

diff --git a/packages/next/src/build/webpack/loaders/lightningcss-loader/src/loader.ts b/packages/next/src/build/webpack/loaders/lightningcss-loader/src/loader.ts
index 00f8f53e4c..1a0b64a259 100644
--- a/packages/next/src/build/webpack/loaders/lightningcss-loader/src/loader.ts
+++ b/packages/next/src/build/webpack/loaders/lightningcss-loader/src/loader.ts
@@ -369,6 +369,14 @@ export async function LightningCssLoader(
     : 0
   const includeMask = (1 | userIncludeMask) & ~userExcludeMask // 1 = Features.Nesting
 
+  // `@custom-media` is draft syntax and behind a parser flag
+  // (`drafts.customMedia`).
+  //
+  // See: https://lightningcss.dev/transpilation.html#custom-media-queries
+  const customMediaMask = featureNamesToMask(['custom-media-queries'])
+  const drafts =
+    (includeMask & customMediaMask) !== 0 ? { customMedia: true } : undefined
+
   try {
     const {
       code,
@@ -392,6 +400,7 @@ export async function LightningCssLoader(
         this.sourceMap && prevMap ? JSON.stringify(prevMap) : undefined,
       include: includeMask,
       exclude: userExcludeMask,
+      drafts,
     })
     let cssCodeAsString = code.toString()
 
diff --git a/turbopack/crates/turbopack-css/src/process.rs b/turbopack/crates/turbopack-css/src/process.rs
index 90317ddd7f..8725a2c9d6 100644
--- a/turbopack/crates/turbopack-css/src/process.rs
+++ b/turbopack/crates/turbopack-css/src/process.rs
@@ -4,7 +4,9 @@ use anyhow::{Result, bail};
 use async_trait::async_trait;
 use lightningcss::{
     css_modules::{CssModuleExport, Pattern, Segment},
-    stylesheet::{MinifyOptions, ParserOptions, PrinterOptions, StyleSheet, ToCssResult},
+    stylesheet::{
+        MinifyOptions, ParserFlags, ParserOptions, PrinterOptions, StyleSheet, ToCssResult,
+    },
     targets::{BrowserslistConfig, Features, Targets},
     traits::ToCss,
     values::url::Url,
@@ -450,7 +452,19 @@ async fn process_content(
         }
     }
 
+    // `@custom-media` is draft syntax and behind a parser flag.
+    //
+    // See: https://lightningcss.dev/transpilation.html#custom-media-queries
+    let mut flags = ParserFlags::empty();
+    let include_features = Features::from_bits_truncate(feature_flags.include)
+        & !Features::from_bits_truncate(feature_flags.exclude);
+    flags.set(
+        ParserFlags::CUSTOM_MEDIA,
+        include_features.contains(Features::CustomMediaQueries),
+    );
+
     let config = ParserOptions {
+        flags,
         css_modules: match ty {
             CssModuleType::Module => Some(lightningcss::css_modules::Config {
                 pattern: Pattern {