Add support for registering webpack build dependencies

src/index.js

@@ -124,6 +124,10 @@ export default async function loader(content, sourceMap, meta) {
       this.addDependency(message.file);
     }
 
+    if (message.type === "build-dependency") {
+      this.addBuildDependency(message.file);
+    }
+
     if (message.type === "asset" && message.content && message.file) {
       this.emitFile(
         message.file,

test/__snapshots__/loader.test.js.snap

@@ -106,6 +106,10 @@ Warning
 ]
 `;
 
+exports[`loader should register dependencies using the "messages" API: errors 1`] = `Array []`;
+
+exports[`loader should register dependencies using the "messages" API: warnings 1`] = `Array []`;
+
 exports[`loader should reuse PostCSS AST: css 1`] = `
 "a {
   color: black;

test/loader.test.js

@@ -1,6 +1,7 @@
 import path from "path";
 
 import postcss from "postcss";
+import { NormalModule } from "webpack";
 
 import {
   compile,
@@ -81,6 +82,54 @@ describe("loader", () => {
     expect(getErrors(stats)).toMatchSnapshot("errors");
   });
 
+  it('should register dependencies using the "messages" API', async () => {
+    const plugin = () => (css, result) => {
+      result.messages.push({
+        type: "build-dependency",
+        file: "build-dep.html",
+        content: "",
+        plugin,
+      });
+    };
+
+    let actualBuildInfo = null;
+
+    const postcssPlugin = postcss.plugin("postcss-plugin", plugin);
+    const compiler = getCompiler(
+      "./css/index.js",
+      {
+        postcssOptions: {
+          plugins: [postcssPlugin()],
+        },
+      },
+      {
+        plugins: [
+          {
+            /** @param {import("webpack").Compiler} compiler */
+            apply(wpcompiler) {
+              wpcompiler.hooks.compilation.tap("plugin", (compilation) => {
+                NormalModule.getCompilationHooks(compilation).beforeLoaders.tap(
+                  "plugin",
+                  (_1, module) => {
+                    actualBuildInfo = module.buildInfo;
+                  }
+                );
+              });
+            },
+          },
+        ],
+      }
+    );
+
+    const stats = await compile(compiler);
+
+    const buildDependencies = [...actualBuildInfo.buildDependencies];
+    expect(buildDependencies).toContain("build-dep.html");
+
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+    expect(getErrors(stats)).toMatchSnapshot("errors");
+  });
+
   it("should reuse PostCSS AST", async () => {
     const spy = jest.fn();
     const compiler = getCompiler(