Add no-deprecations build test

Fails if the site build logs any Hugo deprecation notices; includes a
seeded-deprecation sanity case. Builds to tmp/ to avoid racing with
tests that read public/.

Author: chalin

.gitignore

@@ -1,4 +1,5 @@
 /public
+/tmp
 resources/
 node_modules/
 package-lock.json

tests/no-deprecations.test.mjs

@@ -0,0 +1,72 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { existsSync, mkdirSync, rmSync, writeFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const siteDir = fileURLToPath(new URL('../', import.meta.url));
+
+// Build to a scratch dir so this test never races with tests that read
+// public/ (node --test runs test files concurrently). Kept after the run
+// for inspection; cleared at start.
+const outDir = join(siteDir, 'tmp', 'test-no-deprecations');
+
+function buildSite() {
+  const res = spawnSync(`npm run build -- -d ${outDir}`, {
+    cwd: siteDir,
+    shell: true,
+    encoding: 'utf8',
+  });
+  const output = `${res.stdout ?? ''}${res.stderr ?? ''}`;
+  const deprecations = output
+    .split('\n')
+    .filter((line) => /deprecated/i.test(line));
+  return { res, output, deprecations };
+}
+
+test('site build logs no Hugo deprecation notices', (t) => {
+  // The `_hugo-dev` script builds with `--logLevel info`, the level at which
+  // Hugo first reports deprecated API usage.
+  rmSync(outDir, { recursive: true, force: true });
+  const { res, output, deprecations } = buildSite();
+  assert.equal(res.status, 0, `Build failed:\n${output}`);
+  assert.deepEqual(deprecations, [], 'Hugo build logged deprecation notice(s)');
+  t.diagnostic(`Scanned ${output.split('\n').length} build-log lines`);
+});
+
+// Sanity check that the test above can actually detect deprecation notices:
+// seed a deprecated API call via the theme's head-end hook and ensure that
+// Hugo reports it. Guards against, e.g., Hugo demoting deprecation notices
+// below the info log level.
+test('build with seeded deprecated API call logs a deprecation notice', (t) => {
+  const hookPath = join(
+    siteDir,
+    'layouts',
+    '_partials',
+    'hooks',
+    'head-end.html',
+  );
+  assert.ok(
+    !existsSync(hookPath),
+    `${hookPath} already exists; this test needs to create it`,
+  );
+  mkdirSync(dirname(hookPath), { recursive: true });
+  writeFileSync(
+    hookPath,
+    '{{/* Temporary file seeded by no-deprecations.test.mjs. */}}\n' +
+      '{{ .Language.LanguageName }}\n',
+  );
+  try {
+    const { res, output, deprecations } = buildSite();
+    assert.equal(res.status, 0, `Seeded build failed:\n${output}`);
+    assert.ok(
+      deprecations.length > 0,
+      'Seeded deprecated API call was not reported by the Hugo build',
+    );
+    t.diagnostic(`Seeded deprecation reported as: ${deprecations[0].trim()}`);
+  } finally {
+    rmSync(hookPath);
+    rmSync(dirname(hookPath), { recursive: true, force: true });
+  }
+});