build_meta: produce informative error when a dist is not found

Previously, when `build_sdist` or `build_wheel` were unable
to build a distribution (and were therefore unable to find the
distribution file), they would throw a

    ValueError: not enough values to unpack (expected 1, got 0)

which did not offer any clues as to where the issue might lie.

2 files changed, 20 insertions, 2 deletions

diff --git a/setuptools/build_meta.py b/setuptools/build_meta.py
index b9e8a2b3..3c45db72 100644
--- a/setuptools/build_meta.py
+++ b/setuptools/build_meta.py
@@ -101,8 +101,12 @@ def _file_with_extension(directory, extension):
         f for f in os.listdir(directory)
         if f.endswith(extension)
     )
-    file, = matching
-    return file
+    try:
+        return next(matching)
+    except StopIteration:
+        raise ValueError('No distribution was found. The distribution was '
+                         'possibly not built. Ensure that your `setup.py` '
+                         'is not empty and that it calls `setup()`.')
 
 
 def _open_setup_script(setup_script):
diff --git a/setuptools/tests/test_build_meta.py b/setuptools/tests/test_build_meta.py
index f33a6968..f776b09d 100644
--- a/setuptools/tests/test_build_meta.py
+++ b/setuptools/tests/test_build_meta.py
@@ -3,6 +3,7 @@ import shutil
 import tarfile
 import importlib
 from concurrent import futures
+import re
 
 import pytest
 from jaraco import path
@@ -442,6 +443,19 @@ class TestBuildMetaBackend:
         with pytest.raises(AssertionError):
             build_backend.build_sdist("temp")
 
+    @pytest.mark.parametrize('build_hook', ('build_sdist', 'build_wheel'))
+    def test_build_with_empty_setuppy(self, build_backend, build_hook):
+        files = {'setup.py': ''}
+        path.build(files)
+
+        with pytest.raises(
+                ValueError,
+                match=re.escape(
+                    'No distribution was found. The distribution was '
+                    'possibly not built. Ensure that your `setup.py` '
+                    'is not empty and that it calls `setup()`.')):
+            getattr(build_backend, build_hook)("temp")
+
 
 class TestBuildMetaLegacyBackend(TestBuildMetaBackend):
     backend_name = 'setuptools.build_meta:__legacy__'