Make pip wheel cache what it built

Just like pip install.

news/6852.feature

@@ -0,0 +1,4 @@
+Cache wheels that ``pip wheel`` built locally, matching what
+``pip install`` does. This particularly helps performance in workflows where
+``pip wheel`` is used for `building before installing
+<https://pip.pypa.io/en/stable/user_guide/#installing-from-local-packages>`_.

src/pip/_internal/wheel.py

@@ -1124,19 +1124,15 @@ def build(
             ):
                 continue
 
-            # Determine where the wheel should go.
-            if should_unpack:
-                if (
-                    cache_available and
-                    should_cache(req, self.check_binary_allowed)
-                ):
-                    output_dir = self.wheel_cache.get_path_for_link(req.link)
-                else:
-                    output_dir = self.wheel_cache.get_ephem_path_for_link(
-                        req.link
-                    )
+            if (
+                cache_available and
+                should_cache(req, self.check_binary_allowed)
+            ):
+                output_dir = self.wheel_cache.get_path_for_link(req.link)
             else:
-                output_dir = self._wheel_dir
+                output_dir = self.wheel_cache.get_ephem_path_for_link(
+                    req.link
+                )
 
             buildset.append((req, output_dir))
 
@@ -1202,6 +1198,21 @@ def build(
                         assert req.link.is_wheel
                         # extract the wheel into the dir
                         unpack_file(req.link.file_path, req.source_dir)
+                    else:
+                        # copy from cache to target durectory
+                        try:
+                            ensure_dir(self._wheel_dir)
+                            shutil.copy(
+                                os.path.join(output_dir, wheel_file),
+                                self._wheel_dir,
+                            )
+                        except OSError as e:
+                            logger.warning(
+                                "Building wheel for %s failed: %s",
+                                req.name, e,
+                            )
+                            build_failure.append(req)
+                            continue
                 else:
                     build_failure.append(req)
 

tests/functional/test_wheel.py

@@ -62,6 +62,30 @@ def test_pip_wheel_success(script, data):
     assert "Successfully built simple" in result.stdout, result.stdout
 
 
+def test_pip_wheel_build_cache(script, data):
+    """
+    Test 'pip wheel' builds and caches.
+    """
+    result = script.pip(
+        'wheel', '--no-index', '-f', data.find_links,
+        'simple==3.0',
+    )
+    wheel_file_name = 'simple-3.0-py%s-none-any.whl' % pyversion[0]
+    wheel_file_path = script.scratch / wheel_file_name
+    assert wheel_file_path in result.files_created, result.stdout
+    assert "Successfully built simple" in result.stdout, result.stdout
+    # remove target file
+    (script.scratch_path / wheel_file_name).unlink()
+    # pip wheel again and test that no build occurs since
+    # we get the wheel from cache
+    result = script.pip(
+        'wheel', '--no-index', '-f', data.find_links,
+        'simple==3.0',
+    )
+    assert wheel_file_path in result.files_created, result.stdout
+    assert "Successfully built simple" not in result.stdout, result.stdout
+
+
 def test_basic_pip_wheel_downloads_wheels(script, data):
     """
     Test 'pip wheel' downloads wheels