Make in-tree-build the default

Add an out-of-tree-build feature flag to ease the transition.

docs/html/cli/pip_install.rst

@@ -490,18 +490,20 @@ You can install local projects by specifying the project path to pip:
 
       py -m pip install path/to/SomeProject
 
-During regular installation, pip will copy the entire project directory to a
-temporary location and install from there. The exception is that pip will
-exclude .tox and .nox directories present in the top level of the project from
-being copied. This approach is the cause of several performance and correctness
-issues, so it is planned that pip 21.3 will change to install directly from the
-local project directory. Depending on the build backend used by the project,
-this may generate secondary build artifacts in the project directory, such as
-the ``.egg-info`` and ``build`` directories in the case of the setuptools
-backend.
-
-To opt in to the future behavior, specify the ``--use-feature=in-tree-build``
-option in pip's command line.
+.. note::
+
+   Depending on the build backend used by the project, this may generate
+   secondary build artifacts in the project directory, such as the
+   ``.egg-info`` and ``build`` directories in the case of the setuptools
+   backend.
+
+   Pip has a legacy behaviour that copies the entire project directory to a
+   temporary location and installs from there. This approach was the cause of
+   several performance and correctness issues, so it is now disabled by
+   default, and it is planned that pip 22.1 will remove it.
+
+   To opt in to the legacy behavior, specify the
+   ``--use-deprecated=out-of-tree-build`` option in pip's command line.
 
 
 .. _`editable-installs`:

src/pip/_internal/cli/cmdoptions.py

@@ -961,7 +961,7 @@ def check_list_path_option(options: Values) -> None:
     metavar="feature",
     action="append",
     default=[],
-    choices=["legacy-resolver"],
+    choices=["legacy-resolver", "out-of-tree-build"],
     help=("Enable deprecated functionality, that will be removed in the future."),
 )
 

src/pip/_internal/cli/req_command.py

@@ -34,6 +34,7 @@
 from pip._internal.req.req_tracker import RequirementTracker
 from pip._internal.resolution.base import BaseResolver
 from pip._internal.self_outdated_check import pip_self_version_check
+from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.temp_dir import (
     TempDirectory,
     TempDirectoryTypeRegistry,
@@ -260,6 +261,20 @@ def make_requirement_preparer(
                     "fast-deps has no effect when used with the legacy resolver."
                 )
 
+        in_tree_build = "out-of-tree-build" not in options.deprecated_features_enabled
+        if "in-tree-build" in options.features_enabled:
+            deprecated(
+                reason=("In-tree builds are now the default."),
+                replacement="to remove the --use-feature=in-tree-build flag",
+                gone_in="22.1",
+            )
+        if "out-of-tree-build" in options.deprecated_features_enabled:
+            deprecated(
+                reason="Out-of-tree builds are deprecated.",
+                replacement=None,
+                gone_in="22.1",
+            )
+
         return RequirementPreparer(
             build_dir=temp_build_dir_path,
             src_dir=options.src_dir,
@@ -272,7 +287,7 @@ def make_requirement_preparer(
             require_hashes=options.require_hashes,
             use_user_site=use_user_site,
             lazy_wheel=lazy_wheel,
-            in_tree_build="in-tree-build" in options.features_enabled,
+            in_tree_build=in_tree_build,
         )
 
     @classmethod

src/pip/_internal/operations/prepare.py

@@ -35,7 +35,6 @@
 from pip._internal.network.session import PipSession
 from pip._internal.req.req_install import InstallRequirement
 from pip._internal.req.req_tracker import RequirementTracker
-from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.filesystem import copy2_fixed
 from pip._internal.utils.hashes import Hashes, MissingHashes
 from pip._internal.utils.logging import indent_log
@@ -197,19 +196,9 @@ def unpack_url(
     #
     # As further cleanup, _copy_source_tree and accompanying tests can
     # be removed.
+    #
+    # TODO when use-deprecated=out-of-tree-build is removed
     if link.is_existing_dir():
-        deprecated(
-            reason=(
-                "pip copied the source tree into a temporary directory "
-                "before building it. This is changing so that packages "
-                "are built in-place "
-                'within the original source tree ("in-tree build").'
-            ),
-            replacement=None,
-            gone_in="21.3",
-            feature_flag="in-tree-build",
-            issue=7555,
-        )
         if os.path.isdir(location):
             rmtree(location)
         _copy_source_tree(link.file_path, location)

tests/functional/test_install.py

@@ -587,7 +587,7 @@ def test_install_from_local_directory_with_symlinks_to_directories(script, data)
     Test installing from a local directory containing symlinks to directories.
     """
     to_install = data.packages.joinpath("symlinks")
-    result = script.pip("install", to_install)
+    result = script.pip("install", "--use-deprecated=out-of-tree-build", to_install)
     pkg_folder = script.site_packages / "symlinks"
     dist_info_folder = script.site_packages / "symlinks-0.1.dev0.dist-info"
     result.did_create(pkg_folder)
@@ -597,10 +597,10 @@ def test_install_from_local_directory_with_symlinks_to_directories(script, data)
 @pytest.mark.usefixtures("with_wheel")
 def test_install_from_local_directory_with_in_tree_build(script, data):
     """
-    Test installing from a local directory with --use-feature=in-tree-build.
+    Test installing from a local directory with default in tree build.
     """
     to_install = data.packages.joinpath("FSPkg")
-    args = ["install", "--use-feature=in-tree-build", to_install]
+    args = ["install", to_install]
 
     in_tree_build_dir = to_install / "build"
     assert not in_tree_build_dir.exists()