Merge PEP 660 implementation (#3488)

7 files changed, 1157 insertions, 57 deletions

diff --git a/setuptools/command/build.py b/setuptools/command/build.py
index 12a43622..283999da 100644
--- a/setuptools/command/build.py
+++ b/setuptools/command/build.py
@@ -1,8 +1,17 @@
-from distutils.command.build import build as _build
+import sys
 import warnings
+from typing import TYPE_CHECKING, List, Dict
+from distutils.command.build import build as _build
 
 from setuptools import SetuptoolsDeprecationWarning
 
+if sys.version_info >= (3, 8):
+    from typing import Protocol
+elif TYPE_CHECKING:
+    from typing_extensions import Protocol
+else:
+    from abc import ABC as Protocol
+
 
 _ORIGINAL_SUBCOMMANDS = {"build_py", "build_clib", "build_ext", "build_scripts"}
 
@@ -22,3 +31,116 @@ class build(_build):
             warnings.warn(msg, SetuptoolsDeprecationWarning)
             self.sub_commands = _build.sub_commands
         super().run()
+
+
+class SubCommand(Protocol):
+    """In order to support editable installations (see :pep:`660`) all
+    build subcommands **SHOULD** implement this protocol. They also **MUST** inherit
+    from ``setuptools.Command``.
+
+    When creating an :pep:`editable wheel <660>`, ``setuptools`` will try to evaluate
+    custom ``build`` subcommands using the following procedure:
+
+    1. ``setuptools`` will set the ``editable_mode`` attribute to ``True``
+    2. ``setuptools`` will execute the ``run()`` command.
+
+       .. important::
+          Subcommands **SHOULD** take advantage of ``editable_mode=True`` to adequate
+          its behaviour or perform optimisations.
+
+          For example, if a subcommand don't need to generate any extra file and
+          everything it does is to copy a source file into the build directory,
+          ``run()`` **SHOULD** simply "early return".
+
+          Similarly, if the subcommand creates files that would be placed alongside
+          Python files in the final distribution, during an editable install
+          the command **SHOULD** generate these files "in place" (i.e. write them to
+          the original source directory, instead of using the build directory).
+          Note that ``get_output_mapping()`` should reflect that and include mappings
+          for "in place" builds accordingly.
+
+    3. ``setuptools`` use any knowledge it can derive from the return values of
+       ``get_outputs()`` and ``get_output_mapping()`` to create an editable wheel.
+       When relevant ``setuptools`` **MAY** attempt to use file links based on the value
+       of ``get_output_mapping()``. Alternatively, ``setuptools`` **MAY** attempt to use
+       :doc:`import hooks <python:reference/import>` to redirect any attempt to import
+       to the directory with the original source code and other files built in place.
+
+    Please note that custom sub-commands **SHOULD NOT** rely on ``run()`` being
+    executed (or not) to provide correct return values for ``get_outputs()``,
+    ``get_output_mapping()`` or ``get_source_files()``. The ``get_*`` methods should
+    work independently of ``run()``.
+    """
+
+    editable_mode: bool = False
+    """Boolean flag that will be set to ``True`` when setuptools is used for an
+    editable installation (see :pep:`660`).
+    Implementations **SHOULD** explicitly set the default value of this attribute to
+    ``False``.
+    When subcommands run, they can use this flag to perform optimizations or change
+    their behaviour accordingly.
+    """
+
+    build_lib: str
+    """String representing the directory where the build artifacts should be stored,
+    e.g. ``build/lib``.
+    For example, if a distribution wants to provide a Python module named ``pkg.mod``,
+    then a corresponding file should be written to ``{build_lib}/package/module.py``.
+    A way of thinking about this is that the files saved under ``build_lib``
+    would be eventually copied to one of the directories in :obj:`site.PREFIXES`
+    upon installation.
+
+    A command that produces platform-independent files (e.g. compiling text templates
+    into Python functions), **CAN** initialize ``build_lib`` by copying its value from
+    the ``build_py`` command. On the other hand, a command that produces
+    platform-specific files **CAN** initialize ``build_lib`` by copying its value from
+    the ``build_ext`` command. In general this is done inside the ``finalize_options``
+    method with the help of the ``set_undefined_options`` command::
+
+        def finalize_options(self):
+            self.set_undefined_options("build_py", ("build_lib", "build_lib"))
+            ...
+    """
+
+    def initialize_options(self):
+        """(Required by the original :class:`setuptools.Command` interface)"""
+
+    def finalize_options(self):
+        """(Required by the original :class:`setuptools.Command` interface)"""
+
+    def run(self):
+        """(Required by the original :class:`setuptools.Command` interface)"""
+
+    def get_source_files(self) -> List[str]:
+        """
+        Return a list of all files that are used by the command to create the expected
+        outputs.
+        For example, if your build command transpiles Java files into Python, you should
+        list here all the Java files.
+        The primary purpose of this function is to help populating the ``sdist``
+        with all the files necessary to build the distribution.
+        All files should be strings relative to the project root directory.
+        """
+
+    def get_outputs(self) -> List[str]:
+        """
+        Return a list of files intended for distribution as they would have been
+        produced by the build.
+        These files should be strings in the form of
+        ``"{build_lib}/destination/file/path"``.
+
+        .. note::
+           The return value of ``get_output()`` should include all files used as keys
+           in ``get_output_mapping()`` plus files that are generated during the build
+           and don't correspond to any source file already present in the project.
+        """
+
+    def get_output_mapping(self) -> Dict[str, str]:
+        """
+        Return a mapping between destination files as they would be produced by the
+        build (dict keys) into the respective existing (source) files (dict values).
+        Existing (source) files should be represented as strings relative to the project
+        root directory.
+        Destination files should be strings in the form of
+        ``"{build_lib}/destination/file/path"``.
+        """
diff --git a/setuptools/command/build_ext.py b/setuptools/command/build_ext.py
index c59eff8b..7ad5a87a 100644
--- a/setuptools/command/build_ext.py
+++ b/setuptools/command/build_ext.py
@@ -2,14 +2,16 @@ import os
 import sys
 import itertools
 from importlib.machinery import EXTENSION_SUFFIXES
+from importlib.util import cache_from_source as _compiled_file_name
+from typing import Dict, Iterator, List, Tuple
+
 from distutils.command.build_ext import build_ext as _du_build_ext
-from distutils.file_util import copy_file
 from distutils.ccompiler import new_compiler
 from distutils.sysconfig import customize_compiler, get_config_var
-from distutils.errors import DistutilsError
 from distutils import log
 
-from setuptools.extension import Library
+from setuptools.errors import BaseError
+from setuptools.extension import Extension, Library
 
 try:
     # Attempt to use Cython for building extensions, if available
@@ -73,6 +75,9 @@ def get_abi3_suffix():
 
 
 class build_ext(_build_ext):
+    editable_mode: bool = False
+    inplace: bool = False
+
     def run(self):
         """Build extensions in build directory, then copy if --inplace"""
         old_inplace, self.inplace = self.inplace, 0
@@ -81,27 +86,61 @@ class build_ext(_build_ext):
         if old_inplace:
             self.copy_extensions_to_source()
 
+    def _get_inplace_equivalent(self, build_py, ext: Extension) -> Tuple[str, str]:
+        fullname = self.get_ext_fullname(ext.name)
+        filename = self.get_ext_filename(fullname)
+        modpath = fullname.split('.')
+        package = '.'.join(modpath[:-1])
+        package_dir = build_py.get_package_dir(package)
+        inplace_file = os.path.join(package_dir, os.path.basename(filename))
+        regular_file = os.path.join(self.build_lib, filename)
+        return (inplace_file, regular_file)
+
     def copy_extensions_to_source(self):
         build_py = self.get_finalized_command('build_py')
         for ext in self.extensions:
-            fullname = self.get_ext_fullname(ext.name)
-            filename = self.get_ext_filename(fullname)
-            modpath = fullname.split('.')
-            package = '.'.join(modpath[:-1])
-            package_dir = build_py.get_package_dir(package)
-            dest_filename = os.path.join(package_dir,
-                                         os.path.basename(filename))
-            src_filename = os.path.join(self.build_lib, filename)
+            inplace_file, regular_file = self._get_inplace_equivalent(build_py, ext)
 
             # Always copy, even if source is older than destination, to ensure
             # that the right extensions for the current Python/platform are
             # used.
-            copy_file(
-                src_filename, dest_filename, verbose=self.verbose,
-                dry_run=self.dry_run
-            )
+            self.copy_file(regular_file, inplace_file, level=self.verbose)
+
             if ext._needs_stub:
-                self.write_stub(package_dir or os.curdir, ext, True)
+                inplace_stub = self._get_equivalent_stub(ext, inplace_file)
+                self._write_stub_file(inplace_stub, ext, compile=True)
+                # Always compile stub and remove the original (leave the cache behind)
+                # (this behaviour was observed in previous iterations of the code)
+
+    def _get_equivalent_stub(self, ext: Extension, output_file: str) -> str:
+        dir_ = os.path.dirname(output_file)
+        _, _, name = ext.name.rpartition(".")
+        return f"{os.path.join(dir_, name)}.py"
+
+    def _get_output_mapping(self) -> Iterator[Tuple[str, str]]:
+        if not self.inplace:
+            return
+
+        build_py = self.get_finalized_command('build_py')
+        opt = self.get_finalized_command('install_lib').optimize or ""
+
+        for ext in self.extensions:
+            inplace_file, regular_file = self._get_inplace_equivalent(build_py, ext)
+            yield (regular_file, inplace_file)
+
+            if ext._needs_stub:
+                # This version of `build_ext` always builds artifacts in another dir,
+                # when "inplace=True" is given it just copies them back.
+                # This is done in the `copy_extensions_to_source` function, which
+                # always compile stub files via `_compile_and_remove_stub`.
+                # At the end of the process, a `.pyc` stub file is created without the
+                # corresponding `.py`.
+
+                inplace_stub = self._get_equivalent_stub(ext, inplace_file)
+                regular_stub = self._get_equivalent_stub(ext, regular_file)
+                inplace_cache = _compiled_file_name(inplace_stub, optimization=opt)
+                output_cache = _compiled_file_name(regular_stub, optimization=opt)
+                yield (output_cache, inplace_cache)
 
     def get_ext_filename(self, fullname):
         so_ext = os.getenv('SETUPTOOLS_EXT_SUFFIX')
@@ -131,6 +170,7 @@ class build_ext(_build_ext):
         self.shlib_compiler = None
         self.shlibs = []
         self.ext_map = {}
+        self.editable_mode = False
 
     def finalize_options(self):
         _build_ext.finalize_options(self)
@@ -161,6 +201,9 @@ class build_ext(_build_ext):
             if ltd and use_stubs and os.curdir not in ext.runtime_library_dirs:
                 ext.runtime_library_dirs.append(os.curdir)
 
+        if self.editable_mode:
+            self.inplace = True
+
     def setup_shlib_compiler(self):
         compiler = self.shlib_compiler = new_compiler(
             compiler=self.compiler, dry_run=self.dry_run, force=self.force
@@ -201,8 +244,8 @@ class build_ext(_build_ext):
                 self.compiler = self.shlib_compiler
             _build_ext.build_extension(self, ext)
             if ext._needs_stub:
-                cmd = self.get_finalized_command('build_py').build_lib
-                self.write_stub(cmd, ext)
+                build_lib = self.get_finalized_command('build_py').build_lib
+                self.write_stub(build_lib, ext)
         finally:
             self.compiler = _compiler
 
@@ -215,8 +258,15 @@ class build_ext(_build_ext):
         pkg = '.'.join(ext._full_name.split('.')[:-1] + [''])
         return any(pkg + libname in libnames for libname in ext.libraries)
 
-    def get_outputs(self):
-        return _build_ext.get_outputs(self) + self.__get_stubs_outputs()
+    def get_outputs(self) -> List[str]:
+        if self.inplace:
+            return list(self.get_output_mapping().keys())
+        return sorted(_build_ext.get_outputs(self) + self.__get_stubs_outputs())
+
+    def get_output_mapping(self) -> Dict[str, str]:
+        """See :class:`setuptools.commands.build.SubCommand`"""
+        mapping = self._get_output_mapping()
+        return dict(sorted(mapping, key=lambda x: x[0]))
 
     def __get_stubs_outputs(self):
         # assemble the base name for each extension that needs a stub
@@ -236,12 +286,13 @@ class build_ext(_build_ext):
             yield '.pyo'
 
     def write_stub(self, output_dir, ext, compile=False):
-        log.info("writing stub loader for %s to %s", ext._full_name,
-                 output_dir)
-        stub_file = (os.path.join(output_dir, *ext._full_name.split('.')) +
-                     '.py')
+        stub_file = os.path.join(output_dir, *ext._full_name.split('.')) + '.py'
+        self._write_stub_file(stub_file, ext, compile)
+
+    def _write_stub_file(self, stub_file: str, ext: Extension, compile=False):
+        log.info("writing stub loader for %s to %s", ext._full_name, stub_file)
         if compile and os.path.exists(stub_file):
-            raise DistutilsError(stub_file + " already exists! Please delete.")
+            raise BaseError(stub_file + " already exists! Please delete.")
         if not self.dry_run:
             f = open(stub_file, 'w')
             f.write(
@@ -274,16 +325,19 @@ class build_ext(_build_ext):
             )
             f.close()
         if compile:
-            from distutils.util import byte_compile
+            self._compile_and_remove_stub(stub_file)
+
+    def _compile_and_remove_stub(self, stub_file: str):
+        from distutils.util import byte_compile
 
-            byte_compile([stub_file], optimize=0,
+        byte_compile([stub_file], optimize=0,
+                     force=True, dry_run=self.dry_run)
+        optimize = self.get_finalized_command('install_lib').optimize
+        if optimize > 0:
+            byte_compile([stub_file], optimize=optimize,
                          force=True, dry_run=self.dry_run)
-            optimize = self.get_finalized_command('install_lib').optimize
-            if optimize > 0:
-                byte_compile([stub_file], optimize=optimize,
-                             force=True, dry_run=self.dry_run)
-            if os.path.exists(stub_file) and not self.dry_run:
-                os.unlink(stub_file)
+        if os.path.exists(stub_file) and not self.dry_run:
+            os.unlink(stub_file)
 
 
 if use_stubs or os.name == 'nt':
diff --git a/setuptools/command/build_py.py b/setuptools/command/build_py.py
index 2fced3d6..923a3232 100644
--- a/setuptools/command/build_py.py
+++ b/setuptools/command/build_py.py
@@ -11,6 +11,8 @@ import itertools
 import stat
 import warnings
 from pathlib import Path
+from typing import Dict, Iterator, List, Optional, Tuple
+
 from setuptools._deprecation_warning import SetuptoolsDeprecationWarning
 from setuptools.extern.more_itertools import unique_everseen
 
@@ -28,6 +30,8 @@ class build_py(orig.build_py):
     Also, this version of the 'build_py' command allows you to specify both
     'py_modules' and 'packages' in the same setup operation.
     """
+    editable_mode: bool = False
+    existing_egg_info_dir: Optional[str] = None  #: Private API, internal use only.
 
     def finalize_options(self):
         orig.build_py.finalize_options(self)
@@ -37,9 +41,19 @@ class build_py(orig.build_py):
             del self.__dict__['data_files']
         self.__updated_files = []
 
+    def copy_file(self, infile, outfile, preserve_mode=1, preserve_times=1,
+                  link=None, level=1):
+        # Overwrite base class to allow using links
+        if link:
+            infile = str(Path(infile).resolve())
+            outfile = str(Path(outfile).resolve())
+        return super().copy_file(infile, outfile, preserve_mode, preserve_times,
+                                 link, level)
+
     def run(self):
         """Build modules, packages, and copy data files to build directory"""
-        if not self.py_modules and not self.packages:
+        # if self.editable_mode or not (self.py_modules and self.packages):
+        if not (self.py_modules or self.packages) or self.editable_mode:
             return
 
         if self.py_modules:
@@ -112,16 +126,41 @@ class build_py(orig.build_py):
         )
         return self.exclude_data_files(package, src_dir, files)
 
-    def build_package_data(self):
-        """Copy data files into build directory"""
+    def get_outputs(self, include_bytecode=1) -> List[str]:
+        """See :class:`setuptools.commands.build.SubCommand`"""
+        if self.editable_mode:
+            return list(self.get_output_mapping().keys())
+        return super().get_outputs(include_bytecode)
+
+    def get_output_mapping(self) -> Dict[str, str]:
+        """See :class:`setuptools.commands.build.SubCommand`"""
+        mapping = itertools.chain(
+            self._get_package_data_output_mapping(),
+            self._get_module_mapping(),
+        )
+        return dict(sorted(mapping, key=lambda x: x[0]))
+
+    def _get_module_mapping(self) -> Iterator[Tuple[str, str]]:
+        """Iterate over all modules producing (dest, src) pairs."""
+        for (package, module, module_file) in self.find_all_modules():
+            package = package.split('.')
+            filename = self.get_module_outfile(self.build_lib, package, module)
+            yield (filename, module_file)
+
+    def _get_package_data_output_mapping(self) -> Iterator[Tuple[str, str]]:
+        """Iterate over package data producing (dest, src) pairs."""
         for package, src_dir, build_dir, filenames in self.data_files:
             for filename in filenames:
                 target = os.path.join(build_dir, filename)
-                self.mkpath(os.path.dirname(target))
                 srcfile = os.path.join(src_dir, filename)
-                outf, copied = self.copy_file(srcfile, target)
-                make_writable(target)
-                srcfile = os.path.abspath(srcfile)
+                yield (target, srcfile)
+
+    def build_package_data(self):
+        """Copy data files into build directory"""
+        for target, srcfile in self._get_package_data_output_mapping():
+            self.mkpath(os.path.dirname(target))
+            _outf, _copied = self.copy_file(srcfile, target)
+            make_writable(target)
 
     def analyze_manifest(self):
         self.manifest_files = mf = {}
@@ -132,10 +171,19 @@ class build_py(orig.build_py):
             # Locate package source directory
             src_dirs[assert_relative(self.get_package_dir(package))] = package
 
-        self.run_command('egg_info')
+        if (
+            getattr(self, 'existing_egg_info_dir', None)
+            and Path(self.existing_egg_info_dir, "SOURCES.txt").exists()
+        ):
+            manifest = Path(self.existing_egg_info_dir, "SOURCES.txt")
+            files = manifest.read_text(encoding="utf-8").splitlines()
+        else:
+            self.run_command('egg_info')
+            ei_cmd = self.get_finalized_command('egg_info')
+            files = ei_cmd.filelist.files
+
         check = _IncludePackageDataAbuse()
-        ei_cmd = self.get_finalized_command('egg_info')
-        for path in ei_cmd.filelist.files:
+        for path in files:
             d, f = os.path.split(assert_relative(path))
             prev = None
             oldf = f
@@ -189,6 +237,8 @@ class build_py(orig.build_py):
     def initialize_options(self):
         self.packages_checked = {}
         orig.build_py.initialize_options(self)
+        self.editable_mode = False
+        self.existing_egg_info_dir = None
 
     def get_package_dir(self, package):
         res = orig.build_py.get_package_dir(self, package)
diff --git a/setuptools/command/dist_info.py b/setuptools/command/dist_info.py
index ca540ad1..0685c945 100644
--- a/setuptools/command/dist_info.py
+++ b/setuptools/command/dist_info.py
@@ -5,12 +5,17 @@ As defined in the wheel specification
 
 import os
 import re
+import shutil
+import sys
 import warnings
+from contextlib import contextmanager
 from inspect import cleandoc
+from pathlib import Path
 
 from distutils.core import Command
 from distutils import log
 from setuptools.extern import packaging
+from setuptools._deprecation_warning import SetuptoolsDeprecationWarning
 
 
 class dist_info(Command):
@@ -19,28 +24,85 @@ class dist_info(Command):
 
     user_options = [
         ('egg-base=', 'e', "directory containing .egg-info directories"
-                           " (default: top of the source tree)"),
+                           " (default: top of the source tree)"
+                           " DEPRECATED: use --output-dir."),
+        ('output-dir=', 'o', "directory inside of which the .dist-info will be"
+                             "created (default: top of the source tree)"),
+        ('tag-date', 'd', "Add date stamp (e.g. 20050528) to version number"),
+        ('tag-build=', 'b', "Specify explicit tag to add to version number"),
+        ('no-date', 'D', "Don't include date stamp [default]"),
+        ('keep-egg-info', None, "*TRANSITIONAL* will be removed in the future"),
     ]
 
+    boolean_options = ['tag-date', 'keep-egg-info']
+    negative_opt = {'no-date': 'tag-date'}
+
     def initialize_options(self):
         self.egg_base = None
+        self.output_dir = None
+        self.name = None
+        self.dist_info_dir = None
+        self.tag_date = None
+        self.tag_build = None
+        self.keep_egg_info = False
 
     def finalize_options(self):
-        pass
+        if self.egg_base:
+            msg = "--egg-base is deprecated for dist_info command. Use --output-dir."
+            warnings.warn(msg, SetuptoolsDeprecationWarning)
+            self.output_dir = self.egg_base or self.output_dir
+
+        dist = self.distribution
+        project_dir = dist.src_root or os.curdir
+        self.output_dir = Path(self.output_dir or project_dir)
+
+        egg_info = self.reinitialize_command("egg_info")
+        egg_info.egg_base = str(self.output_dir)
+
+        if self.tag_date:
+            egg_info.tag_date = self.tag_date
+        else:
+            self.tag_date = egg_info.tag_date
+
+        if self.tag_build:
+            egg_info.tag_build = self.tag_build
+        else:
+            self.tag_build = egg_info.tag_build
 
-    def run(self):
-        egg_info = self.get_finalized_command('egg_info')
-        egg_info.egg_base = self.egg_base
         egg_info.finalize_options()
-        egg_info.run()
-        name = _safe(self.distribution.get_name())
-        version = _version(self.distribution.get_version())
-        base = self.egg_base or os.curdir
-        dist_info_dir = os.path.join(base, f"{name}-{version}.dist-info")
-        log.info("creating '{}'".format(os.path.abspath(dist_info_dir)))
+        self.egg_info = egg_info
+
+        name = _safe(dist.get_name())
+        version = _version(dist.get_version())
+        self.name = f"{name}-{version}"
+        self.dist_info_dir = os.path.join(self.output_dir, f"{self.name}.dist-info")
+
+    @contextmanager
+    def _maybe_bkp_dir(self, dir_path: str, requires_bkp: bool):
+        if requires_bkp:
+            bkp_name = f"{dir_path}.__bkp__"
+            _rm(bkp_name, ignore_errors=True)
+            _copy(dir_path, bkp_name, dirs_exist_ok=True, symlinks=True)
+            try:
+                yield
+            finally:
+                _rm(dir_path, ignore_errors=True)
+                shutil.move(bkp_name, dir_path)
+        else:
+            yield
 
+    def run(self):
+        self.output_dir.mkdir(parents=True, exist_ok=True)
+        self.egg_info.run()
+        egg_info_dir = self.egg_info.egg_info
+        assert os.path.isdir(egg_info_dir), ".egg-info dir should have been created"
+
+        log.info("creating '{}'".format(os.path.abspath(self.dist_info_dir)))
         bdist_wheel = self.get_finalized_command('bdist_wheel')
-        bdist_wheel.egg2dist(egg_info.egg_info, dist_info_dir)
+
+        # TODO: if bdist_wheel if merged into setuptools, just add "keep_egg_info" there
+        with self._maybe_bkp_dir(egg_info_dir, self.keep_egg_info):
+            bdist_wheel.egg2dist(egg_info_dir, self.dist_info_dir)
 
 
 def _safe(component: str) -> str:
@@ -67,3 +129,14 @@ def _version(version: str) -> str:
         """
         warnings.warn(cleandoc(msg))
         return _safe(v).strip("_")
+
+
+def _rm(dir_name, **opts):
+    if os.path.isdir(dir_name):
+        shutil.rmtree(dir_name, **opts)
+
+
+def _copy(src, dst, **opts):
+    if sys.version_info < (3, 8):
+        opts.pop("dirs_exist_ok", None)
+    shutil.copytree(src, dst, **opts)
diff --git a/setuptools/command/editable_wheel.py b/setuptools/command/editable_wheel.py
new file mode 100644
index 00000000..a44d24be
--- /dev/null
+++ b/setuptools/command/editable_wheel.py
@@ -0,0 +1,776 @@
+"""
+Create a wheel that, when installed, will make the source package 'editable'
+(add it to the interpreter's path, including metadata) per PEP 660. Replaces
+'setup.py develop'.
+
+.. note::
+   One of the mechanisms briefly mentioned in PEP 660 to implement editable installs is
+   to create a separated directory inside ``build`` and use a .pth file to point to that
+   directory. In the context of this file such directory is referred as
+   *auxiliary build directory* or ``auxiliary_dir``.
+"""
+
+import logging
+import os
+import re
+import shutil
+import sys
+import traceback
+import warnings
+from contextlib import suppress
+from enum import Enum
+from inspect import cleandoc
+from itertools import chain
+from pathlib import Path
+from tempfile import TemporaryDirectory
+from typing import (
+    TYPE_CHECKING,
+    Dict,
+    Iterable,
+    Iterator,
+    List,
+    Mapping,
+    Optional,
+    Tuple,
+    TypeVar,
+    Union,
+)
+
+from setuptools import Command, SetuptoolsDeprecationWarning, errors, namespaces
+from setuptools.discovery import find_package_path
+from setuptools.dist import Distribution
+
+if TYPE_CHECKING:
+    from wheel.wheelfile import WheelFile  # noqa
+
+if sys.version_info >= (3, 8):
+    from typing import Protocol
+elif TYPE_CHECKING:
+    from typing_extensions import Protocol
+else:
+    from abc import ABC as Protocol
+
+_Path = Union[str, Path]
+_P = TypeVar("_P", bound=_Path)
+_logger = logging.getLogger(__name__)
+
+
+class _EditableMode(Enum):
+    """
+    Possible editable installation modes:
+    `lenient` (new files automatically added to the package - DEFAULT);
+    `strict` (requires a new installation when files are added/removed); or
+    `compat` (attempts to emulate `python setup.py develop` - DEPRECATED).
+    """
+
+    STRICT = "strict"
+    LENIENT = "lenient"
+    COMPAT = "compat"  # TODO: Remove `compat` after Dec/2022.
+
+    @classmethod
+    def convert(cls, mode: Optional[str]) -> "_EditableMode":
+        if not mode:
+            return _EditableMode.LENIENT  # default
+
+        _mode = mode.upper()
+        if _mode not in _EditableMode.__members__:
+            raise errors.OptionError(f"Invalid editable mode: {mode!r}. Try: 'strict'.")
+
+        if _mode == "COMPAT":
+            msg = """
+            The 'compat' editable mode is transitional and will be removed
+            in future versions of `setuptools`.
+            Please adapt your code accordingly to use either the 'strict' or the
+            'lenient' modes.
+
+            For more information, please check:
+            https://setuptools.pypa.io/en/latest/userguide/development_mode.html
+            """
+            warnings.warn(msg, SetuptoolsDeprecationWarning)
+
+        return _EditableMode[_mode]
+
+
+_STRICT_WARNING = """
+New or renamed files may not be automatically picked up without a new installation.
+"""
+
+_LENIENT_WARNING = """
+Options like `package-data`, `include/exclude-package-data` or
+`packages.find.exclude/include` may have no effect.
+"""
+
+
+class editable_wheel(Command):
+    """Build 'editable' wheel for development.
+    (This command is reserved for internal use of setuptools).
+    """
+
+    description = "create a PEP 660 'editable' wheel"
+
+    user_options = [
+        ("dist-dir=", "d", "directory to put final built distributions in"),
+        ("dist-info-dir=", "I", "path to a pre-build .dist-info directory"),
+        ("mode=", None, cleandoc(_EditableMode.__doc__ or "")),
+    ]
+
+    def initialize_options(self):
+        self.dist_dir = None
+        self.dist_info_dir = None
+        self.project_dir = None
+        self.mode = None
+
+    def finalize_options(self):
+        dist = self.distribution
+        self.project_dir = dist.src_root or os.curdir
+        self.package_dir = dist.package_dir or {}
+        self.dist_dir = Path(self.dist_dir or os.path.join(self.project_dir, "dist"))
+
+    def run(self):
+        try:
+            self.dist_dir.mkdir(exist_ok=True)
+            self._ensure_dist_info()
+
+            # Add missing dist_info files
+            bdist_wheel = self.reinitialize_command("bdist_wheel")
+            bdist_wheel.write_wheelfile(self.dist_info_dir)
+
+            self._create_wheel_file(bdist_wheel)
+        except Exception as ex:
+            traceback.print_exc()
+            msg = """
+            Support for editable installs via PEP 660 was recently introduced
+            in `setuptools`. If you are seeing this error, please report to:
+
+            https://github.com/pypa/setuptools/issues
+
+            Meanwhile you can try the legacy behavior by setting an
+            environment variable and trying to install again:
+
+            SETUPTOOLS_ENABLE_FEATURES="legacy-editable"
+            """
+            raise errors.InternalError(cleandoc(msg)) from ex
+
+    def _ensure_dist_info(self):
+        if self.dist_info_dir is None:
+            dist_info = self.reinitialize_command("dist_info")
+            dist_info.output_dir = self.dist_dir
+            dist_info.finalize_options()
+            dist_info.run()
+            self.dist_info_dir = dist_info.dist_info_dir
+        else:
+            assert str(self.dist_info_dir).endswith(".dist-info")
+            assert Path(self.dist_info_dir, "METADATA").exists()
+
+    def _install_namespaces(self, installation_dir, pth_prefix):
+        # XXX: Only required to support the deprecated namespace practice
+        dist = self.distribution
+        if not dist.namespace_packages:
+            return
+
+        src_root = Path(self.project_dir, self.pakcage_dir.get("", ".")).resolve()
+        installer = _NamespaceInstaller(dist, installation_dir, pth_prefix, src_root)
+        installer.install_namespaces()
+
+    def _find_egg_info_dir(self) -> Optional[str]:
+        parent_dir = Path(self.dist_info_dir).parent if self.dist_info_dir else Path()
+        candidates = map(str, parent_dir.glob("*.egg-info"))
+        return next(candidates, None)
+
+    def _configure_build(
+        self, name: str, unpacked_wheel: _Path, build_lib: _Path, tmp_dir: _Path
+    ):
+        """Configure commands to behave in the following ways:
+
+        - Build commands can write to ``build_lib`` if they really want to...
+          (but this folder is expected to be ignored and modules are expected to live
+          in the project directory...)
+        - Binary extensions should be built in-place (editable_mode = True)
+        - Data/header/script files are not part of the "editable" specification
+          so they are written directly to the unpacked_wheel directory.
+        """
+        # Non-editable files (data, headers, scripts) are written directly to the
+        # unpacked_wheel
+
+        dist = self.distribution
+        wheel = str(unpacked_wheel)
+        build_lib = str(build_lib)
+        data = str(Path(unpacked_wheel, f"{name}.data", "data"))
+        headers = str(Path(unpacked_wheel, f"{name}.data", "headers"))
+        scripts = str(Path(unpacked_wheel, f"{name}.data", "scripts"))
+
+        # egg-info may be generated again to create a manifest (used for package data)
+        egg_info = dist.reinitialize_command("egg_info", reinit_subcommands=True)
+        egg_info.egg_base = str(tmp_dir)
+        egg_info.ignore_egg_info_in_manifest = True
+
+        build = dist.reinitialize_command("build", reinit_subcommands=True)
+        install = dist.reinitialize_command("install", reinit_subcommands=True)
+
+        build.build_platlib = build.build_purelib = build.build_lib = build_lib
+        install.install_purelib = install.install_platlib = install.install_lib = wheel
+        install.install_scripts = build.build_scripts = scripts
+        install.install_headers = headers
+        install.install_data = data
+
+        install_scripts = dist.get_command_obj("install_scripts")
+        install_scripts.no_ep = True
+
+        build.build_temp = str(tmp_dir)
+
+        build_py = dist.get_command_obj("build_py")
+        build_py.compile = False
+        build_py.existing_egg_info_dir = self._find_egg_info_dir()
+
+        self._set_editable_mode()
+
+        build.ensure_finalized()
+        install.ensure_finalized()
+
+    def _set_editable_mode(self):
+        """Set the ``editable_mode`` flag in the build sub-commands"""
+        dist = self.distribution
+        build = dist.get_command_obj("build")
+        for cmd_name in build.get_sub_commands():
+            cmd = dist.get_command_obj(cmd_name)
+            if hasattr(cmd, "editable_mode"):
+                cmd.editable_mode = True
+
+    def _collect_build_outputs(self) -> Tuple[List[str], Dict[str, str]]:
+        files: List[str] = []
+        mapping: Dict[str, str] = {}
+        build = self.get_finalized_command("build")
+
+        for cmd_name in build.get_sub_commands():
+            cmd = self.get_finalized_command(cmd_name)
+            if hasattr(cmd, "get_outputs"):
+                files.extend(cmd.get_outputs() or [])
+            if hasattr(cmd, "get_output_mapping"):
+                mapping.update(cmd.get_output_mapping() or {})
+
+        return files, mapping
+
+    def _run_build_commands(
+        self, dist_name: str, unpacked_wheel: _Path, build_lib: _Path, tmp_dir: _Path
+    ) -> Tuple[List[str], Dict[str, str]]:
+        self._configure_build(dist_name, unpacked_wheel, build_lib, tmp_dir)
+        self.run_command("build")
+        files, mapping = self._collect_build_outputs()
+        self._run_install("headers")
+        self._run_install("scripts")
+        self._run_install("data")
+        return files, mapping
+
+    def _create_wheel_file(self, bdist_wheel):
+        from wheel.wheelfile import WheelFile
+
+        dist_info = self.get_finalized_command("dist_info")
+        dist_name = dist_info.name
+        tag = "-".join(bdist_wheel.get_tag())
+        build_tag = "0.editable"  # According to PEP 427 needs to start with digit
+        archive_name = f"{dist_name}-{build_tag}-{tag}.whl"
+        wheel_path = Path(self.dist_dir, archive_name)
+        if wheel_path.exists():
+            wheel_path.unlink()
+
+        unpacked_wheel = TemporaryDirectory(suffix=archive_name)
+        build_lib = TemporaryDirectory(suffix=".build-lib")
+        build_tmp = TemporaryDirectory(suffix=".build-temp")
+
+        with unpacked_wheel as unpacked, build_lib as lib, build_tmp as tmp:
+            unpacked_dist_info = Path(unpacked, Path(self.dist_info_dir).name)
+            shutil.copytree(self.dist_info_dir, unpacked_dist_info)
+            self._install_namespaces(unpacked, dist_info.name)
+            files, mapping = self._run_build_commands(dist_name, unpacked, lib, tmp)
+            strategy = self._select_strategy(dist_name, tag, lib)
+            with strategy, WheelFile(wheel_path, "w") as wheel_obj:
+                strategy(wheel_obj, files, mapping)
+                wheel_obj.write_files(unpacked)
+
+        return wheel_path
+
+    def _run_install(self, category: str):
+        has_category = getattr(self.distribution, f"has_{category}", None)
+        if has_category and has_category():
+            _logger.info(f"Installing {category} as non editable")
+            self.run_command(f"install_{category}")
+
+    def _select_strategy(
+        self,
+        name: str,
+        tag: str,
+        build_lib: _Path,
+    ) -> "EditableStrategy":
+        """Decides which strategy to use to implement an editable installation."""
+        build_name = f"__editable__.{name}-{tag}"
+        project_dir = Path(self.project_dir)
+        mode = _EditableMode.convert(self.mode)
+
+        if mode is _EditableMode.STRICT:
+            auxiliary_dir = _empty_dir(Path(self.project_dir, "build", build_name))
+            return _LinkTree(self.distribution, name, auxiliary_dir, build_lib)
+
+        packages = _find_packages(self.distribution)
+        has_simple_layout = _simple_layout(packages, self.package_dir, project_dir)
+        is_compat_mode = mode is _EditableMode.COMPAT
+        if set(self.package_dir) == {""} and has_simple_layout or is_compat_mode:
+            # src-layout(ish) is relatively safe for a simple pth file
+            src_dir = self.package_dir.get("", ".")
+            return _StaticPth(self.distribution, name, [Path(project_dir, src_dir)])
+
+        # Use a MetaPathFinder to avoid adding accidental top-level packages/modules
+        return _TopLevelFinder(self.distribution, name)
+
+
+class EditableStrategy(Protocol):
+    def __call__(self, wheel: "WheelFile", files: List[str], mapping: Dict[str, str]):
+        ...
+
+    def __enter__(self):
+        ...
+
+    def __exit__(self, _exc_type, _exc_value, _traceback):
+        ...
+
+
+class _StaticPth:
+    def __init__(self, dist: Distribution, name: str, path_entries: List[Path]):
+        self.dist = dist
+        self.name = name
+        self.path_entries = path_entries
+
+    def __call__(self, wheel: "WheelFile", files: List[str], mapping: Dict[str, str]):
+        entries = "\n".join((str(p.resolve()) for p in self.path_entries))
+        contents = bytes(f"{entries}\n", "utf-8")
+        wheel.writestr(f"__editable__.{self.name}.pth", contents)
+
+    def __enter__(self):
+        msg = f"""
+        Editable install will be performed using .pth file to extend `sys.path` with:
+        {self.path_entries!r}
+        """
+        _logger.warning(msg + _LENIENT_WARNING)
+        return self
+
+    def __exit__(self, _exc_type, _exc_value, _traceback):
+        ...
+
+
+class _LinkTree(_StaticPth):
+    """
+    Creates a ``.pth`` file that points to a link tree in the ``auxiliary_dir``.
+
+    This strategy will only link files (not dirs), so it can be implemented in
+    any OS, even if that means using hardlinks instead of symlinks.
+
+    By collocating ``auxiliary_dir`` and the original source code, limitations
+    with hardlinks should be avoided.
+    """
+    def __init__(
+        self, dist: Distribution,
+        name: str,
+        auxiliary_dir: _Path,
+        build_lib: _Path,
+    ):
+        self.auxiliary_dir = Path(auxiliary_dir)
+        self.build_lib = Path(build_lib).resolve()
+        self._file = dist.get_command_obj("build_py").copy_file
+        super().__init__(dist, name, [self.auxiliary_dir])
+
+    def __call__(self, wheel: "WheelFile", files: List[str], mapping: Dict[str, str]):
+        self._create_links(files, mapping)
+        super().__call__(wheel, files, mapping)
+
+    def _normalize_output(self, file: str) -> Optional[str]:
+        # Files relative to build_lib will be normalized to None
+        with suppress(ValueError):
+            path = Path(file).resolve().relative_to(self.build_lib)
+            return str(path).replace(os.sep, '/')
+        return None
+
+    def _create_file(self, relative_output: str, src_file: str, link=None):
+        dest = self.auxiliary_dir / relative_output
+        if not dest.parent.is_dir():
+            dest.parent.mkdir(parents=True)
+        self._file(src_file, dest, link=link)
+
+    def _create_links(self, outputs, output_mapping):
+        self.auxiliary_dir.mkdir(parents=True, exist_ok=True)
+        link_type = "sym" if _can_symlink_files(self.auxiliary_dir) else "hard"
+        mappings = {
+            self._normalize_output(k): v
+            for k, v in output_mapping.items()
+        }
+        mappings.pop(None, None)  # remove files that are not relative to build_lib
+
+        for output in outputs:
+            relative = self._normalize_output(output)
+            if relative and relative not in mappings:
+                self._create_file(relative, output)
+
+        for relative, src in mappings.items():
+            self._create_file(relative, src, link=link_type)
+
+    def __enter__(self):
+        msg = "Strict editable install will be performed using a link tree.\n"
+        _logger.warning(msg + _STRICT_WARNING)
+        return self
+
+    def __exit__(self, _exc_type, _exc_value, _traceback):
+        msg = f"""\n
+        Strict editable installation performed using the auxiliary directory:
+            {self.auxiliary_dir}
+
+        Please be careful to not remove this directory, otherwise you might not be able
+        to import/use your package.
+        """
+        warnings.warn(msg, InformationOnly)
+
+
+class _TopLevelFinder:
+    def __init__(self, dist: Distribution, name: str):
+        self.dist = dist
+        self.name = name
+
+    def __call__(self, wheel: "WheelFile", files: List[str], mapping: Dict[str, str]):
+        src_root = self.dist.src_root or os.curdir
+        top_level = chain(_find_packages(self.dist), _find_top_level_modules(self.dist))
+        package_dir = self.dist.package_dir or {}
+        roots = _find_package_roots(top_level, package_dir, src_root)
+
+        namespaces_: Dict[str, List[str]] = dict(chain(
+            _find_namespaces(self.dist.packages, roots),
+            ((ns, []) for ns in _find_virtual_namespaces(roots)),
+        ))
+
+        name = f"__editable__.{self.name}.finder"
+        finder = _make_identifier(name)
+        content = bytes(_finder_template(name, roots, namespaces_), "utf-8")
+        wheel.writestr(f"{finder}.py", content)
+
+        content = bytes(f"import {finder}; {finder}.install()", "utf-8")
+        wheel.writestr(f"__editable__.{self.name}.pth", content)
+
+    def __enter__(self):
+        msg = "Editable install will be performed using a meta path finder.\n"
+        _logger.warning(msg + _LENIENT_WARNING)
+        return self
+
+    def __exit__(self, _exc_type, _exc_value, _traceback):
+        ...
+
+
+def _can_symlink_files(base_dir: Path) -> bool:
+    with TemporaryDirectory(dir=str(base_dir.resolve())) as tmp:
+        path1, path2 = Path(tmp, "file1.txt"), Path(tmp, "file2.txt")
+        path1.write_text("file1", encoding="utf-8")
+        with suppress(AttributeError, NotImplementedError, OSError):
+            os.symlink(path1, path2)
+            if path2.is_symlink() and path2.read_text(encoding="utf-8") == "file1":
+                return True
+
+        try:
+            os.link(path1, path2)  # Ensure hard links can be created
+        except Exception as ex:
+            msg = (
+                "File system does not seem to support either symlinks or hard links. "
+                "Strict editable installs require one of them to be supported."
+            )
+            raise LinksNotSupported(msg) from ex
+        return False
+
+
+def _simple_layout(
+    packages: Iterable[str], package_dir: Dict[str, str], project_dir: Path
+) -> bool:
+    """Return ``True`` if:
+    - all packages are contained by the same parent directory, **and**
+    - all packages become importable if the parent directory is added to ``sys.path``.
+
+    >>> _simple_layout(['a'], {"": "src"}, "/tmp/myproj")
+    True
+    >>> _simple_layout(['a', 'a.b'], {"": "src"}, "/tmp/myproj")
+    True
+    >>> _simple_layout(['a', 'a.b'], {}, "/tmp/myproj")
+    True
+    >>> _simple_layout(['a', 'a.a1', 'a.a1.a2', 'b'], {"": "src"}, "/tmp/myproj")
+    True
+    >>> _simple_layout(['a', 'a.a1', 'a.a1.a2', 'b'], {"a": "a", "b": "b"}, ".")
+    True
+    >>> _simple_layout(['a', 'a.a1', 'a.a1.a2', 'b'], {"a": "_a", "b": "_b"}, ".")
+    False
+    >>> _simple_layout(['a', 'a.a1', 'a.a1.a2', 'b'], {"a": "_a"}, "/tmp/myproj")
+    False
+    >>> _simple_layout(['a', 'a.a1', 'a.a1.a2', 'b'], {"a.a1.a2": "_a2"}, ".")
+    False
+    >>> _simple_layout(['a', 'a.b'], {"": "src", "a.b": "_ab"}, "/tmp/myproj")
+    False
+    """
+    layout = {
+        pkg: find_package_path(pkg, package_dir, project_dir)
+        for pkg in packages
+    }
+    if not layout:
+        return False
+    parent = os.path.commonpath([_parent_path(k, v) for k, v in layout.items()])
+    return all(
+        _normalize_path(Path(parent, *key.split('.'))) == _normalize_path(value)
+        for key, value in layout.items()
+    )
+
+
+def _parent_path(pkg, pkg_path):
+    """Infer the parent path containing a package, that if added to ``sys.path`` would
+    allow importing that package.
+    When ``pkg`` is directly mapped into a directory with a different name, return its
+    own path.
+    >>> _parent_path("a", "src/a")
+    'src'
+    >>> _parent_path("b", "src/c")
+    'src/c'
+    """
+    parent = pkg_path[:-len(pkg)] if pkg_path.endswith(pkg) else pkg_path
+    return parent.rstrip("/" + os.sep)
+
+
+def _find_packages(dist: Distribution) -> Iterator[str]:
+    yield from iter(dist.packages or [])
+
+    py_modules = dist.py_modules or []
+    nested_modules = [mod for mod in py_modules if "." in mod]
+    if dist.ext_package:
+        yield dist.ext_package
+    else:
+        ext_modules = dist.ext_modules or []
+        nested_modules += [x.name for x in ext_modules if "." in x.name]
+
+    for module in nested_modules:
+        package, _, _ = module.rpartition(".")
+        yield package
+
+
+def _find_top_level_modules(dist: Distribution) -> Iterator[str]:
+    py_modules = dist.py_modules or []
+    yield from (mod for mod in py_modules if "." not in mod)
+
+    if not dist.ext_package:
+        ext_modules = dist.ext_modules or []
+        yield from (x.name for x in ext_modules if "." not in x.name)
+
+
+def _find_package_roots(
+    packages: Iterable[str],
+    package_dir: Mapping[str, str],
+    src_root: _Path,
+) -> Dict[str, str]:
+    pkg_roots: Dict[str, str] = {
+        pkg: _absolute_root(find_package_path(pkg, package_dir, src_root))
+        for pkg in sorted(packages)
+    }
+
+    return _remove_nested(pkg_roots)
+
+
+def _absolute_root(path: _Path) -> str:
+    """Works for packages and top-level modules"""
+    path_ = Path(path)
+    parent = path_.parent
+
+    if path_.exists():
+        return str(path_.resolve())
+    else:
+        return str(parent.resolve() / path_.name)
+
+
+def _find_virtual_namespaces(pkg_roots: Dict[str, str]) -> Iterator[str]:
+    """By carefully designing ``package_dir``, it is possible to implement the logical
+    structure of PEP 420 in a package without the corresponding directories.
+    This function will try to find this kind of namespaces.
+    """
+    for pkg in pkg_roots:
+        if "." not in pkg:
+            continue
+        parts = pkg.split(".")
+        for i in range(len(parts) - 1, 0, -1):
+            partial_name = ".".join(parts[:i])
+            path = Path(find_package_path(partial_name, pkg_roots, ""))
+            if not path.exists():
+                yield partial_name
+
+
+def _find_namespaces(
+    packages: List[str], pkg_roots: Dict[str, str]
+) -> Iterator[Tuple[str, List[str]]]:
+    for pkg in packages:
+        path = find_package_path(pkg, pkg_roots, "")
+        if Path(path).exists() and not Path(path, "__init__.py").exists():
+            yield (pkg, [path])
+
+
+def _remove_nested(pkg_roots: Dict[str, str]) -> Dict[str, str]:
+    output = dict(pkg_roots.copy())
+
+    for pkg, path in reversed(list(pkg_roots.items())):
+        if any(
+            pkg != other and _is_nested(pkg, path, other, other_path)
+            for other, other_path in pkg_roots.items()
+        ):
+            output.pop(pkg)
+
+    return output
+
+
+def _is_nested(pkg: str, pkg_path: str, parent: str, parent_path: str) -> bool:
+    """
+    Return ``True`` if ``pkg`` is nested inside ``parent`` both logically and in the
+    file system.
+    >>> _is_nested("a.b", "path/a/b", "a", "path/a")
+    True
+    >>> _is_nested("a.b", "path/a/b", "a", "otherpath/a")
+    False
+    >>> _is_nested("a.b", "path/a/b", "c", "path/c")
+    False
+    """
+    norm_pkg_path = _normalize_path(pkg_path)
+    rest = pkg.replace(parent, "").strip(".").split(".")
+    return (
+        pkg.startswith(parent)
+        and norm_pkg_path == _normalize_path(Path(parent_path, *rest))
+    )
+
+
+def _normalize_path(filename: _Path) -> str:
+    """Normalize a file/dir name for comparison purposes"""
+    # See pkg_resources.normalize_path
+    file = os.path.abspath(filename) if sys.platform == 'cygwin' else filename
+    return os.path.normcase(os.path.realpath(os.path.normpath(file)))
+
+
+def _empty_dir(dir_: _P) -> _P:
+    """Create a directory ensured to be empty. Existing files may be removed."""
+    shutil.rmtree(dir_, ignore_errors=True)
+    os.makedirs(dir_)
+    return dir_
+
+
+def _make_identifier(name: str) -> str:
+    """Make a string safe to be used as Python identifier.
+    >>> _make_identifier("12abc")
+    '_12abc'
+    >>> _make_identifier("__editable__.myns.pkg-78.9.3_local")
+    '__editable___myns_pkg_78_9_3_local'
+    """
+    safe = re.sub(r'\W|^(?=\d)', '_', name)
+    assert safe.isidentifier()
+    return safe
+
+
+class _NamespaceInstaller(namespaces.Installer):
+    def __init__(self, distribution, installation_dir, editable_name, src_root):
+        self.distribution = distribution
+        self.src_root = src_root
+        self.installation_dir = installation_dir
+        self.editable_name = editable_name
+        self.outputs = []
+
+    def _get_target(self):
+        """Installation target."""
+        return os.path.join(self.installation_dir, self.editable_name)
+
+    def _get_root(self):
+        """Where the modules/packages should be loaded from."""
+        return repr(str(self.src_root))
+
+
+_FINDER_TEMPLATE = """\
+import sys
+from importlib.machinery import ModuleSpec
+from importlib.machinery import all_suffixes as module_suffixes
+from importlib.util import spec_from_file_location
+from itertools import chain
+from pathlib import Path
+
+MAPPING = {mapping!r}
+NAMESPACES = {namespaces!r}
+PATH_PLACEHOLDER = {name!r} + ".__path_hook__"
+
+
+class _EditableFinder:  # MetaPathFinder
+    @classmethod
+    def find_spec(cls, fullname, path=None, target=None):
+        for pkg, pkg_path in reversed(list(MAPPING.items())):
+            if fullname.startswith(pkg):
+                rest = fullname.replace(pkg, "").strip(".").split(".")
+                return cls._find_spec(fullname, Path(pkg_path, *rest))
+
+        return None
+
+    @classmethod
+    def _find_spec(cls, fullname, candidate_path):
+        init = candidate_path / "__init__.py"
+        candidates = (candidate_path.with_suffix(x) for x in module_suffixes())
+        for candidate in chain([init], candidates):
+            if candidate.exists():
+                return spec_from_file_location(fullname, candidate)
+
+
+class _EditableNamespaceFinder:  # PathEntryFinder
+    @classmethod
+    def _path_hook(cls, path):
+        if path == PATH_PLACEHOLDER:
+            return cls
+        raise ImportError
+
+    @classmethod
+    def _paths(cls, fullname):
+        # Ensure __path__ is not empty for the spec to be considered a namespace.
+        return NAMESPACES[fullname] or MAPPING.get(fullname) or [PATH_PLACEHOLDER]
+
+    @classmethod
+    def find_spec(cls, fullname, target=None):
+        if fullname in NAMESPACES:
+            spec = ModuleSpec(fullname, None, is_package=True)
+            spec.submodule_search_locations = cls._paths(fullname)
+            return spec
+        return None
+
+    @classmethod
+    def find_module(cls, fullname):
+        return None
+
+
+def install():
+    if not any(finder == _EditableFinder for finder in sys.meta_path):
+        sys.meta_path.append(_EditableFinder)
+
+    if not NAMESPACES:
+        return
+
+    if not any(hook == _EditableNamespaceFinder._path_hook for hook in sys.path_hooks):
+        # PathEntryFinder is needed to create NamespaceSpec without private APIS
+        sys.path_hooks.append(_EditableNamespaceFinder._path_hook)
+    if PATH_PLACEHOLDER not in sys.path:
+        sys.path.append(PATH_PLACEHOLDER)  # Used just to trigger the path hook
+"""
+
+
+def _finder_template(
+    name: str, mapping: Mapping[str, str], namespaces: Dict[str, List[str]]
+) -> str:
+    """Create a string containing the code for the``MetaPathFinder`` and
+    ``PathEntryFinder``.
+    """
+    mapping = dict(sorted(mapping.items(), key=lambda p: p[0]))
+    return _FINDER_TEMPLATE.format(name=name, mapping=mapping, namespaces=namespaces)
+
+
+class InformationOnly(UserWarning):
+    """Currently there is no clear way of displaying messages to the users
+    that use the setuptools backend directly via ``pip``.
+    The only thing that might work is a warning, although it is not the
+    most appropriate tool for the job...
+    """
+
+
+class LinksNotSupported(errors.FileError):
+    """File system does not seem to support either symlinks or hard links."""
diff --git a/setuptools/command/egg_info.py b/setuptools/command/egg_info.py
index 42a0178f..25888ed8 100644
--- a/setuptools/command/egg_info.py
+++ b/setuptools/command/egg_info.py
@@ -182,6 +182,7 @@ class egg_info(InfoCommon, Command):
         self.egg_info = None
         self.egg_version = None
         self.broken_egg_info = False
+        self.ignore_egg_info_in_manifest = False
 
     ####################################
     # allow the 'tag_svn_revision' to be detected and
@@ -310,6 +311,7 @@ class egg_info(InfoCommon, Command):
         """Generate SOURCES.txt manifest file"""
         manifest_filename = os.path.join(self.egg_info, "SOURCES.txt")
         mm = manifest_maker(self.distribution)
+        mm.ignore_egg_info_dir = self.ignore_egg_info_in_manifest
         mm.manifest = manifest_filename
         mm.run()
         self.filelist = mm.filelist
@@ -333,6 +335,10 @@ class egg_info(InfoCommon, Command):
 class FileList(_FileList):
     # Implementations of the various MANIFEST.in commands
 
+    def __init__(self, warn=None, debug_print=None, ignore_egg_info_dir=False):
+        super().__init__(warn, debug_print)
+        self.ignore_egg_info_dir = ignore_egg_info_dir
+
     def process_template_line(self, line):
         # Parse the line: split it up, make sure the right number of words
         # is there, and return the relevant words.  'action' is always
@@ -522,6 +528,10 @@ class FileList(_FileList):
             return False
 
         try:
+            # ignore egg-info paths
+            is_egg_info = ".egg-info" in u_path or b".egg-info" in utf8_path
+            if self.ignore_egg_info_dir and is_egg_info:
+                return False
             # accept is either way checks out
             if os.path.exists(u_path) or os.path.exists(utf8_path):
                 return True
@@ -538,12 +548,13 @@ class manifest_maker(sdist):
         self.prune = 1
         self.manifest_only = 1
         self.force_manifest = 1
+        self.ignore_egg_info_dir = False
 
     def finalize_options(self):
         pass
 
     def run(self):
-        self.filelist = FileList()
+        self.filelist = FileList(ignore_egg_info_dir=self.ignore_egg_info_dir)
         if not os.path.exists(self.manifest):
             self.write_manifest()  # it must exist so it'll get in the list
         self.add_defaults()
diff --git a/setuptools/command/sdist.py b/setuptools/command/sdist.py
index 0ffeacf3..4a8cde7e 100644
--- a/setuptools/command/sdist.py
+++ b/setuptools/command/sdist.py
@@ -4,10 +4,12 @@ import os
 import sys
 import io
 import contextlib
+from itertools import chain
 
 from .py36compat import sdist_add_defaults
 
 from .._importlib import metadata
+from .build import _ORIGINAL_SUBCOMMANDS
 
 _default_revctrl = list
 
@@ -100,6 +102,10 @@ class sdist(sdist_add_defaults, orig.sdist):
             if orig_val is not NoValue:
                 setattr(os, 'link', orig_val)
 
+    def add_defaults(self):
+        super().add_defaults()
+        self._add_defaults_build_sub_commands()
+
     def _add_defaults_optional(self):
         super()._add_defaults_optional()
         if os.path.isfile('pyproject.toml'):
@@ -112,6 +118,14 @@ class sdist(sdist_add_defaults, orig.sdist):
             self.filelist.extend(build_py.get_source_files())
             self._add_data_files(self._safe_data_files(build_py))
 
+    def _add_defaults_build_sub_commands(self):
+        build = self.get_finalized_command("build")
+        missing_cmds = set(build.get_sub_commands()) - _ORIGINAL_SUBCOMMANDS
+        # ^-- the original built-in sub-commands are already handled by default.
+        cmds = (self.get_finalized_command(c) for c in missing_cmds)
+        files = (c.get_source_files() for c in cmds if hasattr(c, "get_source_files"))
+        self.filelist.extend(chain.from_iterable(files))
+
     def _safe_data_files(self, build_py):
         """
         Since the ``sdist`` class is also used to compute the MANIFEST