Special-case PyPy's lib location differences

src/pip/_internal/locations/__init__.py

@@ -1,5 +1,6 @@
 import logging
 import pathlib
+import sys
 import sysconfig
 from typing import List, Optional
 
@@ -41,6 +42,19 @@ def _default_base(*, user: bool) -> str:
     return base
 
 
+def _pypy_home_lib_correct(old: pathlib.Path, new: pathlib.Path) -> bool:
+    """Special-case PyPy's purelib and platlib paths in a posix_home scheme.
+
+    distutils incorrectly put PyPy packages under ``../site-packages/python``
+    in the ``posix_home`` scheme, but PyPy devs said they expect the last
+    directory name to be ``pypy`` instead. So we treat this as a bug fix and
+    not warn about it.
+    """
+    if old.parent != new.parent:
+        return False
+    return old.name == "python" and new.name == "pypy"
+
+
 def _warn_if_mismatch(old: pathlib.Path, new: pathlib.Path, *, key: str) -> bool:
     if old == new:
         return False
@@ -99,6 +113,14 @@ def get_scheme(
         # Extra join because distutils can return relative paths.
         old_v = pathlib.Path(base, getattr(old, k))
         new_v = pathlib.Path(getattr(new, k))
+        skip_pypy_special_case = (
+            sys.implementation.name == "pypy"
+            and home is not None
+            and k in ("platlib", "purelib")
+            and _pypy_home_lib_correct(old_v, new_v)
+        )
+        if skip_pypy_special_case:
+            continue
         warned.append(_warn_if_mismatch(old_v, new_v, key=f"scheme.{k}"))
 
     if any(warned):