Deprecate install fallback when bdist_wheel fails

Author: sbidoul

news/8368.removal

@@ -0,0 +1,2 @@
+Deprecate legacy setup.py install when building a wheel failed for source
+distributions without pyproject.toml

src/pip/_internal/commands/install.py

@@ -359,9 +359,6 @@ def run(self, options, args):
 
             # If we're using PEP 517, we cannot do a direct install
             # so we fail here.
-            # We don't care about failures building legacy
-            # requirements, as we'll fall through to a direct
-            # install for those.
             pep517_build_failures = [
                 r for r in build_failures if r.use_pep517
             ]
@@ -371,6 +368,26 @@ def run(self, options, args):
                     " PEP 517 and cannot be installed directly".format(
                         ", ".join(r.name for r in pep517_build_failures)))
 
+            # For now, we just warn about failures building legacy
+            # requirements, as we'll fall through to a direct
+            # install for those.
+            legacy_build_failures = [
+                r for r in build_failures if not r.use_pep517
+            ]
+            if legacy_build_failures:
+                deprecated(
+                    reason=(
+                        "Could not build wheels for {} which do not use "
+                        "PEP 517. pip will fall back to legacy setup.py "
+                        "install for these.".format(
+                            ", ".join(r.name for r in legacy_build_failures)
+                        )
+                    ),
+                    replacement="to fix the wheel build issue reported above",
+                    gone_in="20.3",
+                    issue=8368,
+                )
+
             to_install = resolver.get_installation_order(
                 requirement_set
             )