Deprecate pip-compile --resolver=legacy (#1724)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Sorin Sbarnea <sorin.sbarnea@gmail.com>
Resolves #1659

README.rst

@@ -571,6 +571,8 @@ This section lists ``pip-tools`` features that are currently deprecated.
   default. Use ``--no-allow-unsafe`` to keep the old behavior. It is
   recommended to pass the ``--allow-unsafe`` now to adapt to the upcoming
   change.
+- Legacy resolver is deprecated and will be removed in future versions.
+  Use ``--resolver=backtracking`` instead.
 
 A Note on Resolvers
 ===================

piptools/scripts/compile.py

@@ -378,6 +378,13 @@ def cli(
         if isinstance(output_file, LazyFile):  # pragma: no cover
             ctx.call_on_close(safecall(output_file.close_intelligently))
 
+    if resolver_name == "legacy":
+        log.warning(
+            "WARNING: using legacy resolver is deprecated and will be removed in "
+            "future versions. The default resolver will be change to 'backtracking' "
+            "in 7.0.0 version. Specify --resolver=backtracking to silence this warning."
+        )
+
     ###
     # Setup
     ###

tests/test_cli_compile.py

@@ -2546,3 +2546,17 @@ def test_failure_of_legacy_resolver_prompts_for_backtracking(
         assert out.exit_code == 0, out
     else:  # pragma: no cover
         raise AssertionError("unreachable")
+
+
+def test_print_deprecation_warning_if_using_legacy_resolver(runner, current_resolver):
+    with open("requirements.in", "w"):
+        pass
+
+    out = runner.invoke(cli)
+    assert out.exit_code == 0, out
+
+    expected_warning = "WARNING: using legacy resolver is deprecated"
+    if current_resolver == "legacy":
+        assert expected_warning in out.stderr
+    else:
+        assert expected_warning not in out.stderr