Prompt user to use the backtracking resolver on errors

piptools/scripts/compile.py

@@ -18,7 +18,7 @@
 
 from .._compat import IS_CLICK_VER_8_PLUS, parse_requirements
 from ..cache import DependencyCache
-from ..exceptions import PipToolsError
+from ..exceptions import NoCandidateFound, PipToolsError
 from ..locations import CACHE_DIR
 from ..logging import log
 from ..repositories import LocalRequirementsRepository, PyPIRepository
@@ -547,6 +547,16 @@ def cli(
         )
         results = resolver.resolve(max_rounds=max_rounds)
         hashes = resolver.resolve_hashes(results) if generate_hashes else None
+    except NoCandidateFound as e:
+        if resolver_cls == LegacyResolver:  # pragma: no branch
+            log.error(
+                "Using legacy resolver. "
+                "Consider using backtracking resolver with "
+                "`--resolver=backtracking`."
+            )
+
+        log.error(str(e))
+        sys.exit(2)
     except PipToolsError as e:
         log.error(str(e))
         sys.exit(2)

tests/test_cli_compile.py

@@ -2489,3 +2489,36 @@ def test_preserve_via_requirements_constrained_dependencies_when_run_twice(
     for output in (first_out, second_out):
         assert output.exit_code == 0, output
         assert output.stderr == expected_output
+
+
+def test_failure_of_legacy_resolver_prompts_for_backtracking(
+    pip_conf, runner, tmpdir, make_package, make_wheel, current_resolver
+):
+    """Test that pip-compile prompts to use the backtracking resolver"""
+    pkgs = [
+        make_package("a", version="0.1", install_requires=["b==0.1"]),
+        make_package("a", version="0.2", install_requires=["b==0.2"]),
+        make_package("b", version="0.1"),
+        make_package("b", version="0.2"),
+        make_package("c", version="1", install_requires=["b==0.1", "a"]),
+    ]
+
+    dists_dir = tmpdir / "dists"
+    for pkg in pkgs:
+        make_wheel(pkg, dists_dir)
+
+    with open("requirements.in", "w") as req_in:
+        req_in.writelines(["c"])
+
+    out = runner.invoke(
+        cli,
+        ["--resolver", current_resolver, "--find-links", str(dists_dir)],
+    )
+
+    if current_resolver == "legacy":
+        assert out.exit_code == 2, out
+        assert "Consider using backtracking resolver with" in out.stderr
+    elif current_resolver == "backtracking":
+        assert out.exit_code == 0, out
+    else:  # pragma: no cover
+        raise AssertionError("unreachable")