Display dependency chain on each Collecting line

This tremendously helps understand why a package is being fetched and
can help investigate and fix dependency resolver backtracking issues
when incoherent constraints/package sets are provided or when new
versions of a package trigger a completely different backtracking
strategy, leading to very hard to debug situations.

Author: iXce

news/C0DAB099-D9CB-438B-9091-146533DD4741.trivial.rst

@@ -269,6 +269,16 @@ def _log_preparing_link(self, req: InstallRequirement) -> None:
             message = "Collecting %s"
             information = str(req.req or req)
 
+        # If we used req.req, inject requirement source if available (this
+        # would already be included if we used req directly)
+        if req.req and req.comes_from:
+            if isinstance(req.comes_from, str):
+                comes_from: Optional[str] = req.comes_from
+            else:
+                comes_from = req.comes_from.from_path()
+            if comes_from:
+                information += f" (from {comes_from})"
+
         if (message, information) != self._previous_requirement_header:
             self._previous_requirement_header = (message, information)
             logger.info(message, information)

src/pip/_internal/operations/prepare.py

@@ -2243,3 +2243,54 @@ def test_install_logs_pip_version_in_debug(
     result = script.pip("install", "-v", fake_package)
     pattern = "Using pip .* from .*"
     assert_re_match(pattern, result.stdout)
+
+
+def test_install_pip_prints_req_chain_local(script: PipTestEnvironment) -> None:
+    """
+    Test installing a local package with a dependency and check that the
+    dependency chain is reported.
+    """
+
+    req_path = script.scratch_path.joinpath("requirements.txt")
+    req_path.write_text("base==0.1.0")
+
+    create_basic_wheel_for_package(
+        script,
+        "base",
+        "0.1.0",
+        depends=["dep"],
+    )
+    dep_path = create_basic_wheel_for_package(
+        script,
+        "dep",
+        "0.1.0",
+    )
+
+    result = script.pip(
+        "install",
+        "--no-cache-dir",
+        "--no-index",
+        "--find-links",
+        script.scratch_path,
+        "-r",
+        req_path,
+    )
+    assert f"Processing ./{os.path.basename(dep_path)} (from base==0.1.0->-r {req_path} (line 1))" in result.stdout
+
+
+@pytest.mark.network
+def test_install_pip_prints_req_chain_pypi(script: PipTestEnvironment) -> None:
+    """
+    Test installing a package with a dependency from PyPI and check that the
+    dependency chain is reported.
+    """
+    req_path = script.scratch_path.joinpath("requirements.txt")
+    req_path.write_text("Paste[openid]==1.7.5.1")
+
+    result = script.pip(
+        "install",
+        "-r",
+        req_path,
+    )
+
+    assert f"Collecting python-openid (from Paste[openid]==1.7.5.1->-r {req_path} (line 1))" in result.stdout