add test for the *existing* `install --dry-run` functionality

tests/functional/test_install.py

@@ -7,7 +7,7 @@ import sysconfig
 import textwrap
 from os.path import curdir, join, pardir
 from pathlib import Path
-from typing import Dict, List, Tuple
+from typing import Callable, Dict, Iterable, List, Optional, Tuple
 
 import pytest
 
@@ -20,6 +20,7 @@ from tests.lib import (
     PipTestEnvironment,
     ResolverVariant,
     TestData,
+    TestPipResult,
     _create_svn_repo,
     _create_test_package,
     create_basic_wheel_for_package,
@@ -2371,14 +2372,68 @@ def test_install_logs_pip_version_in_debug(
     assert_re_match(pattern, result.stdout)
 
 
-def test_install_dry_run(script: PipTestEnvironment, data: TestData) -> None:
-    """Test that pip install --dry-run logs what it would install."""
-    result = script.pip(
-        "install", "--dry-run", "--find-links", data.find_links, "simple"
-    )
+@pytest.fixture
+def install_find_links(
+    script: PipTestEnvironment,
+    data: TestData,
+) -> Callable[[Iterable[str], bool, Optional[Path]], TestPipResult]:
+    def install(
+        args: Iterable[str], dry_run: bool, target_dir: Optional[Path]
+    ) -> TestPipResult:
+        return script.pip(
+            "install",
+            *(
+                (
+                    "--target",
+                    str(target_dir),
+                )
+                if target_dir is not None
+                else ()
+            ),
+            *(("--dry-run",) if dry_run else ()),
+            "--no-index",
+            "--find-links",
+            data.find_links,
+            *args,
+        )
+
+    return install
+
+
+@pytest.mark.parametrize(
+    "with_target_dir",
+    (True, False),
+)
+def test_install_dry_run_nothing_installed(
+    script: PipTestEnvironment,
+    tmpdir: Path,
+    install_find_links: Callable[[Iterable[str], bool, Optional[Path]], TestPipResult],
+    with_target_dir: bool,
+) -> None:
+    """Test that pip install --dry-run logs what it would install, but doesn't actually
+    install anything."""
+    if with_target_dir:
+        install_dir = tmpdir / "fake-install"
+        install_dir.mkdir()
+    else:
+        install_dir = None
+
+    result = install_find_links(["simple"], True, install_dir)
     assert "Would install simple-3.0" in result.stdout
     assert "Successfully installed" not in result.stdout
 
+    script.assert_not_installed("simple")
+    if with_target_dir:
+        assert not os.listdir(install_dir)
+
+    # Ensure that the same install command would normally have worked if not for
+    # --dry-run.
+    install_find_links(["simple"], False, install_dir)
+    if with_target_dir:
+        assert os.listdir(install_dir)
+    else:
+        script.assert_installed(simple="3.0")
+
 
 @pytest.mark.skipif(
     sys.version_info < (3, 11),