poetry run: deprecate uninstalled entry points

src/poetry/console/commands/run.py

@@ -63,6 +63,9 @@ def run_script(self, script: str | dict[str, str], args: list[str]) -> int:
             if script_path.exists():
                 args = [str(script_path), *args[1:]]
                 break
+        else:
+            # If we reach this point, the script is not installed
+            self._warning_not_installed_script(args[0])
 
         if isinstance(script, dict):
             script = script["callable"]
@@ -81,3 +84,14 @@ def run_script(self, script: str | dict[str, str], args: list[str]) -> int:
         ]
 
         return self.env.execute(*cmd)
+
+    def _warning_not_installed_script(self, script: str) -> None:
+        message = f"""\
+Warning: '{script}' is an entry point defined in pyproject.toml, but it's not \
+installed as a script. You may get improper `sys.argv[0]`.
+
+The support to run uninstalled scripts will be removed in a future release.
+
+Run `poetry install` to resolve and get rid of this message.
+"""
+        self.line_error(message, style="warning")

tests/console/commands/test_run.py

@@ -180,3 +180,17 @@ def test_run_script_sys_argv0(
     )
     argv1 = "absolute" if installed_script else "relative"
     assert tester.execute(f"check-argv0 {argv1}") == 0
+
+    if installed_script:
+        expected_message = ""
+    else:
+        expected_message = """\
+Warning: 'check-argv0' is an entry point defined in pyproject.toml, but it's not \
+installed as a script. You may get improper `sys.argv[0]`.
+
+The support to run uninstalled scripts will be removed in a future release.
+
+Run `poetry install` to resolve and get rid of this message.
+
+"""
+    assert tester.io.fetch_error() == expected_message