Allow transparent cell magics (#8911)

## Summary

This PR updates the logic for `is_magic_cell` to include certain cell
magics. These cell magics would contain Python code following the line
defining the command. The code could define a variable which can then be
referenced in other cells. Currently, we would ignore the cell
completely leading to undefined-name violation.

As discussed in
https://github.com/astral-sh/ruff/issues/8354#issuecomment-1832221009

## Test Plan

Add new test case to validate this scenario.

crates/ruff_linter/src/snapshots/ruff_linter__linter__tests__ipy_escape_command.snap

@@ -19,5 +19,20 @@ ipy_escape_command.ipynb:cell 1:5:8: F401 [*] `os` imported but unused
 5   |-import os
 6 5 | 
 7 6 | _ = math.pi
+8 7 | %%timeit
+
+ipy_escape_command.ipynb:cell 2:2:8: F401 [*] `sys` imported but unused
+  |
+1 | %%timeit
+2 | import sys
+  |        ^^^ F401
+  |
+  = help: Remove unused import: `sys`
+
+ℹ Safe fix
+6 6 | 
+7 7 | _ = math.pi
+8 8 | %%timeit
+9   |-import sys
 
 

crates/ruff_notebook/resources/test/fixtures/jupyter/cell/cell_magic.json

@@ -4,5 +4,10 @@
   "id": "1",
   "metadata": {},
   "outputs": [],
-  "source": ["%%timeit\n", "print('hello world')"]
+  "source": [
+    "%%script bash\n",
+    "for i in 1 2 3; do\n",
+    "  echo $i\n",
+    "done"
+  ]
 }

crates/ruff_notebook/resources/test/fixtures/jupyter/cell/valid_cell_magic.json

@@ -0,0 +1,11 @@
+{
+  "execution_count": null,
+  "cell_type": "code",
+  "id": "1",
+  "metadata": {},
+  "outputs": [],
+  "source": [
+    "%%timeit\n",
+    "print('hello world')"
+  ]
+}

crates/ruff_notebook/resources/test/fixtures/jupyter/ipy_escape_command.ipynb

@@ -26,6 +26,18 @@
     "%%timeit\n",
     "import sys"
    ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "36dedfd1-6c03-4894-bea6-6c1687b82b3c",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "%%random\n",
+    "# This cell is ignored\n",
+    "import pathlib"
+   ]
   }
  ],
  "metadata": {

crates/ruff_notebook/resources/test/fixtures/jupyter/ipy_escape_command_expected.ipynb

@@ -22,8 +22,19 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "%%timeit\n",
+    "%%timeit"
-    "import sys"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "4b6d7faa-72b3-4087-8670-fe6d35e41fb6",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "%%random\n",
+    "# This cell is ignored\n",
+    "import pathlib"
    ]
   }
  ],

crates/ruff_notebook/src/cell.rs

@@ -170,7 +170,50 @@ impl Cell {
         }
 
         // Detect cell magics (which operate on multiple lines).
-        lines.any(|line| line.trim_start().starts_with("%%"))
+        lines.any(|line| {
+            line.split_whitespace().next().is_some_and(|first| {
+                if first.len() < 2 {
+                    return false;
+                }
+                let (token, command) = first.split_at(2);
+                // These cell magics are special in that the lines following them are valid
+                // Python code and the variables defined in that scope are available to the
+                // rest of the notebook.
+                //
+                // For example:
+                //
+                // Cell 1:
+                // ```python
+                // x = 1
+                // ```
+                //
+                // Cell 2:
+                // ```python
+                // %%time
+                // y = x
+                // ```
+                //
+                // Cell 3:
+                // ```python
+                // print(y)  # Here, `y` is available.
+                // ```
+                //
+                // This is to avoid false positives when these variables are referenced
+                // elsewhere in the notebook.
+                token == "%%"
+                    && !matches!(
+                        command,
+                        "capture"
+                            | "debug"
+                            | "prun"
+                            | "pypy"
+                            | "python"
+                            | "python3"
+                            | "time"
+                            | "timeit"
+                    )
+            })
+        })
     }
 }
 

crates/ruff_notebook/src/notebook.rs

@@ -426,6 +426,7 @@ mod tests {
     #[test_case(Path::new("code_and_magic.json"), true; "code_and_magic")]
     #[test_case(Path::new("only_code.json"), true; "only_code")]
     #[test_case(Path::new("cell_magic.json"), false; "cell_magic")]
+    #[test_case(Path::new("valid_cell_magic.json"), true; "valid_cell_magic")]
     #[test_case(Path::new("automagic.json"), false; "automagic")]
     #[test_case(Path::new("automagics.json"), false; "automagics")]
     #[test_case(Path::new("automagic_before_code.json"), false; "automagic_before_code")]