Compute NotebookIndex for Diagnostics on stdin (#7663)

## Summary

This PR fixes the bug where the `NotebookIndex` was not being computed
when
using stdin as the input source.

## Test Plan

On `main`, the diagnostic output won't include the cell number when
using stdin
while it'll be included after this fix.

### `main`

```console
$ cat ~/playground/ruff/notebooks/test.ipynb | cargo run --bin ruff -- check --isolated --no-cache - --stdin-filename ~/playground/ruff/notebooks/test.ipynb
/Users/dhruv/playground/ruff/notebooks/test.ipynb:2:8: F401 [*] `math` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:7:8: F811 Redefinition of unused `random` from line 1
/Users/dhruv/playground/ruff/notebooks/test.ipynb:8:8: F401 [*] `pprint` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:12:4: F632 [*] Use `==` to compare constant literals
/Users/dhruv/playground/ruff/notebooks/test.ipynb:13:38: F632 [*] Use `==` to compare constant literals
Found 5 errors.
[*] 4 potentially fixable with the --fix option.
```

### `dhruv/notebook-index-stdin`

```console
$ cat ~/playground/ruff/notebooks/test.ipynb | cargo run --bin ruff -- check --isolated --no-cache - --stdin-filename ~/playground/ruff/notebooks/test.ipynb       
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 3:2:8: F401 [*] `math` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 5:1:8: F811 Redefinition of unused `random` from line 1
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 5:2:8: F401 [*] `pprint` imported but unused
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 6:2:4: F632 [*] Use `==` to compare constant literals
/Users/dhruv/playground/ruff/notebooks/test.ipynb:cell 6:3:38: F632 [*] Use `==` to compare constant literals
Found 5 errors.
[*] 4 potentially fixable with the --fix option.
```

crates/ruff_cli/src/diagnostics.rs

@@ -8,7 +8,7 @@ use std::ops::AddAssign;
 use std::os::unix::fs::PermissionsExt;
 use std::path::Path;
 
-use anyhow::{anyhow, Context, Result};
+use anyhow::{Context, Result};
 use colored::Colorize;
 use filetime::FileTime;
 use log::{debug, error, warn};
@@ -342,13 +342,7 @@ pub(crate) fn lint_path(
     }
 
     let notebook_indexes = if let SourceKind::IpyNotebook(notebook) = source_kind {
-        FxHashMap::from_iter([(
-            path.to_str()
-                .ok_or_else(|| anyhow!("Unable to parse filename: {:?}", path))?
-                .to_string(),
-            // Index needs to be computed always to store in cache.
-            notebook.index().clone(),
-        )])
+        FxHashMap::from_iter([(path.to_string_lossy().to_string(), notebook.into_index())])
     } else {
         FxHashMap::default()
     };
@@ -474,14 +468,23 @@ pub(crate) fn lint_stdin(
         );
     }
 
+    let notebook_indexes = if let SourceKind::IpyNotebook(notebook) = source_kind {
+        FxHashMap::from_iter([(
+            path.map_or_else(|| "-".into(), |path| path.to_string_lossy().to_string()),
+            notebook.into_index(),
+        )])
+    } else {
+        FxHashMap::default()
+    };
+
     Ok(Diagnostics {
         messages,
         fixed: FxHashMap::from_iter([(
             fs::relativize_path(path.unwrap_or_else(|| Path::new("-"))),
             fixed,
         )]),
         imports,
-        notebook_indexes: FxHashMap::default(),
+        notebook_indexes,
     })
 }
 

crates/ruff_notebook/src/notebook.rs

@@ -369,6 +369,15 @@ impl Notebook {
         self.index.get_or_init(|| self.build_index())
     }
 
+    /// Return the Jupyter notebook index, consuming the notebook.
+    ///
+    /// The index is built only once when required. This is only used to
+    /// report diagnostics, so by that time all of the fixes must have
+    /// been applied if `--fix` was passed.
+    pub fn into_index(mut self) -> NotebookIndex {
+        self.index.take().unwrap_or_else(|| self.build_index())
+    }
+
     /// Return the cell offsets for the concatenated source code corresponding
     /// the Jupyter notebook.
     pub fn cell_offsets(&self) -> &[TextSize] {