Attribute panics to the mdtests that cause them (#15241)

This updates the mdtest harness to catch any panics that occur during
type checking, and to display the panic message as an mdtest failure.
(We don't know which specific line causes the failure, so we attribute
panics to the first line of the test case.)

crates/ruff_db/src/lib.rs

@@ -10,6 +10,7 @@ pub mod diagnostic;
 pub mod display;
 pub mod file_revision;
 pub mod files;
+pub mod panic;
 pub mod parsed;
 pub mod source;
 pub mod system;

crates/ruff_db/src/panic.rs

@@ -0,0 +1,46 @@
+#[derive(Default, Debug)]
+pub struct PanicError {
+    pub info: String,
+    pub backtrace: Option<std::backtrace::Backtrace>,
+}
+
+impl std::fmt::Display for PanicError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        writeln!(f, "{}", self.info)?;
+        if let Some(backtrace) = &self.backtrace {
+            writeln!(f, "Backtrace: {backtrace}")
+        } else {
+            Ok(())
+        }
+    }
+}
+
+thread_local! {
+    static LAST_PANIC: std::cell::Cell<Option<PanicError>> = const { std::cell::Cell::new(None) };
+}
+
+/// [`catch_unwind`](std::panic::catch_unwind) wrapper that sets a custom [`set_hook`](std::panic::set_hook)
+/// to extract the backtrace. The original panic-hook gets restored before returning.
+pub fn catch_unwind<F, R>(f: F) -> Result<R, PanicError>
+where
+    F: FnOnce() -> R + std::panic::UnwindSafe,
+{
+    let prev = std::panic::take_hook();
+    std::panic::set_hook(Box::new(|info| {
+        let info = info.to_string();
+        let backtrace = std::backtrace::Backtrace::force_capture();
+        LAST_PANIC.with(|cell| {
+            cell.set(Some(PanicError {
+                info,
+                backtrace: Some(backtrace),
+            }));
+        });
+    }));
+
+    let result = std::panic::catch_unwind(f)
+        .map_err(|_| LAST_PANIC.with(std::cell::Cell::take).unwrap_or_default());
+
+    std::panic::set_hook(prev);
+
+    result
+}