stop panic on to_string, report using write_unraisable instead

newsfragments/3060.fixed.md

@@ -0,0 +1 @@
+Stop panic on `fmt::Display`, instead return `"<unprintable object>"` string and report error via `sys.unraisablehook()`

src/gil.rs

@@ -463,6 +463,14 @@ pub(crate) fn ensure_gil_unchecked() -> EnsureGIL {
     }
 }
 
+/// Yields access to the GIL only if it is already held
+pub(crate) fn try_ensure_gil() -> Option<EnsureGIL> {
+    if gil_is_acquired() {
+        Some(EnsureGIL(None))
+    } else {
+        None
+    }
+}
 /// Struct used internally which avoids acquiring the GIL where it's not necessary.
 pub(crate) struct EnsureGIL(Option<GILGuard>);
 

src/marker.rs

@@ -354,6 +354,14 @@ impl Python<'_> {
     {
         f(gil::ensure_gil_unchecked().python())
     }
+
+    /// Similar to [`Self::with_gil`] but only provides access to the GIL if it is already held.
+    pub fn try_with_gil<F, R>(f: F) -> Option<R>
+    where
+        F: for<'py> FnOnce(Python<'py>) -> R,
+    {
+        gil::try_ensure_gil().map(|gil| f(unsafe { gil.python() }))
+    }
 }
 
 impl<'py> Python<'py> {

src/types/mod.rs

@@ -99,8 +99,15 @@ macro_rules! pyobject_native_type_base(
             fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>)
                    -> ::std::result::Result<(), ::std::fmt::Error>
             {
-                let s = self.str().or(::std::result::Result::Err(::std::fmt::Error))?;
-                f.write_str(&s.to_string_lossy())
+                match self.str() {
+                    ::std::result::Result::Ok(s) => return f.write_str(&s.to_string_lossy()),
+                    ::std::result::Result::Err(err) => err.write_unraisable(self.py(), ::std::option::Option::Some(self)),
+                }
+
+                match self.get_type().name() {
+                    ::std::result::Result::Ok(name) => ::std::write!(f, "<unprintable {} object>", name),
+                    ::std::result::Result::Err(_err) => f.write_str("<unprintable object>"),
+                }
             }
         }