Fallback to debug-printing generic info

crates/rune/src/runtime/shared.rs

@@ -205,6 +205,12 @@ impl<T: ?Sized> Shared<T> {
     }
 }
 
+impl<T: ?Sized> fmt::Pointer for Shared<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Pointer::fmt(&self.inner.as_ptr(), fmt)
+    }
+}
+
 impl<T: ?Sized> TryClone for Shared<T> {
     #[inline]
     fn try_clone(&self) -> alloc::Result<Self> {

crates/rune/src/runtime/value.rs

@@ -851,11 +851,16 @@ impl Value {
                 vm_write!(f, "{:?}", value);
             }
             _ => {
+                // reborrow f to avoid moving it
                 let result =
-                    vm_try!(caller.call_protocol_fn(Protocol::STRING_DEBUG, self.clone(), (f,)));
+                    caller.call_protocol_fn(Protocol::STRING_DEBUG, self.clone(), (&mut *f,));
 
-                vm_try!(<()>::from_value(result));
-                return VmResult::Ok(());
+                if let VmResult::Ok(result) = result {
+                    vm_try!(<()>::from_value(result));
+                } else {
+                    let type_info = vm_try!(self.type_info());
+                    vm_write!(f, "<{} object at {:p}>", type_info, self.inner);
+                }
             }
         };
 

crates/rune/src/tests.rs

@@ -6,6 +6,7 @@
 pub(crate) mod prelude {
     pub(crate) use crate as rune;
     pub(crate) use crate::alloc;
+    pub(crate) use crate::alloc::fmt::TryWrite;
     pub(crate) use crate::alloc::prelude::*;
     pub(crate) use crate::ast;
     pub(crate) use crate::compile::{self, ErrorKind, Item, ItemBuf, Located, Named};
@@ -14,9 +15,9 @@ pub(crate) mod prelude {
     pub(crate) use crate::module::InstallWith;
     pub(crate) use crate::parse;
     pub(crate) use crate::runtime::{
-        self, AnyTypeInfo, Bytes, FullTypeOf, Function, MaybeTypeOf, Mut, Object, OwnedTuple,
-        Protocol, RawRef, RawStr, Ref, Stack, Tuple, TypeInfo, TypeOf, UnsafeToRef, ValueKind,
-        VecTuple, VmErrorKind, VmResult,
+        self, AnyTypeInfo, Bytes, Formatter, FullTypeOf, Function, MaybeTypeOf, Mut, Object,
+        OwnedTuple, Protocol, RawRef, RawStr, Ref, Stack, Tuple, TypeInfo, TypeOf, UnsafeToRef,
+        ValueKind, VecTuple, VmErrorKind, VmResult,
     };
     pub(crate) use crate::support::Result;
     pub(crate) use crate::tests::run;
@@ -450,6 +451,7 @@ mod reference_error;
 mod rename_type;
 mod result;
 mod stmt_reordering;
+mod string_debug;
 mod tuple;
 mod type_name_native;
 mod type_name_rune;

crates/rune/src/tests/string_debug.rs

@@ -0,0 +1,60 @@
+//! Tests for `std::any::type_name_of_val(v)` for native types
+
+prelude!();
+
+#[derive(Any, Debug)]
+#[rune(item = ::native_crate)]
+pub struct NativeStructWithProtocol;
+
+impl NativeStructWithProtocol {
+    #[rune::function(protocol = STRING_DEBUG)]
+    fn string_debug(&self, f: &mut Formatter) -> VmResult<()> {
+        vm_write!(f, "{:?}", self);
+
+        VmResult::Ok(())
+    }
+}
+
+#[derive(Any)]
+#[rune(item = ::native_crate)]
+pub struct NativeStructWithoutProtocol;
+
+fn make_native_module() -> Result<Module, ContextError> {
+    let mut module = Module::with_crate("native_crate")?;
+    module.ty::<NativeStructWithProtocol>()?;
+    module.function_meta(NativeStructWithProtocol::string_debug)?;
+    module.ty::<NativeStructWithoutProtocol>()?;
+
+    Ok(module)
+}
+
+#[test]
+fn test_with_string_debug() {
+    let t1 = NativeStructWithProtocol;
+    assert_eq!(
+        rune_n! {
+            make_native_module().expect("failed making native module"),
+            (t1, ),
+            String =>
+            pub fn main(v) { format!("{:?}", v) }
+        },
+        "NativeStructWithProtocol"
+    );
+}
+
+#[test]
+fn test_without_string_debug() {
+    let t1 = NativeStructWithoutProtocol;
+    let result = rune_n! {
+        make_native_module().expect("failed making native module"),
+        (t1, ),
+        String =>
+            pub fn main(v) { format!("{:?}", v) }
+    };
+
+    assert!(
+        result.starts_with("<NativeStructWithoutProtocol object at 0x"),
+        "Expected '<NativeStructWithoutProtocol object at 0x', got: {:?}",
+        result
+    );
+}