Attempt to use a default method for a trait crashes with SIGSEGV

[mhammond]

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With the patch below, the Python test crashes with fish: Job 1, 'PYTHONPATH=/Users/skip/src/moz/…' terminated by signal SIGSEGV (Address boundary error)

(When running the test you just see a failure but no output - to see the output I needed to run PYTHONPATH=target/tmp/uniffi_proc_macro-xxx python fixtures/proc-macro/tests/bindings/test_proc_macro.py)

The patch:

diff --git a/fixtures/proc-macro/src/lib.rs b/fixtures/proc-macro/src/lib.rs
index 9d1f18b4ce..141199737d 100644
--- a/fixtures/proc-macro/src/lib.rs
+++ b/fixtures/proc-macro/src/lib.rs
@@ -58,6 +58,10 @@
     // incompatible with callback interfaces
     #[allow(clippy::ptr_arg)]
     fn concat_strings(&self, a: &str, b: &str) -> String;
+
+    fn optional(&self) -> String {
+        "trait".to_string()
+    }
 }
 
 struct TraitImpl {}
diff --git a/fixtures/proc-macro/tests/bindings/test_proc_macro.py b/fixtures/proc-macro/tests/bindings/test_proc_macro.py
index 6148f91dff..65625ec9d6 100644
--- a/fixtures/proc-macro/tests/bindings/test_proc_macro.py
+++ b/fixtures/proc-macro/tests/bindings/test_proc_macro.py
@@ -32,6 +32,7 @@
 assert concat_strings_by_ref(trait_impl, "foo", "bar") == "foobar"
 assert issubclass(StructWithTrait, Trait)
 assert StructWithTrait("me").concat_strings("foo", "bar") == "me: foobar"
+assert StructWithTrait("").optional() == "trait"
 
 trait_impl2 = obj.get_trait_with_foreign(None)
 # This is an instance of `TraitWithForeignImpl` - `TraitWithForeign` is used primarily for subclassing.
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