Use nested bridging when parse specialization fails.

Previously, we would always unwrap an error out of
`parse_rs_std_template_specialization`. For types like `Option<Proto>`
this would lead to failing to format the type because specialization
would try to format it in location `Other` when we need to format it as
`NestedBridgeable`. We fix this by only unwrapping our error when our
location doesn't admit bridging.

PiperOrigin-RevId: 896082192

Author: thunderseethe

cc_bindings_from_rs/generate_bindings/format_type.rs

@@ -292,12 +292,15 @@ pub fn format_ty_for_cc<'tcx>(
 
             let specialization = db.parse_rs_std_template_specialization(ty);
             if specialization.as_ref().is_some_and(|specialization| {
-                specialization.is_err()
-                    || specialization.as_ref().is_ok_and(|rs_std_enum| {
-                        (matches!(rs_std_enum.kind, TemplateSpecializationKind::Option { .. })
-                            && !location.is_bridgeable())
-                            || matches!(rs_std_enum.kind, TemplateSpecializationKind::Result { .. })
-                    })
+                // We only want to consider errors when bridging could not occur.
+                // Otherwise, fallthrough to the normal bridging logic.
+                let error_occurred = !location.is_bridgeable() && specialization.is_err();
+                let is_option_or_result = specialization.as_ref().is_ok_and(|rs_std_enum| {
+                    !location.is_bridgeable()
+                        && matches!(rs_std_enum.kind, TemplateSpecializationKind::Option { .. })
+                        || matches!(rs_std_enum.kind, TemplateSpecializationKind::Result { .. })
+                });
+                error_occurred || is_option_or_result
             }) {
                 let rs_std_enum = specialization.unwrap()?;
                 let tokens = rs_std_enum.core.self_ty_cc.clone().into_tokens(&mut prereqs);

cc_bindings_from_rs/generate_bindings/generate_struct_and_union_test.rs

@@ -89,7 +89,8 @@ fn test_format_bridged_type_in_generic_types() {
         let err = result.unwrap_err();
         assert_eq!(
             err,
-            "Error handling parameter #0 of type `std::option::Option<std::boxed::Box<std::result::Result<RustType, ()>>>`: Generic types are not supported yet (b/259749095)" 
+            "Error handling parameter #0 of type `std::option::Option<std::boxed::Box<std::result::Result<RustType, ()>>>`: \
+            Failed to construct CrubitAbiType for std::boxed::Box<std::result::Result<RustType, ()>> because it does not have a move ctor or assignment operator."
         );
     });
 }

cc_bindings_from_rs/test/enums/option.rs

@@ -171,3 +171,12 @@ pub struct OptionWithSizeTypes {
     pub uval: Option<usize>,
     pub ival: Option<isize>,
 }
+
+#[doc = "CRUBIT_ANNOTATE: cpp_type=int"]
+#[doc = "CRUBIT_ANNOTATE: include_path=unused_include.h"]
+#[repr(transparent)]
+pub struct BridgedType(i32);
+
+pub fn take_option_bridged(x: Option<BridgedType>) -> i32 {
+    x.map(|b| b.0).unwrap_or(-1)
+}

cc_bindings_from_rs/test/enums/option_cc_api.h

@@ -32,6 +32,12 @@
 
 namespace option {
 struct HasOptions;
+
+// Error generating bindings for `option_golden::BridgedType` defined at
+// cc_bindings_from_rs/test/enums/option.rs;l=178:
+// Type bindings for option_golden::BridgedType suppressed due to being mapped
+// to an existing C++ type (int)
+
 // Generated from:
 // cc_bindings_from_rs/test/enums/option.rs;l=120
 struct CRUBIT_INTERNAL_RUST_TYPE(":: option_golden :: CloneNoDefault") alignas(
@@ -297,6 +303,14 @@ OptionWithSizeTypes final {
 // cc_bindings_from_rs/test/enums/option.rs;l=158:
 // Zero-sized types (ZSTs) are not supported (b/258259459)
 
+// Error generating bindings for `option_golden::take_option_bridged` defined at
+// cc_bindings_from_rs/test/enums/option.rs;l=180:
+// Error handling parameter #0 of type
+// `std::option::Option<option_golden::BridgedType>`: Failed to format type for
+// the definition of `option_golden::BridgedType`: Error formatting the
+// fully-qualified C++ name of `BridgedType`: `int` is a C++ reserved keyword
+// and can't be used as a C++ identifier
+
 }  // namespace option
 
 #ifndef _CRUBIT_BINDINGS_FOR_rs_ustd_x00000020_x0000003a_x0000003a_x00000020Option_x00000020_x0000003c_x00000020_x0000003a_x0000003a_x00000020option_x00000020_x0000003a_x0000003a_x00000020CloneNoDefault_x00000020_x0000003e