Update env_var_to_ffi() to take an Option<EnvVar>

It wasn't possible to handle cases where vars.get() returned `None` then forward
that to C++, but now we can.

fish-rust/src/env/environment.rs

@@ -34,8 +34,12 @@ lazy_static! {
 static UVARS_LOCALLY_MODIFIED: RelaxedAtomicBool = RelaxedAtomicBool::new(false);
 
 /// Convert an EnvVar to an FFI env_var_t.
-fn env_var_to_ffi(var: EnvVar) -> cxx::UniquePtr<ffi::env_var_t> {
-    ffi::env_var_t::new_ffi(Box::into_raw(Box::from(var)).cast()).within_unique_ptr()
+pub fn env_var_to_ffi(var: Option<EnvVar>) -> cxx::UniquePtr<ffi::env_var_t> {
+    if let Some(var) = var {
+        ffi::env_var_t::new_ffi(Box::into_raw(Box::from(var)).cast()).within_unique_ptr()
+    } else {
+        cxx::UniquePtr::null()
+    }
 }
 
 /// An environment is read-only access to variable values.

fish-rust/src/flog.rs

@@ -150,7 +150,7 @@ pub trait FloggableDisplay {
 
 impl<T: std::fmt::Display> FloggableDisplay for T {
     fn to_flog_str(&self) -> String {
-        format!("{}", self)
+        self.to_string()
     }
 }