melib: add as_any() method to MailBackend trait

Cast the trait object into an &Any object. Then we can downcast it to
its actual type with downcast_ref().

melib/src/backends.rs

@@ -36,6 +36,7 @@ use self::maildir::MaildirType;
 #[cfg(feature = "mbox_backend")]
 use self::mbox::MboxType;
 use super::email::{Envelope, EnvelopeHash, Flag};
+use std::any::Any;
 use std::fmt;
 use std::fmt::Debug;
 use std::ops::Deref;
@@ -191,6 +192,8 @@ pub trait MailBackend: ::std::fmt::Debug + Send + Sync {
     fn folder_operation(&mut self, _path: &str, _op: FolderOperation) -> Result<()> {
         Ok(())
     }
+
+    fn as_any(&self) -> &dyn Any;
 }
 
 /// A `BackendOp` manages common operations for the various mail backends. They only live for the

melib/src/backends/imap.rs

@@ -358,6 +358,10 @@ impl MailBackend for ImapType {
         }
         Ok(())
     }
+
+    fn as_any(&self) -> &dyn::std::any::Any {
+        self
+    }
 }
 
 macro_rules! get_conf_val {

melib/src/backends/maildir/backend.rs

@@ -552,6 +552,10 @@ impl MailBackend for MaildirType {
             folder
         )))
     }
+
+    fn as_any(&self) -> &dyn::std::any::Any {
+        self
+    }
 }
 
 impl MaildirType {

melib/src/backends/mbox.rs

@@ -548,6 +548,10 @@ impl MailBackend for MboxType {
     fn save(&self, _bytes: &[u8], _folder: &str, _flags: Option<Flag>) -> Result<()> {
         unimplemented!();
     }
+
+    fn as_any(&self) -> &dyn::std::any::Any {
+        self
+    }
 }
 
 impl MboxType {