rollback rust edition (#303)

src/backends/imap/imap_flag.rs

@@ -1,5 +1,9 @@
 use anyhow::{anyhow, Error, Result};
-use std::{convert::TryFrom, fmt, ops::Deref};
+use std::{
+    convert::{TryFrom, TryInto},
+    fmt,
+    ops::Deref,
+};
 
 /// Represents the imap flag variants.
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize)]

src/msg/msg_entity.rs

@@ -5,7 +5,7 @@ use html_escape;
 use lettre::message::{header::ContentType, Attachment, MultiPart, SinglePart};
 use log::{debug, info, trace};
 use regex::Regex;
-use std::{collections::HashSet, env::temp_dir, fmt::Debug, fs, path::PathBuf};
+use std::{collections::HashSet, convert::TryInto, env::temp_dir, fmt::Debug, fs, path::PathBuf};
 use uuid::Uuid;
 
 use crate::{