Replace textnonce with rand

The textnonce dependency pulls in quite a few transitive dependencies.
However, we only use the dependency in a single location, to generate
MIME boundaries. For this, we can use `rand` directly (which is already
a transitive dependency anyways, since it's required by uuid).

This reduces the dependency count for a standard build from 117 to 105.

Cargo.toml

@@ -33,11 +33,11 @@ nom = { version = "5", optional = true }
 once_cell = "1"
 quoted_printable = { version = "0.4", optional = true }
 r2d2 = { version = "0.8", optional = true }
+rand = { version = "0.7", optional = true, default-features = false }
 regex = "1"
 rustls = { version = "0.17", optional = true }
 serde = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }
-textnonce = { version = "0.7", optional = true }
 uuid = { version = "0.8", features = ["v4"] }
 webpki = { version = "0.21", optional = true }
 webpki-roots = { version = "0.19", optional = true }
@@ -54,7 +54,7 @@ name = "transport_smtp"
 
 [features]
 async = ["async-std", "async-trait", "async-attributes"]
-builder = ["mime", "base64", "hyperx", "textnonce", "quoted_printable"]
+builder = ["mime", "base64", "hyperx", "rand", "quoted_printable"]
 default = ["file-transport", "smtp-transport", "rustls-tls", "hostname", "r2d2", "sendmail-transport", "builder"]
 file-transport = ["serde", "serde_json"]
 rustls-tls = ["webpki", "webpki-roots", "rustls"]

src/message/mimebody.rs

@@ -4,7 +4,7 @@ use crate::message::{
     EmailFormat,
 };
 use mime::Mime;
-use textnonce::TextNonce;
+use rand::Rng;
 
 /// MIME part variants
 ///
@@ -194,11 +194,19 @@ pub enum MultiPartKind {
     Related,
 }
 
+/// Create a random MIME boundary.
+fn make_boundary() -> String {
+    rand::thread_rng()
+        .sample_iter(&rand::distributions::Alphanumeric)
+        .take(68)
+        .collect()
+}
+
 impl MultiPartKind {
     fn to_mime<S: AsRef<str>>(self, boundary: Option<S>) -> Mime {
         let boundary = boundary
             .map(|s| s.as_ref().into())
-            .unwrap_or_else(|| TextNonce::sized(68).unwrap().into_string());
+            .unwrap_or_else(make_boundary);
 
         use self::MultiPartKind::*;
         format!(
@@ -601,4 +609,20 @@ mod test {
                            "int main() { return 0; }\r\n",
                            "--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK--\r\n"));
     }
+
+    #[test]
+    fn test_make_boundary() {
+        let mut boundaries = std::collections::HashSet::with_capacity(10);
+        for _ in 0..1000 {
+            boundaries.insert(make_boundary());
+        }
+
+        // Ensure there are no duplicates
+        assert_eq!(1000, boundaries.len());
+
+        // Ensure correct length
+        for boundary in boundaries {
+            assert_eq!(68, boundary.len());
+        }
+    }
 }