fix(rfc2047): improve output in some cases

src/headers/rfc2047.rs

@@ -39,10 +39,16 @@ pub fn encode(mut s: &str, w: &mut EmailWriter<'_>) -> fmt::Result {
         let mut word =
             utils::truncate_to_char_boundary(s, unencoded_remaining_line_len.min(s.len()));
         if word.is_empty() {
-            if wrote {
+            if wrote || w.has_spaces() {
                 // No space remaining on this line, go to a new one
                 w.new_line()?;
-                w.space();
+                if !w.has_spaces() {
+                    // The last write before this call to `encode` most
+                    // likely wasn't rfc2047 so we must write a "soft"
+                    // space to let the decoder know we're still within the
+                    // same header
+                    w.space();
+                }
                 continue;
             }
 

src/headers/writer.rs

@@ -58,6 +58,10 @@ impl<'a> EmailWriter<'a> {
         self.spaces = 0;
     }
 
+    pub(super) fn has_spaces(&mut self) -> bool {
+        self.spaces >= 1
+    }
+
     /// Get the length in bytes of the last line written to the inner writer.
     pub fn line_len(&self) -> usize {
         self.line_len
@@ -329,8 +333,8 @@ mod tests {
         assert_eq!(
             s,
             concat!(
-                "Subject: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA BBBBBBBBBBBBB =?utf-8?b?cw==?=\r\n",
-                " =?utf-8?b?w6lsZWN0aW9u?=",
+                "Subject: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA BBBBBBBBBBBBB\r\n",
+                " =?utf-8?b?c8OpbGVjdGlvbg==?=",
             )
         );
     }