Omit check of a successive line in loop

I think that s == "" is the only edge case (as it makes iter.next() return None the first time). The early return is necessary so that the last character of 'out' isn't popped if s == "" && !frag.need_backline

Author: vacuus

Diff for: src/librustdoc/clean/types.rs

@@ -939,21 +939,25 @@ crate enum DocFragmentKind {
 //   `need_backline` field).
 fn add_doc_fragment(out: &mut String, frag: &DocFragment) {
     let s = frag.doc.as_str();
-    let mut iter = s.lines().peekable();
+    let mut iter = s.lines();
+    if s == "" {
+        if frag.need_backline {
+            out.push('\n');
+        }
+        return;
+    }
     while let Some(line) = iter.next() {
         if line.chars().any(|c| !c.is_whitespace()) {
             assert!(line.len() >= frag.indent);
             out.push_str(&line[frag.indent..]);
         } else {
             out.push_str(line);
         }
-        if iter.peek().is_some() {
-            out.push('\n');
-        }
-    }
-    if frag.need_backline {
         out.push('\n');
     }
+    if !frag.need_backline {
+        out.pop();
+    }
 }
 
 /// Collapse a collection of [`DocFragment`]s into one string,