Fix invalid removal of backlines from doc comments

Author: GuillaumeGomez

compiler/rustc_ast/src/util/comments.rs

@@ -34,18 +34,11 @@ pub fn beautify_doc_string(data: Symbol) -> Symbol {
             i += 1;
         }
 
-        while i < j && lines[i].trim().is_empty() {
-            i += 1;
-        }
         // like the first, a last line of all stars should be omitted
         if j > i && !lines[j - 1].is_empty() && lines[j - 1].chars().all(|c| c == '*') {
             j -= 1;
         }
 
-        while j > i && lines[j - 1].trim().is_empty() {
-            j -= 1;
-        }
-
         if i != 0 || j != lines.len() { Some((i, j)) } else { None }
     }
 

src/test/rustdoc/mixing-doc-comments-and-attrs.S1_top-doc.html

@@ -1,4 +1,4 @@
-<div class="docblock"><p>Hello world!
-Goodbye!
+<div class="docblock"><p>Hello world!</p>
+<p>Goodbye!
 Hello again!</p>
 </div>