Split rustdoc summary lines in a smarter way

Previously it would fail on a trivial case like

    /// Summary line
    /// <trailing space>
    /// Regular content

Compliant markdown preprocessor would render that as two separate paragraphs, but our summary line
extractor would interpret both lines as the same paragraph and include both into the short summary.

src/librustdoc/html/render.rs

@@ -407,7 +407,7 @@ fn build_index(krate: &clean::Crate, cache: &mut Cache) -> io::Result<String> {
                         ty: shortty(item),
                         name: item.name.clone().unwrap(),
                         path: fqp[..fqp.len() - 1].connect("::"),
-                        desc: shorter(item.doc_value()).to_string(),
+                        desc: shorter(item.doc_value()),
                         parent: Some(did),
                     });
                 },
@@ -876,7 +876,7 @@ impl DocFolder for Cache {
                         ty: shortty(&item),
                         name: s.to_string(),
                         path: path.connect("::").to_string(),
-                        desc: shorter(item.doc_value()).to_string(),
+                        desc: shorter(item.doc_value()),
                         parent: parent,
                     });
                 }
@@ -1467,13 +1467,14 @@ fn full_path(cx: &Context, item: &clean::Item) -> String {
     return s
 }
 
-fn shorter<'a>(s: Option<&'a str>) -> &'a str {
+fn shorter<'a>(s: Option<&'a str>) -> String {
     match s {
-        Some(s) => match s.find("\n\n") {
-            Some(pos) => &s[..pos],
-            None => s,
-        },
-        None => ""
+        Some(s) => s.lines().take_while(|line|{
+            (*line).chars().any(|chr|{
+                !chr.is_whitespace()
+            })
+        }).collect::<Vec<_>>().connect("\n"),
+        None => "".to_string()
     }
 }
 
@@ -1603,7 +1604,7 @@ fn item_module(w: &mut fmt::Formatter, cx: &Context,
                     </tr>
                 ",
                 *myitem.name.as_ref().unwrap(),
-                Markdown(shorter(myitem.doc_value())),
+                Markdown(&shorter(myitem.doc_value())[..]),
                 class = shortty(myitem),
                 href = item_path(myitem),
                 title = full_path(cx, myitem),