Fix rustdoc item summaries that are headers

src/librustdoc/html/render.rs

@@ -1819,7 +1819,9 @@ fn full_path(cx: &Context, item: &clean::Item) -> String {
 
 fn shorter<'a>(s: Option<&'a str>) -> String {
     match s {
-        Some(s) => s.lines().take_while(|line|{
+        Some(s) => s.lines()
+            .skip_while(|s| s.chars().all(|c| c.is_whitespace()))
+            .take_while(|line|{
             (*line).chars().any(|chr|{
                 !chr.is_whitespace()
             })

src/test/rustdoc/issue-46377.rs

@@ -0,0 +1,13 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// @has 'issue_46377/index.html' '//*[@class="docblock-short"]' 'Check out this struct!'
+/// # Check out this struct!
+pub struct SomeStruct;