Fix syntax error when rendering topbar for source page

Apparently `'krate' in __tera_context` checks for the string 'krate' in
the *serialized context*, not the field `krate`. This is utterly
horrifying and I refuse to be convinced otherwise.

Author: jyn514

src/web/source.rs

@@ -305,4 +305,18 @@ mod tests {
             Ok(())
         })
     }
+    #[test]
+    fn literal_krate_description() {
+        wrapper(|env| {
+            env.fake_release()
+                .name("rustc-ap-syntax")
+                .version("178.0.0")
+                .description("some stuff with krate")
+                .source_file("fold.rs", b"fn foo() {}")
+                .create()?;
+            let web = env.frontend();
+            assert_success("/crate/rustc-ap-syntax/178.0.0/source/fold.rs", web)?;
+            Ok(())
+        })
+    }
 }

templates/rustdoc/topbar.html

@@ -6,7 +6,7 @@
 {%- include "header/topbar_begin.html" -%}{#
 extra whitespace unremovable, need to use html tags unaffacted by whitespace T_T
 #}<ul class="pure-menu-list">
-    {%- if 'krate' in __tera_context -%}
+    {%- if krate -%}
         <li class="pure-menu-item pure-menu-has-children">
             <a href="#" class="pure-menu-link crate-name" title="{{ krate.description }}">
                 {{ "cube" | fas }}