Display empty impl blocks if they have documentations

Author: GuillaumeGomez

src/librustdoc/html/render/mod.rs

@@ -1600,6 +1600,13 @@ fn render_impl(
         }
 
         if let Some(ref dox) = i.impl_item.collapsed_doc_value() {
+            if trait_.is_none() && i.inner_impl().items.is_empty() {
+                w.write_str(
+                    "<div class=\"item-info\">\
+                    <div class=\"stab empty-impl\">This impl block contains no items.</div>
+                </div>",
+                );
+            }
             write!(
                 w,
                 "<div class=\"docblock\">{}</div>",

src/librustdoc/html/static/css/themes/ayu.css

@@ -283,7 +283,8 @@ details.undocumented > summary::before {
 
 .stab.unstable,
 .stab.deprecated,
-.stab.portability {
+.stab.portability,
+.stab.empty-impl {
 	color: #c5c5c5;
 	background: #314559 !important;
 	border-style: none !important;

src/librustdoc/html/static/css/themes/dark.css

@@ -266,6 +266,7 @@ details.undocumented > summary::before {
 	color: #ddd;
 }
 
+.stab.empty-impl { background: #FFF5D6; border-color: #FFC600; color: #2f2f2f; }
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; color: #2f2f2f; }
 .stab.deprecated { background: #ffc4c4; border-color: #db7b7b; color: #2f2f2f; }
 .stab.portability { background: #F3DFFF; border-color: #b07bdb; color: #2f2f2f; }

src/librustdoc/html/static/css/themes/light.css

@@ -255,6 +255,7 @@ details.undocumented > summary::before {
 	color: #000;
 }
 
+.stab.empty-impl { background: #FFF5D6; border-color: #FFC600; }
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; }
 .stab.deprecated { background: #ffc4c4; border-color: #db7b7b; }
 .stab.portability { background: #F3DFFF; border-color: #b07bdb; }

src/librustdoc/passes/stripper.rs

@@ -124,8 +124,9 @@ pub(crate) struct ImplStripper<'a> {
 impl<'a> DocFolder for ImplStripper<'a> {
     fn fold_item(&mut self, i: Item) -> Option<Item> {
         if let clean::ImplItem(ref imp) = *i.kind {
-            // emptied none trait impls can be stripped
-            if imp.trait_.is_none() && imp.items.is_empty() {
+            // Impl blocks can be skipped if they are: empty; not a trait impl; and have no
+            // documentation.
+            if imp.trait_.is_none() && imp.items.is_empty() && i.doc_value().is_none() {
                 return None;
             }
             if let Some(did) = imp.for_.def_id(self.cache) {