Rollup merge of #90905 - GuillaumeGomez:empty-impl-blocks, r=jsha

Add empty impl blocks if they have documentation

Fixes https://github.com/rust-lang/rust/issues/90866.

The update for the test script is needed to count the number of impl blocks we have with only the struct. To be noted that with https://github.com/rust-lang/rust/pull/89676 merged, it wouldn't be needed (I don't know what is the status of it btw. cc ```@Mark-Simulacrum).```

It looks like this:

![Screenshot from 2021-11-14 16-51-28](https://user-images.githubusercontent.com/3050060/141689100-e57123c0-bf50-4c42-adf5-d991e169a0e4.png)

cc ```@jyn514```
r? ```@camelid```

7 files changed, 68 insertions, 19 deletions

diff --git a/src/etc/htmldocck.py b/src/etc/htmldocck.py
index 5eb70ab13db..f762e389005 100644
--- a/src/etc/htmldocck.py
+++ b/src/etc/htmldocck.py
@@ -94,6 +94,10 @@ There are a number of supported commands:
   in the specified file. The number of occurrences must match the given
   count.
 
+* `@count PATH XPATH TEXT COUNT` checks for the occurrence of the given XPath
+  with the given text in the specified file. The number of occurrences must
+  match the given count.
+
 * `@snapshot NAME PATH XPATH` creates a snapshot test named NAME.
   A snapshot test captures a subtree of the DOM, at the location
   determined by the XPath, and compares it to a pre-recorded value
@@ -382,9 +386,10 @@ def check_tree_attr(tree, path, attr, pat, regexp):
     return ret
 
 
-def check_tree_text(tree, path, pat, regexp):
+# Returns the number of occurences matching the regex (`regexp`) and the text (`pat`).
+def check_tree_text(tree, path, pat, regexp, stop_at_first):
     path = normalize_xpath(path)
-    ret = False
+    match_count = 0
     try:
         for e in tree.findall(path):
             try:
@@ -392,13 +397,14 @@ def check_tree_text(tree, path, pat, regexp):
             except KeyError:
                 continue
             else:
-                ret = check_string(value, pat, regexp)
-                if ret:
-                    break
+                if check_string(value, pat, regexp):
+                    match_count += 1
+                    if stop_at_first:
+                        break
     except Exception:
         print('Failed to get path "{}"'.format(path))
         raise
-    return ret
+    return match_count
 
 
 def get_tree_count(tree, path):
@@ -518,6 +524,19 @@ def print_err(lineno, context, err, message=None):
         stderr("\t{}".format(context))
 
 
+def get_nb_matching_elements(cache, c, regexp, stop_at_first):
+    tree = cache.get_tree(c.args[0])
+    pat, sep, attr = c.args[1].partition('/@')
+    if sep:  # attribute
+        tree = cache.get_tree(c.args[0])
+        return check_tree_attr(tree, pat, attr, c.args[2], False)
+    else:  # normalized text
+        pat = c.args[1]
+        if pat.endswith('/text()'):
+            pat = pat[:-7]
+        return check_tree_text(cache.get_tree(c.args[0]), pat, c.args[2], regexp, stop_at_first)
+
+
 ERR_COUNT = 0
 
 
@@ -538,16 +557,7 @@ def check_command(c, cache):
                 ret = check_string(cache.get_file(c.args[0]), c.args[1], regexp)
             elif len(c.args) == 3:  # @has/matches <path> <pat> <match> = XML tree test
                 cerr = "`XPATH PATTERN` did not match"
-                tree = cache.get_tree(c.args[0])
-                pat, sep, attr = c.args[1].partition('/@')
-                if sep:  # attribute
-                    tree = cache.get_tree(c.args[0])
-                    ret = check_tree_attr(tree, pat, attr, c.args[2], regexp)
-                else:  # normalized text
-                    pat = c.args[1]
-                    if pat.endswith('/text()'):
-                        pat = pat[:-7]
-                    ret = check_tree_text(cache.get_tree(c.args[0]), pat, c.args[2], regexp)
+                ret = get_nb_matching_elements(cache, c, regexp, True) != 0
             else:
                 raise InvalidCheck('Invalid number of @{} arguments'.format(c.cmd))
 
@@ -557,6 +567,11 @@ def check_command(c, cache):
                 found = get_tree_count(cache.get_tree(c.args[0]), c.args[1])
                 cerr = "Expected {} occurrences but found {}".format(expected, found)
                 ret = expected == found
+            elif len(c.args) == 4:  # @count <path> <pat> <text> <count> = count test
+                expected = int(c.args[3])
+                found = get_nb_matching_elements(cache, c, False, False)
+                cerr = "Expected {} occurrences but found {}".format(expected, found)
+                ret = found == expected
             else:
                 raise InvalidCheck('Invalid number of @{} arguments'.format(c.cmd))
 
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 23ce634cf28..cb887d16906 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/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>",
diff --git a/src/librustdoc/html/static/css/themes/ayu.css b/src/librustdoc/html/static/css/themes/ayu.css
index 7303cecc0d6..8e0521d9ad6 100644
--- a/src/librustdoc/html/static/css/themes/ayu.css
+++ b/src/librustdoc/html/static/css/themes/ayu.css
@@ -281,9 +281,13 @@ details.undocumented > summary::before {
 	color: #000;
 }
 
+/* Created this empty rule to satisfy the theme checks. */
+.stab.empty-impl {}
+
 .stab.unstable,
 .stab.deprecated,
-.stab.portability {
+.stab.portability,
+.stab.empty-impl {
 	color: #c5c5c5;
 	background: #314559 !important;
 	border-style: none !important;
diff --git a/src/librustdoc/html/static/css/themes/dark.css b/src/librustdoc/html/static/css/themes/dark.css
index 34a4f446b56..071ad006ed3 100644
--- a/src/librustdoc/html/static/css/themes/dark.css
+++ b/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; }
diff --git a/src/librustdoc/html/static/css/themes/light.css b/src/librustdoc/html/static/css/themes/light.css
index aa6ad2f5473..5c3789bf463 100644
--- a/src/librustdoc/html/static/css/themes/light.css
+++ b/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; }
diff --git a/src/librustdoc/passes/stripper.rs b/src/librustdoc/passes/stripper.rs
index d5db919dc4b..0fd124e6154 100644
--- a/src/librustdoc/passes/stripper.rs
+++ b/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) {
diff --git a/src/test/rustdoc/empty-impl-block.rs b/src/test/rustdoc/empty-impl-block.rs
new file mode 100644
index 00000000000..6a2a254f63a
--- /dev/null
+++ b/src/test/rustdoc/empty-impl-block.rs
@@ -0,0 +1,20 @@
+#![crate_name = "foo"]
+
+// @has 'foo/struct.Foo.html'
+pub struct Foo;
+
+// @has - '//*[@class="docblock"]' 'Hello empty impl block!'
+// @has - '//*[@class="item-info"]' 'This impl block contains no items.'
+/// Hello empty impl block!
+impl Foo {}
+// We ensure that this empty impl block without doc isn't rendered.
+// @count - '//*[@class="impl has-srclink"]' 'impl Foo' 1
+impl Foo {}
+
+// Just to ensure that empty trait impl blocks are rendered.
+pub struct Another;
+pub trait Bar {}
+
+// @has 'foo/struct.Another.html'
+// @has - '//h3[@class="code-header in-band"]' 'impl Bar for Another'
+impl Bar for Another {}