Support non-exhaustive enum variants in rustdoc.

This commit adds support for non-exhaustive enum variants in rustdoc,
extending the existing support for non-exhaustive enums and structs.

src/librustdoc/clean/mod.rs

@@ -421,6 +421,9 @@ impl Item {
     pub fn is_enum(&self) -> bool {
         self.type_() == ItemType::Enum
     }
+    pub fn is_variant(&self) -> bool {
+        self.type_() == ItemType::Variant
+    }
     pub fn is_associated_type(&self) -> bool {
         self.type_() == ItemType::AssociatedType
     }

src/librustdoc/html/render.rs

@@ -2596,7 +2596,15 @@ fn document_non_exhaustive_header(item: &clean::Item) -> &str {
 fn document_non_exhaustive(w: &mut fmt::Formatter<'_>, item: &clean::Item) -> fmt::Result {
     if item.is_non_exhaustive() {
         write!(w, "<div class='docblock non-exhaustive non-exhaustive-{}'>", {
-            if item.is_struct() { "struct" } else if item.is_enum() { "enum" } else { "type" }
+            if item.is_struct() {
+                "struct"
+            } else if item.is_enum() {
+                "enum"
+            } else if item.is_variant() {
+                "variant"
+            } else {
+                "type"
+            }
         })?;
 
         if item.is_struct() {
@@ -2609,6 +2617,10 @@ fn document_non_exhaustive(w: &mut fmt::Formatter<'_>, item: &clean::Item) -> fm
             write!(w, "Non-exhaustive enums could have additional variants added in future. \
                        Therefore, when matching against variants of non-exhaustive enums, an \
                        extra wildcard arm must be added to account for any future variants.")?;
+        } else if item.is_variant() {
+            write!(w, "Non-exhaustive enum variants could have additional fields added in future. \
+                       Therefore, non-exhaustive enum variants cannot be constructed in external \
+                       crates and cannot be matched against.")?;
         } else {
             write!(w, "This type will require a wildcard arm in any match statements or \
                        constructors.")?;
@@ -3671,6 +3683,7 @@ fn item_enum(w: &mut fmt::Formatter<'_>, cx: &Context, it: &clean::Item,
             }
             write!(w, "</code></span>")?;
             document(w, cx, variant)?;
+            document_non_exhaustive(w, variant)?;
 
             use crate::clean::{Variant, VariantKind};
             if let clean::VariantItem(Variant {

src/librustdoc/html/static/main.js

@@ -2247,6 +2247,8 @@ if (!DOMTokenList.prototype.remove) {
                     otherMessage += "struct";
                 } else if (hasClass(e, "non-exhaustive-enum")) {
                     otherMessage += "enum";
+                } else if (hasClass(e, "non-exhaustive-variant")) {
+                    otherMessage += "enum variant";
                 } else if (hasClass(e, "non-exhaustive-type")) {
                     otherMessage += "type";
                 }
@@ -2264,6 +2266,9 @@ if (!DOMTokenList.prototype.remove) {
             if (hasClass(e, "type-decl") === true && showItemDeclarations === true) {
                 collapseDocs(e.previousSibling.childNodes[0], "toggle");
             }
+            if (hasClass(e, "non-exhaustive") === true) {
+                collapseDocs(e.previousSibling.childNodes[0], "toggle");
+            }
         }
     }