Rollup merge of rust-lang#91687 - euclio:tuple-variant-field-section, r=GuillaumeGomez

rustdoc: do not emit tuple variant fields if none are documented

Fixes rust-lang#90824.

Before:

![2021-12-15T22:26:41](https://user-images.githubusercontent.com/1372438/146302871-4d265433-b9aa-4e53-adfb-e7cb92107180.png)

After:

![2021-12-15T22:27:01](https://user-images.githubusercontent.com/1372438/146302872-e39eda3d-2fb2-4fb9-aae7-2008e4e1b4dd.png)

Author: matthiaskrgr

src/librustdoc/html/render/print_item.rs

@@ -1126,18 +1126,27 @@ fn item_enum(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, e: &clean::Enum
             w.write_str("</h3>");
 
             use crate::clean::Variant;
-            if let Some((extra, fields)) = match *variant.kind {
-                clean::VariantItem(Variant::Struct(ref s)) => Some(("", &s.fields)),
-                clean::VariantItem(Variant::Tuple(ref fields)) => Some(("Tuple ", fields)),
+
+            let heading_and_fields = match &*variant.kind {
+                clean::VariantItem(Variant::Struct(s)) => Some(("Fields", &s.fields)),
+                // Documentation on tuple variant fields is rare, so to reduce noise we only emit
+                // the section if at least one field is documented.
+                clean::VariantItem(Variant::Tuple(fields))
+                    if fields.iter().any(|f| f.doc_value().is_some()) =>
+                {
+                    Some(("Tuple Fields", fields))
+                }
                 _ => None,
-            } {
+            };
+
+            if let Some((heading, fields)) = heading_and_fields {
                 let variant_id = cx.derive_id(format!(
                     "{}.{}.fields",
                     ItemType::Variant,
                     variant.name.as_ref().unwrap()
                 ));
                 write!(w, "<div class=\"sub-variant\" id=\"{id}\">", id = variant_id);
-                write!(w, "<h4>{extra}Fields</h4>", extra = extra,);
+                write!(w, "<h4>{heading}</h4>", heading = heading);
                 document_non_exhaustive(w, variant);
                 for field in fields {
                     match *field.kind {

src/test/rustdoc/issue-88600.rs

@@ -18,17 +18,18 @@ pub enum FooEnum {
     // @has - '//*[@id="variant.MixedHiddenFirst"]//code' 'MixedHiddenFirst(_, S)'
     // @count - '//*[@id="variant.MixedHiddenFirst.field.0"]' 0
     // @has - '//*[@id="variant.MixedHiddenFirst.field.1"]' '1: S'
-    MixedHiddenFirst(#[doc(hidden)] H, S),
+    MixedHiddenFirst(#[doc(hidden)] H, /** dox */ S),
     // @has - '//*[@id="variant.MixedHiddenLast"]//code' 'MixedHiddenLast(S, _)'
     // @has - '//*[@id="variant.MixedHiddenLast.field.0"]' '0: S'
     // @count - '//*[@id="variant.MixedHiddenLast.field.1"]' 0
-    MixedHiddenLast(S, #[doc(hidden)] H),
+    MixedHiddenLast(/** dox */ S, #[doc(hidden)] H),
     // @has - '//*[@id="variant.HiddenStruct"]//code' 'HiddenStruct'
     // @count - '//*[@id="variant.HiddenStruct.field.h"]' 0
     // @has - '//*[@id="variant.HiddenStruct.field.s"]' 's: S'
     HiddenStruct {
         #[doc(hidden)]
         h: H,
+        /// dox
         s: S,
     },
 }

src/test/rustdoc/tuple-struct-fields-doc.rs

@@ -24,6 +24,9 @@ pub struct Foo(
 // @has - '//*[@id="variant.BarVariant.field.0"]' '0: String'
 // @has - '//*[@id="variant.BarVariant.fields"]//*[@class="docblock"]' 'Hello docs'
 // @matches - '//*[@id="variant.FooVariant.fields"]/h4' '^Fields$'
+// @has - '//*[@id="variant.BazVariant.fields"]//*[@class="docblock"]' 'dox'
+// @has - '//*[@id="variant.OtherVariant.fields"]//*[@class="docblock"]' 'dox'
+// @!matches - '//*[@id="variant.QuuxVariant.fields"]/h4' '^Tuple Fields$'
 pub enum Bar {
     BarVariant(
         /// Hello docs
@@ -33,4 +36,15 @@ pub enum Bar {
        /// hello
        x: u32,
     },
+    BazVariant(
+        String,
+        /// dox
+        u32,
+    ),
+    OtherVariant(
+        /// dox
+        String,
+        u32,
+    ),
+    QuuxVariant(String),
 }