[rustdoc] Add highlighting for comments in items declaration

src/librustdoc/html/render/print_item.rs

@@ -1509,7 +1509,7 @@ fn print_tuple_struct_fields<'a, 'cx: 'a>(
                 matches!(*field.kind, clean::StrippedItem(box clean::StructFieldItem(..)))
             })
         {
-            return f.write_str("/* private fields */");
+            return f.write_str("<span class=\"comment\">/* private fields */</span>");
         }
 
         for (i, ty) in s.iter().enumerate() {
@@ -1666,7 +1666,7 @@ fn render_enum_fields(
         }
 
         if variants_stripped && !is_non_exhaustive {
-            w.write_str("    // some variants omitted\n");
+            w.write_str("    <span class=\"comment\">// some variants omitted</span>\n");
         }
         if toggle {
             toggle_close(&mut w);
@@ -1811,15 +1811,21 @@ fn item_proc_macro(
         let name = it.name.expect("proc-macros always have names");
         match m.kind {
             MacroKind::Bang => {
-                write!(buffer, "{name}!() {{ /* proc-macro */ }}").unwrap();
+                write!(buffer, "{name}!() {{ <span class=\"comment\">/* proc-macro */</span> }}")
+                    .unwrap();
             }
             MacroKind::Attr => {
                 write!(buffer, "#[{name}]").unwrap();
             }
             MacroKind::Derive => {
                 write!(buffer, "#[derive({name})]").unwrap();
                 if !m.helpers.is_empty() {
-                    buffer.write_str("\n{\n    // Attributes available to this derive:\n").unwrap();
+                    buffer
+                        .write_str(
+                            "\n{\n    \
+                        <span class=\"comment\">// Attributes available to this derive:</span>\n",
+                        )
+                        .unwrap();
                     for attr in &m.helpers {
                         writeln!(buffer, "    #[{attr}]").unwrap();
                     }
@@ -2181,7 +2187,7 @@ fn render_union<'a, 'cx: 'a>(
         }
 
         if it.has_stripped_entries().unwrap() {
-            write!(f, "    /* private fields */\n")?;
+            write!(f, "    <span class=\"comment\">/* private fields */</span>\n")?;
         }
         if toggle {
             toggle_close(&mut f);
@@ -2267,11 +2273,11 @@ fn render_struct_fields(
 
             if has_visible_fields {
                 if has_stripped_entries {
-                    write!(w, "\n{tab}    /* private fields */");
+                    write!(w, "\n{tab}    <span class=\"comment\">/* private fields */</span>");
                 }
                 write!(w, "\n{tab}");
             } else if has_stripped_entries {
-                write!(w, " /* private fields */ ");
+                write!(w, " <span class=\"comment\">/* private fields */</span> ");
             }
             if toggle {
                 toggle_close(&mut w);
@@ -2285,7 +2291,7 @@ fn render_struct_fields(
                     matches!(*field.kind, clean::StrippedItem(box clean::StructFieldItem(..)))
                 })
             {
-                write!(w, "/* private fields */");
+                write!(w, "<span class=\"comment\">/* private fields */</span>");
             } else {
                 for (i, field) in fields.iter().enumerate() {
                     if i > 0 {

tests/rustdoc-gui/item-decl-comment-highlighting.goml

@@ -0,0 +1,73 @@
+// This test checks that comments in item declarations are highlighted.
+go-to: "file://" + |DOC_PATH| + "/test_docs/private/enum.Enum.html"
+show-text: true
+
+define-function: (
+    "check-item-decl-comment",
+    (theme, url, comment_color),
+    block {
+        go-to: |url|
+        set-local-storage: {"rustdoc-theme": |theme|, "rustdoc-use-system-theme": "false"}
+        reload:
+        assert-css: (".item-decl .comment", {"color": |comment_color|}, ALL)
+    }
+)
+
+define-function: (
+    "check-items-for-theme",
+    (theme, comment_color),
+    block {
+        call-function: ("check-item-decl-comment", {
+            "theme": |theme|,
+            "url": "file://" + |DOC_PATH| + "/test_docs/private/enum.Enum.html",
+            "comment_color": |comment_color|,
+        })
+        call-function: ("check-item-decl-comment", {
+            "theme": |theme|,
+            "url": "file://" + |DOC_PATH| + "/test_docs/private/struct.Struct.html",
+            "comment_color": |comment_color|,
+        })
+        call-function: ("check-item-decl-comment", {
+            "theme": |theme|,
+            "url": "file://" + |DOC_PATH| + "/test_docs/private/struct.Tuple.html",
+            "comment_color": |comment_color|,
+        })
+        call-function: ("check-item-decl-comment", {
+            "theme": |theme|,
+            "url": "file://" + |DOC_PATH| + "/test_docs/private/union.Union.html",
+            "comment_color": |comment_color|,
+        })
+        call-function: ("check-item-decl-comment", {
+            "theme": |theme|,
+            "url": "file://" + |DOC_PATH| + "/proc_macro_test/macro.make_answer.html",
+            "comment_color": |comment_color|,
+        })
+        call-function: ("check-item-decl-comment", {
+            "theme": |theme|,
+            "url": "file://" + |DOC_PATH| + "/proc_macro_test/derive.HelperAttr.html",
+            "comment_color": |comment_color|,
+        })
+    }
+)
+
+call-function: (
+    "check-items-for-theme",
+    {
+        "theme": "ayu",
+        "comment_color": "#788797",
+    }
+)
+call-function: (
+    "check-items-for-theme",
+    {
+        "theme": "dark",
+        "comment_color": "#8d8d8b",
+    }
+)
+call-function: (
+    "check-items-for-theme",
+    {
+        "theme": "light",
+        "comment_color": "#8e908c",
+    }
+)

tests/rustdoc-gui/sidebar-source-code.goml

@@ -73,7 +73,7 @@ assert: "//*[@class='dir-entry' and @open]/*[text()='sub_mod']"
 // Only "another_folder" should be "open" in "lib2".
 assert: "//*[@class='dir-entry' and not(@open)]/*[text()='another_mod']"
 // All other trees should be collapsed.
-assert-count: ("//*[@id='src-sidebar']/details[not(text()='lib2') and not(@open)]", 10)
+assert-count: ("//*[@id='src-sidebar']/details[not(text()='lib2') and not(@open)]", 11)
 
 // We now switch to mobile mode.
 set-window-size: (600, 600)

tests/rustdoc-gui/src/proc_macro_test/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "proc_macro_test"
+version = "0.1.0"

tests/rustdoc-gui/src/proc_macro_test/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "proc_macro_test"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+path = "lib.rs"
+proc-macro = true

tests/rustdoc-gui/src/proc_macro_test/lib.rs

@@ -0,0 +1,11 @@
+use proc_macro::TokenStream;
+
+#[proc_macro]
+pub fn make_answer(_item: TokenStream) -> TokenStream {
+    "fn answer() -> u32 { 42 }".parse().unwrap()
+}
+
+#[proc_macro_derive(HelperAttr, attributes(helper))]
+pub fn derive_helper_attr(_item: TokenStream) -> TokenStream {
+    TokenStream::new()
+}

tests/rustdoc-gui/src/test_docs/lib.rs

@@ -593,3 +593,21 @@ pub mod foreign_impl_order {
         fn f(&mut self, fg: [u8; 3]) {}
     }
 }
+
+pub mod private {
+    pub struct Tuple(u32, u8);
+    pub struct Struct {
+        a: u8,
+    }
+
+    pub union Union {
+        a: u8,
+        b: u16,
+    }
+
+    pub enum Enum {
+        A,
+        #[doc(hidden)]
+        B,
+    }
+}

tests/rustdoc/where.SWhere_Simd_item-decl.html

@@ -1,3 +1,3 @@
-<pre class="rust item-decl"><code>pub struct Simd&lt;T&gt;(/* private fields */)
+<pre class="rust item-decl"><code>pub struct Simd&lt;T&gt;(<span class="comment">/* private fields */</span>)
 <span class="where">where
-    T: <a class="trait" href="trait.MyTrait.html" title="trait foo::MyTrait">MyTrait</a></span>;</code></pre>
+    T: <a class="trait" href="trait.MyTrait.html" title="trait foo::MyTrait">MyTrait</a></span>;</code></pre>

tests/rustdoc/where.alpha_trait_decl.html

@@ -1,3 +1,3 @@
-<code>pub struct Alpha&lt;A&gt;(/* private fields */)
+<code>pub struct Alpha&lt;A&gt;(<span class="comment">/* private fields */</span>)
 <span class="where">where
     A: <a class="trait" href="trait.MyTrait.html" title="trait foo::MyTrait">MyTrait</a></span>;</code>

tests/rustdoc/whitespace-after-where-clause.union.html

@@ -1,4 +1,4 @@
 <pre class="rust item-decl"><code>pub union Union&lt;'a, B&gt;<div class="where">where
     B: <a class="trait" href="trait.ToOwned.html" title="trait foo::ToOwned">ToOwned</a>&lt;<a class="primitive" href="{{channel}}/std/primitive.unit.html">()</a>&gt; + ?<a class="trait" href="{{channel}}/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + 'a,</div>{
-    /* private fields */
+    <span class="comment">/* private fields */</span>
 }</code></pre>

tests/rustdoc/whitespace-after-where-clause.union2.html

@@ -1,3 +1,3 @@
 <pre class="rust item-decl"><code>pub union Union2&lt;'a, B: ?<a class="trait" href="{{channel}}/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="trait.ToOwned.html" title="trait foo::ToOwned">ToOwned</a>&lt;<a class="primitive" href="{{channel}}/std/primitive.unit.html">()</a>&gt; + 'a&gt; {
-    /* private fields */
+    <span class="comment">/* private fields */</span>
 }</code></pre>