Improve display of const unstable feature

src/librustdoc/html/render/mod.rs

@@ -684,6 +684,12 @@ enum ShortItemInfo {
     Portability {
         message: String,
     },
+    /// The feature corresponding to a const unstable item, and optionally
+    /// a tracking issue URL and number.
+    ConstUnstable {
+        feature: String,
+        tracking: Option<u32>,
+    },
 }
 
 /// Render the stability, deprecation and portability information that is displayed at the top of
@@ -723,10 +729,10 @@ fn short_item_info(
         extra_info.push(ShortItemInfo::Deprecation { message });
     }
 
+    let stability = item.stability(cx.tcx());
     // Render unstable items. But don't render "rustc_private" crates (internal compiler crates).
     // Those crates are permanently unstable so it makes no sense to render "unstable" everywhere.
-    if let Some((StabilityLevel::Unstable { reason: _, issue, .. }, feature)) = item
-        .stability(cx.tcx())
+    if let Some((StabilityLevel::Unstable { reason: _, issue, .. }, feature)) = stability
         .as_ref()
         .filter(|stab| stab.feature != sym::rustc_private)
         .map(|stab| (stab.level, stab.feature))
@@ -740,6 +746,18 @@ fn short_item_info(
         extra_info.push(ShortItemInfo::Unstable { feature: feature.to_string(), tracking });
     }
 
+    // Only display const unstable if NOT entirely unstable.
+    if stability.and_then(|stability| stability.stable_since()).is_some()
+        && let Some(ConstStability {
+            level: StabilityLevel::Unstable { issue, .. }, feature, ..
+        }) = item.const_stability(cx.tcx())
+    {
+        extra_info.push(ShortItemInfo::ConstUnstable {
+            feature: feature.to_string(),
+            tracking: issue.map(|issue| issue.get()),
+        });
+    }
+
     if let Some(message) = portability(item, parent) {
         extra_info.push(ShortItemInfo::Portability { message });
     }

src/librustdoc/html/templates/short_item_info.html

@@ -18,6 +18,18 @@
                 ) {# #}
             </span> {# #}
         </div>
+    {% when Self::ConstUnstable with { feature, tracking } %}
+        <div class="stab unstable"> {# #}
+            <span class="emoji">🔬</span> {# #}
+            <span> {# #}
+                The const version is a nightly-only experimental API. ({# #}
+                <code>{{feature}}</code>
+                {% if let Some(num) = tracking %}
+                    &nbsp;<a href="https://github.com/rust-lang/rust/issues/{{num}}">#{{num}}</a>
+                {% endif %}
+                ) {# #}
+            </span> {# #}
+        </div>
     {% when Self::Portability with { message } %}
         <div class="stab portability">{{message|safe}}</div>
 {% endmatch %}

tests/rustdoc/const-display.rs

@@ -14,7 +14,7 @@ pub const fn foo() -> u32 { 42 }
 //@ has 'foo/fn.foo_unsafe.html' '//pre' 'pub unsafe fn foo_unsafe() -> u32'
 //@ has - '//span[@class="since"]' '1.0.0 (const: unstable)'
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature="foo", issue = "none")]
+#[rustc_const_unstable(feature="foo", issue = "111")]
 pub const unsafe fn foo_unsafe() -> u32 { 42 }
 
 //@ has 'foo/fn.foo2.html' '//pre' 'pub const fn foo2() -> u32'