rustdoc: Elide const-unstable if also unstable overall

It's confusing because if a function is unstable overall, there's no
need to highlight the constness is also unstable. Technically, these
attributes (overall stability and const-stability) are separate, but in
practice, we don't even show the const-unstable's feature flag (it's
normally the same as the overall function).

src/librustdoc/html/render/mod.rs

@@ -1016,18 +1016,23 @@ fn render_stability_since_raw_with_extra(
                 .map(|since| (format!("const since {since}"), format!("const: {since}")))
         }
         Some(ConstStability { level: StabilityLevel::Unstable { issue, .. }, feature, .. }) => {
-            let unstable = if let Some(n) = issue {
-                format!(
-                    "<a \
+            if stable_version.is_none() {
+                // don't display const unstable if entirely unstable
+                None
+            } else {
+                let unstable = if let Some(n) = issue {
+                    format!(
+                        "<a \
                         href=\"https://github.com/rust-lang/rust/issues/{n}\" \
                         title=\"Tracking issue for {feature}\"\
                        >unstable</a>"
-                )
-            } else {
-                String::from("unstable")
-            };
+                    )
+                } else {
+                    String::from("unstable")
+                };
 
-            Some((String::from("const unstable"), format!("const: {unstable}")))
+                Some((String::from("const unstable"), format!("const: {unstable}")))
+            }
         }
         _ => None,
     };

tests/rustdoc/const-display.rs

@@ -24,6 +24,12 @@ pub const unsafe fn foo_unsafe() -> u32 { 42 }
 #[unstable(feature = "humans", issue = "none")]
 pub const fn foo2() -> u32 { 42 }
 
+// @has 'foo/fn.foo3.html' '//pre' 'pub fn foo3() -> u32'
+// @!hasraw - '//span[@class="since"]'
+#[unstable(feature = "humans", issue = "none")]
+#[rustc_const_unstable(feature = "humans", issue = "none")]
+pub const fn foo3() -> u32 { 42 }
+
 // @has 'foo/fn.bar2.html' '//pre' 'pub const fn bar2() -> u32'
 // @has - //span '1.0.0 (const: 1.0.0)'
 #[stable(feature = "rust1", since = "1.0.0")]