add stable_since convenience

compiler/rustc_attr/src/builtin.rs

@@ -80,6 +80,10 @@ impl Stability {
     pub fn is_stable(&self) -> bool {
         self.level.is_stable()
     }
+
+    pub fn stable_since(&self) -> Option<StableSince> {
+        self.level.stable_since()
+    }
 }
 
 /// Represents the `#[rustc_const_unstable]` and `#[rustc_const_stable]` attributes.
@@ -170,6 +174,12 @@ impl StabilityLevel {
     pub fn is_stable(&self) -> bool {
         matches!(self, StabilityLevel::Stable { .. })
     }
+    pub fn stable_since(&self) -> Option<StableSince> {
+        match *self {
+            StabilityLevel::Stable { since, .. } => Some(since),
+            StabilityLevel::Unstable { .. } => None,
+        }
+    }
 }
 
 #[derive(Encodable, Decodable, PartialEq, Copy, Clone, Debug, Eq, Hash)]

src/librustdoc/clean/types.rs

@@ -638,10 +638,7 @@ impl Item {
     }
 
     pub(crate) fn stable_since(&self, tcx: TyCtxt<'_>) -> Option<StableSince> {
-        match self.stability(tcx)?.level {
+        self.stability(tcx).and_then(|stability| stability.stable_since())
-            StabilityLevel::Stable { since, .. } => Some(since),
-            StabilityLevel::Unstable { .. } => None,
-        }
     }
 
     pub(crate) fn is_non_exhaustive(&self) -> bool {