Fix `Interner::with_capacity` for default keys (#13092)

The new method did not account for allocating the default key, causing
code that then tried to use it to panic.

crates/circuit/src/interner.rs

@@ -105,9 +105,7 @@ where
     /// `Interner::get_default` to reliably work correctly without a hash lookup (though ideally
     /// we'd just use specialisation to do that).
     pub fn new() -> Self {
-        let mut set = IndexSet::with_capacity_and_hasher(1, Default::default());
-        set.insert(Default::default());
-        Self(set)
+        Self::with_capacity(1)
     }
 
     /// Retrieve the key corresponding to the default store, without any hash or equality lookup.
@@ -126,11 +124,14 @@ where
         }
     }
 
+    /// Create an interner with enough space to hold `capacity` entries.
+    ///
+    /// Note that the default item of the interner is always allocated and given a key immediately,
+    /// which will use one slot of the capacity.
     pub fn with_capacity(capacity: usize) -> Self {
-        Self(IndexSet::with_capacity_and_hasher(
-            capacity,
-            ::ahash::RandomState::new(),
-        ))
+        let mut set = IndexSet::with_capacity_and_hasher(capacity, ::ahash::RandomState::new());
+        set.insert(Default::default());
+        Self(set)
     }
 }
 
@@ -196,3 +197,21 @@ where
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn default_key_exists() {
+        let mut interner = Interner::<[u32]>::new();
+        assert_eq!(interner.get_default(), interner.get_default());
+        assert_eq!(interner.get(interner.get_default()), &[]);
+        assert_eq!(interner.insert_owned(Vec::new()), interner.get_default());
+        assert_eq!(interner.insert(&[]), interner.get_default());
+
+        let capacity = Interner::<str>::with_capacity(4);
+        assert_eq!(capacity.get_default(), capacity.get_default());
+        assert_eq!(capacity.get(capacity.get_default()), "");
+    }
+}