move empty to trait

utils/pattern/src/common.rs

@@ -115,6 +115,9 @@ pub trait PatternBackend: crate::private::Sealed + 'static + core::fmt::Debug {
     /// Writes the placeholder into the pattern string, the inverse of `Self::PlaceholderKey::from_str`
     #[cfg(feature = "alloc")]
     fn escape_placeholder(s: &mut String, placeholder: Self::PlaceholderKey<'_>);
+
+    /// The store for the empty pattern, used to implement `Default`
+    fn empty() -> &'static Self::Store;
 }
 
 /// Default annotation for the literal portion of a pattern.

utils/pattern/src/double.rs

@@ -389,6 +389,10 @@ impl PatternBackend for DoublePlaceholder {
             DoublePlaceholderKey::Place1 => "{1}",
         });
     }
+
+    fn empty() -> &'static Self::Store {
+        "\0\u{1}"
+    }
 }
 
 #[doc(hidden)] // TODO(#4467): Should be internal

utils/pattern/src/frontend/mod.rs

@@ -93,17 +93,38 @@ impl<B: PatternBackend> core::fmt::Debug for Pattern<B> {
     }
 }
 
+impl<B: PatternBackend> Default for &'static Pattern<B> {
+    fn default() -> Self {
+        Pattern::from_ref_store_unchecked(B::empty())
+    }
+}
+
 #[cfg(feature = "alloc")]
 impl<B: PatternBackend> Default for Box<Pattern<B>>
 where
-    Box<B::Store>: Default,
+    Box<B::Store>: for<'a> From<&'a B::Store>,
 {
     fn default() -> Self {
-        #[allow(clippy::unwrap_used)]
-        Pattern::try_from_items(core::iter::empty()).unwrap()
+        Pattern::from_ref_store_unchecked(B::empty()).to_owned()
     }
 }
 
+#[test]
+fn test_defaults() {
+    assert_eq!(
+        Box::<Pattern::<crate::SinglePlaceholder>>::default(),
+        Pattern::try_from_items(core::iter::empty()).unwrap()
+    );
+    assert_eq!(
+        Box::<Pattern::<crate::DoublePlaceholder>>::default(),
+        Pattern::try_from_items(core::iter::empty()).unwrap()
+    );
+    assert_eq!(
+        Box::<Pattern::<crate::MultiNamedPlaceholder>>::default(),
+        Pattern::try_from_items(core::iter::empty()).unwrap()
+    );
+}
+
 #[cfg(feature = "alloc")]
 impl<B: PatternBackend> ToOwned for Pattern<B>
 where

utils/pattern/src/multi_named.rs

@@ -366,6 +366,10 @@ impl PatternBackend for MultiNamedPlaceholder {
         s.push_str(placeholder.0);
         s.push('}');
     }
+
+    fn empty() -> &'static Self::Store {
+        ""
+    }
 }
 
 #[derive(Debug)]

utils/pattern/src/single.rs

@@ -249,6 +249,10 @@ impl PatternBackend for SinglePlaceholder {
     fn escape_placeholder(s: &mut String, _: SinglePlaceholderKey) {
         s.push_str("{0}");
     }
+
+    fn empty() -> &'static Self::Store {
+        "\0"
+    }
 }
 
 #[doc(hidden)] // TODO(#4467): Should be internal