default

utils/pattern/src/common.rs

@@ -111,6 +111,9 @@ pub trait PatternBackend: crate::private::Sealed + 'static + core::fmt::Debug {
     /// Iterates over the pattern items in a store.
     #[doc(hidden)] // TODO(#4467): Should be internal
     fn iter_items(store: &Self::Store) -> Self::Iter<'_>;
+
+    /// 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

@@ -381,6 +381,10 @@ impl PatternBackend for DoublePlaceholder {
 
         Ok(result.into_boxed_str())
     }
+
+    fn empty() -> &'static Self::Store {
+        "\0\u{1}"
+    }
 }
 
 #[doc(hidden)] // TODO(#4467): Should be internal

utils/pattern/src/frontend/mod.rs

@@ -94,6 +94,38 @@ 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>: for<'a> From<&'a B::Store>,
+{
+    fn default() -> Self {
+        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/frontend/serde.rs

@@ -59,12 +59,10 @@ impl<P: PatternBackend> core::ops::Deref for PatternString<P> {
 impl<B: PatternBackend> Default for PatternString<B>
 where
     B: PatternBackend,
-    for<'a> B::PlaceholderKeyCow<'a>: core::str::FromStr,
-    for<'a> <B::PlaceholderKeyCow<'a> as core::str::FromStr>::Err: core::fmt::Debug,
+    Box<B::Store>: for<'a> From<&'a B::Store>,
 {
     fn default() -> Self {
-        #[allow(clippy::unwrap_used)] // todo does this work?
-        Self(Pattern::<B>::try_from_str("", Default::default()).unwrap())
+        Self(Box::<Pattern<B>>::default())
     }
 }
 

utils/pattern/src/multi_named.rs

@@ -358,6 +358,10 @@ impl PatternBackend for MultiNamedPlaceholder {
         }
         Ok(string.into_boxed_str())
     }
+
+    fn empty() -> &'static Self::Store {
+        ""
+    }
 }
 
 #[derive(Debug)]

utils/pattern/src/single.rs

@@ -244,6 +244,10 @@ impl PatternBackend for SinglePlaceholder {
         }
         Ok(result.into_boxed_str())
     }
+
+    fn empty() -> &'static Self::Store {
+        "\0"
+    }
 }
 
 #[doc(hidden)] // TODO(#4467): Should be internal