From 5f02346ee09779bff97d5e30b79d7aaa44de5d24 Mon Sep 17 00:00:00 2001
From: "Shane F. Carr" <shane@unicode.org>
Date: Tue, 26 Dec 2023 19:13:23 -0800
Subject: [PATCH 1/3] Add as_pattern_item_ule

---
 components/datetime/src/pattern/item/ule.rs | 37 ++++++++++++++++++---
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/components/datetime/src/pattern/item/ule.rs b/components/datetime/src/pattern/item/ule.rs
index 0e71cd91e2b..84579ceb8f1 100644
--- a/components/datetime/src/pattern/item/ule.rs
+++ b/components/datetime/src/pattern/item/ule.rs
@@ -215,6 +215,28 @@ impl GenericPatternItemULE {
             char::try_from(u).is_ok()
         }
     }
+
+    /// Converts this [`GenericPatternItemULE`] to a [`PatternItemULE`]
+    /// (if a Literal) or returns the placeholder value.
+    #[allow(dead_code)] // #4415
+    #[inline]
+    pub(crate) fn as_pattern_item_ule(&self) -> Result<&PatternItemULE, u8> {
+        if Self::determine_field_from_u8(self.0[0]) {
+            Err(self.0[2])
+        } else {
+            if cfg!(debug_assertions) {
+                let GenericPatternItem::Literal(c) = GenericPatternItem::from_unaligned(*self)
+                else {
+                    unreachable!("expected a literal!")
+                };
+                let pattern_item_ule = PatternItem::Literal(c).to_unaligned();
+                debug_assert_eq!(self.0, pattern_item_ule.0);
+            }
+            // Safety: when a Literal, the two ULEs have the same repr,
+            // as shown in the above assertion (and the class docs).
+            Ok(unsafe { core::mem::transmute(self) })
+        }
+    }
 }
 
 // Safety (based on the safety checklist on the ULE trait):
@@ -242,11 +264,9 @@ unsafe impl ULE for GenericPatternItemULE {
     }
 }
 
-impl AsULE for GenericPatternItem {
-    type ULE = GenericPatternItemULE;
-
+impl GenericPatternItem {
     #[inline]
-    fn to_unaligned(self) -> Self::ULE {
+    pub(crate) const fn to_unaligned_const(self) -> <Self as AsULE>::ULE {
         match self {
             Self::Placeholder(idx) => GenericPatternItemULE([0b1000_0000, 0x00, idx]),
             Self::Literal(ch) => {
@@ -256,6 +276,15 @@ impl AsULE for GenericPatternItem {
             }
         }
     }
+}
+
+impl AsULE for GenericPatternItem {
+    type ULE = GenericPatternItemULE;
+
+    #[inline]
+    fn to_unaligned(self) -> Self::ULE {
+        self.to_unaligned_const()
+    }
 
     #[inline]
     fn from_unaligned(unaligned: Self::ULE) -> Self {

From 54857bbf9f9016b828178aa13548805932a620c6 Mon Sep 17 00:00:00 2001
From: "Shane F. Carr" <shane@unicode.org>
Date: Tue, 26 Dec 2023 19:18:50 -0800
Subject: [PATCH 2/3] Revert unrelated changes

---
 components/datetime/src/pattern/item/ule.rs | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/components/datetime/src/pattern/item/ule.rs b/components/datetime/src/pattern/item/ule.rs
index 84579ceb8f1..f726c26f44f 100644
--- a/components/datetime/src/pattern/item/ule.rs
+++ b/components/datetime/src/pattern/item/ule.rs
@@ -264,9 +264,11 @@ unsafe impl ULE for GenericPatternItemULE {
     }
 }
 
-impl GenericPatternItem {
+impl AsULE for GenericPatternItem {
+    type ULE = GenericPatternItemULE;
+
     #[inline]
-    pub(crate) const fn to_unaligned_const(self) -> <Self as AsULE>::ULE {
+    fn to_unaligned(self) -> Self::ULE {
         match self {
             Self::Placeholder(idx) => GenericPatternItemULE([0b1000_0000, 0x00, idx]),
             Self::Literal(ch) => {
@@ -276,15 +278,6 @@ impl GenericPatternItem {
             }
         }
     }
-}
-
-impl AsULE for GenericPatternItem {
-    type ULE = GenericPatternItemULE;
-
-    #[inline]
-    fn to_unaligned(self) -> Self::ULE {
-        self.to_unaligned_const()
-    }
 
     #[inline]
     fn from_unaligned(unaligned: Self::ULE) -> Self {

From cde1a5b4bf9745827827b392d2c964ab76ed38c8 Mon Sep 17 00:00:00 2001
From: "Shane F. Carr" <shane@unicode.org>
Date: Wed, 27 Dec 2023 09:07:41 -0800
Subject: [PATCH 3/3] More docs

---
 components/datetime/src/pattern/item/ule.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/components/datetime/src/pattern/item/ule.rs b/components/datetime/src/pattern/item/ule.rs
index f726c26f44f..96970705d61 100644
--- a/components/datetime/src/pattern/item/ule.rs
+++ b/components/datetime/src/pattern/item/ule.rs
@@ -25,7 +25,8 @@ use zerovec::ule::{AsULE, ZeroVecError, ULE};
 ///
 /// If the discriminant is not set, the bottom three bits of the first byte,
 /// together with the next two bytes, contain all 21 bits required to encode
-/// any [`Unicode Code Point`].
+/// any [`Unicode Code Point`]. By design, the representation of a code point
+/// is the same between [`PatternItemULE`] and [`GenericPatternItemULE`].
 ///
 /// # Diagram
 ///
@@ -160,7 +161,8 @@ impl AsULE for PatternItem {
 ///
 /// If the discriminant is not set, the bottom three bits of the first byte,
 /// together with the next two bytes, contain all 21 bits required to encode
-/// any [`Unicode Code Point`].
+/// any [`Unicode Code Point`]. By design, the representation of a code point
+/// is the same between [`PatternItemULE`] and [`GenericPatternItemULE`].
 ///
 /// # Diagram
 ///