From e14701c1503dbfdb755359af4eed70abc86c9401 Mon Sep 17 00:00:00 2001
From: Don Isaac <donald.isaac@gmail.com>
Date: Mon, 30 Sep 2024 18:42:22 -0400
Subject: [PATCH] feat(allocator): add `Vec::into_boxed_slice`

---
 crates/oxc_allocator/src/boxed.rs | 12 +++++++++
 crates/oxc_allocator/src/vec.rs   | 41 ++++++++++++++++++++++++++++++-
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/crates/oxc_allocator/src/boxed.rs b/crates/oxc_allocator/src/boxed.rs
index 5ef4eef87a7e8..f003acaa39035 100644
--- a/crates/oxc_allocator/src/boxed.rs
+++ b/crates/oxc_allocator/src/boxed.rs
@@ -76,6 +76,18 @@ impl<'alloc, T> Box<'alloc, T> {
     }
 }
 
+impl<'alloc, T: ?Sized> Box<'alloc, T> {
+    /// Create a [`Box`] from a raw pointer to a value in the arena.
+    ///
+    /// # SAFETY
+    /// It is only safe to create a `Box` this way with pointers taken out of
+    /// another `Box` in the same arena.
+    #[inline]
+    pub(crate) const unsafe fn from_non_null(ptr: NonNull<T>) -> Self {
+        Self(ptr, PhantomData)
+    }
+}
+
 impl<'alloc, T: ?Sized> ops::Deref for Box<'alloc, T> {
     type Target = T;
 
diff --git a/crates/oxc_allocator/src/vec.rs b/crates/oxc_allocator/src/vec.rs
index 551faaf3f1613..5ea602df2a356 100644
--- a/crates/oxc_allocator/src/vec.rs
+++ b/crates/oxc_allocator/src/vec.rs
@@ -14,7 +14,7 @@ use bumpalo::Bump;
 #[cfg(any(feature = "serialize", test))]
 use serde::{ser::SerializeSeq, Serialize, Serializer};
 
-use crate::Allocator;
+use crate::{Allocator, Box};
 
 /// Bumpalo Vec
 #[derive(Debug, PartialEq, Eq)]
@@ -101,6 +101,33 @@ impl<'alloc, T> Vec<'alloc, T> {
         vec.extend(iter);
         Self(vec)
     }
+
+    /// Converts the vector into [`Box<[T]>`][owned slice].
+    ///
+    /// If the vector has excess capacity, its items will be moved into a
+    /// newly-allocated buffer with exactly the right capacity. That is, any
+    /// excess capacity will be removed.
+    ///
+    /// [owned slice]: Box
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use oxc_allocator::{Allocator, Vec};
+    ///
+    /// let allocator = Allocator::default();
+    /// let v = Vec::from_iter_in([1, 2, 3], &allocator);
+    /// let b = v.into_boxed_slice();
+    ///
+    /// assert_eq!(&*b, &[1, 2, 3]);
+    /// ```
+    pub fn into_boxed_slice(self) -> Box<'alloc, [T]> {
+        let b = self.0.into_boxed_slice();
+        let (ptr, _) = allocator_api2::boxed::Box::into_non_null(b);
+        // SAFETY: this is effectively a transmute from allocator_api2's box to
+        // our own representation. Ptr is non-null and points to initialized, well-aligned memory.
+        unsafe { Box::from_non_null(ptr) }
+    }
 }
 
 impl<'alloc, T> ops::Deref for Vec<'alloc, T> {
@@ -219,4 +246,16 @@ mod test {
             program
         }
     }
+
+    #[test]
+    fn test_vec_to_boxed_slice() {
+        // file: oxc_allocator/src/vec.rs
+        use crate::boxed::Box;
+
+        let allocator = Allocator::default();
+        let v = Vec::from_iter_in([1, 2, 3], &allocator);
+        let b: Box<[i32]> = v.into_boxed_slice(); // <- compiler error
+
+        assert_eq!(&*b, &[1, 2, 3]);
+    }
 }