feat(allocator): Vec<u8>::into_string (#8017)

Bumpalo has a method `String::from_utf8_unchecked` to covert a `Vec<u8>` to a `String`. But we can't use it because we use Bumpalo's `String` as our arena string type, but allocator_api2's `Vec` as our arena vec.

Provide the same functionality, that works around this incompatibility. Also use the faster `simdutf8` for checking that `Vec` contains a valid UTF-8 string.

Cargo.lock

@@ -1452,6 +1452,7 @@ dependencies = [
  "bumpalo",
  "serde",
  "serde_json",
+ "simdutf8",
 ]
 
 [[package]]

crates/oxc_allocator/Cargo.toml

@@ -21,6 +21,7 @@ doctest = false
 [dependencies]
 allocator-api2 = { workspace = true }
 bumpalo = { workspace = true, features = ["allocator-api2", "collections"] }
+simdutf8 = { workspace = true }
 
 serde = { workspace = true, optional = true }
 

crates/oxc_allocator/src/vec.rs

@@ -15,8 +15,9 @@ use allocator_api2::vec;
 use bumpalo::Bump;
 #[cfg(any(feature = "serialize", test))]
 use serde::{ser::SerializeSeq, Serialize, Serializer};
+use simdutf8::basic::{from_utf8, Utf8Error};
 
-use crate::{Allocator, Box};
+use crate::{Allocator, Box, String};
 
 /// A `Vec` without [`Drop`], which stores its data in the arena allocator.
 ///
@@ -180,6 +181,38 @@ impl<'alloc, T> Vec<'alloc, T> {
     }
 }
 
+impl<'alloc> Vec<'alloc, u8> {
+    /// Convert `Vec<u8>` into `String`.
+    ///
+    /// # Errors
+    /// Returns [`Err`] if the `Vec` does not comprise a valid UTF-8 string.
+    pub fn into_string(self) -> Result<String<'alloc>, Utf8Error> {
+        // Check vec comprises a valid UTF-8 string.
+        from_utf8(&self.0)?;
+        // SAFETY: We just checked it's a valid UTF-8 string
+        let s = unsafe { self.into_string_unchecked() };
+        Ok(s)
+    }
+
+    /// Convert `Vec<u8>` into [`String`], without checking bytes comprise a valid UTF-8 string.
+    ///
+    /// Does not copy the contents of the `Vec`, converts in place. This is a zero-cost operation.
+    ///
+    /// # SAFETY
+    /// Caller must ensure this `Vec<u8>` comprises a valid UTF-8 string.
+    #[expect(clippy::missing_safety_doc, clippy::unnecessary_safety_comment)]
+    #[inline] // `#[inline]` because this is a no-op at runtime
+    pub unsafe fn into_string_unchecked(self) -> String<'alloc> {
+        // Cannot use `bumpalo::String::from_utf8_unchecked` because it takes a `bumpalo::collections::Vec`,
+        // and our inner `Vec` type is `allocator_api2::vec::Vec`.
+        // SAFETY: Conversion is safe because both types store data in arena in same way.
+        // Lifetime of returned `String` is same as lifetime of original `Vec<u8>`.
+        let inner = ManuallyDrop::into_inner(self.0);
+        let (ptr, len, cap, bump) = inner.into_raw_parts_with_alloc();
+        String::from_raw_parts_in(ptr, len, cap, bump)
+    }
+}
+
 impl<'alloc, T> ops::Deref for Vec<'alloc, T> {
     type Target = vec::Vec<T, &'alloc Bump>;