feat(span): add Atom::from_strs_array_in method (#11261)

Add `Atom::from_strs_array_in` method, to complement `String::from_strs_array_in`. In many places we were using `String::from_strs_array_in` only as a means to get an `Atom`. Cut out the middleman!

This prepares the way for #11260, and getting rid of `String` entirely.

crates/oxc_allocator/src/allocator.rs

@@ -1,5 +1,9 @@
+use std::{alloc::Layout, ptr, slice, str};
+
 use bumpalo::Bump;
 
+use oxc_data_structures::assert_unchecked;
+
 /// A bump-allocated memory arena.
 ///
 /// # Anatomy of an Allocator
@@ -288,6 +292,109 @@ impl Allocator {
         self.bump.alloc_str(src)
     }
 
+    /// Create new `&str` from a fixed-size array of `&str`s concatenated together,
+    /// allocated in the given `allocator`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the sum of length of all strings exceeds `isize::MAX`.
+    ///
+    /// # Example
+    /// ```
+    /// use oxc_allocator::Allocator;
+    ///
+    /// let allocator = Allocator::new();
+    /// let s = allocator.alloc_concat_strs_array(["hello", " ", "world", "!"]);
+    /// assert_eq!(s, "hello world!");
+    /// ```
+    // `#[inline(always)]` because want compiler to be able to remove checked addition where some of
+    // `strings` are statically known.
+    #[expect(clippy::inline_always)]
+    #[inline(always)]
+    pub fn alloc_concat_strs_array<'a, const N: usize>(&'a self, strings: [&str; N]) -> &'a str {
+        // Calculate total length of all the strings concatenated.
+        //
+        // We have to use `checked_add` here to guard against additions wrapping around
+        // if some of the input `&str`s are very long, or there's many of them.
+        //
+        // However, `&str`s have max length of `isize::MAX`.
+        // https://users.rust-lang.org/t/does-str-reliably-have-length-isize-max/126777
+        // Use `assert_unchecked!` to communicate this invariant to compiler, which allows it to
+        // optimize out the overflow checks where some of `strings` are static, so their size is known.
+        //
+        // e.g. `allocator.from_strs_array_in(["__vite_ssr_import_", str, "__"])`, for example,
+        // requires no checks at all, because the static parts have total length of 20 bytes,
+        // and `str` has max length of `isize::MAX`. `isize::MAX as usize + 20` cannot overflow `usize`.
+        // Compiler can see that, and removes the overflow check.
+        // https://godbolt.org/z/MGh44Yz5d
+        #[expect(clippy::checked_conversions)]
+        let total_len = strings.iter().fold(0usize, |total_len, s| {
+            let len = s.len();
+            // SAFETY: `&str`s have maximum length of `isize::MAX`
+            unsafe { assert_unchecked!(len <= (isize::MAX as usize)) };
+            total_len.checked_add(len).unwrap()
+        });
+        assert!(
+            isize::try_from(total_len).is_ok(),
+            "attempted to create a string longer than `isize::MAX` bytes"
+        );
+
+        // Create actual `&str` in a separate function, to ensure that `alloc_concat_strs_array`
+        // is inlined, so that compiler has knowledge to remove the overflow checks above.
+        // When some of `strings` are static, this function is usually only a few instructions.
+        // Compiler can choose whether or not to inline `alloc_concat_strs_array_with_total_len_in`.
+        // SAFETY: `total_len` has been calculated correctly above.
+        // `total_len` is `<= isize::MAX`.
+        unsafe { self.alloc_concat_strs_array_with_total_len_in(strings, total_len) }
+    }
+
+    /// Create a new `&str` from a fixed-size array of `&str`s concatenated together,
+    /// allocated in the given `allocator`, with provided `total_len`.
+    ///
+    /// # SAFETY
+    /// * `total_len` must be the total length of all `strings` concatenated.
+    /// * `total_len` must be `<= isize::MAX`.
+    unsafe fn alloc_concat_strs_array_with_total_len_in<'a, const N: usize>(
+        &'a self,
+        strings: [&str; N],
+        total_len: usize,
+    ) -> &'a str {
+        if total_len == 0 {
+            return "";
+        }
+
+        // Allocate `total_len` bytes.
+        // SAFETY: Caller guarantees `total_len <= isize::MAX`.
+        let layout = unsafe { Layout::from_size_align_unchecked(total_len, 1) };
+        let start_ptr = self.bump().alloc_layout(layout);
+
+        let mut end_ptr = start_ptr;
+        for str in strings {
+            let src_ptr = str.as_ptr();
+            let len = str.len();
+
+            // SAFETY:
+            // `src` is obtained from a `&str` with length `len`, so is valid for reading `len` bytes.
+            // `end_ptr` is within bounds of the allocation. So is `end_ptr + len`.
+            // `u8` has no alignment requirements, so `src_ptr` and `end_ptr` are sufficiently aligned.
+            // No overlapping, because we're copying from an existing `&str` to a newly allocated buffer.
+            unsafe { ptr::copy_nonoverlapping(src_ptr, end_ptr.as_ptr(), len) };
+
+            // SAFETY: We allocated sufficient capacity for all the strings concatenated.
+            // So `end_ptr.add(len)` cannot go out of bounds.
+            end_ptr = unsafe { end_ptr.add(len) };
+        }
+
+        debug_assert_eq!(end_ptr.as_ptr() as usize - start_ptr.as_ptr() as usize, total_len);
+
+        // SAFETY: We have allocated and filled `total_len` bytes starting at `start_ptr`.
+        // Concatenating multiple `&str`s results in a valid UTF-8 string.
+        unsafe {
+            let slice = slice::from_raw_parts(start_ptr.as_ptr(), total_len);
+            str::from_utf8_unchecked(slice)
+        }
+    }
+
     /// Reset this allocator.
     ///
     /// Performs mass deallocation on everything allocated in this arena by resetting the pointer
@@ -469,4 +576,48 @@ mod test {
         }
         allocator.reset();
     }
+
+    #[test]
+    fn string_from_array_len_1() {
+        let allocator = Allocator::default();
+        let s = allocator.alloc_concat_strs_array(["hello"]);
+        assert_eq!(s, "hello");
+    }
+
+    #[test]
+    fn string_from_array_len_2() {
+        let allocator = Allocator::default();
+        let s = allocator.alloc_concat_strs_array(["hello", "world!"]);
+        assert_eq!(s, "helloworld!");
+    }
+
+    #[test]
+    fn string_from_array_len_3() {
+        let hello = "hello";
+        let world = std::string::String::from("world");
+        let allocator = Allocator::default();
+        let s = allocator.alloc_concat_strs_array([hello, &world, "!"]);
+        assert_eq!(s, "helloworld!");
+    }
+
+    #[test]
+    fn string_from_empty_array() {
+        let allocator = Allocator::default();
+        let s = allocator.alloc_concat_strs_array([]);
+        assert_eq!(s, "");
+    }
+
+    #[test]
+    fn string_from_array_of_empty_strs() {
+        let allocator = Allocator::default();
+        let s = allocator.alloc_concat_strs_array(["", "", ""]);
+        assert_eq!(s, "");
+    }
+
+    #[test]
+    fn string_from_array_containing_some_empty_strs() {
+        let allocator = Allocator::default();
+        let s = allocator.alloc_concat_strs_array(["", "hello", ""]);
+        assert_eq!(s, "hello");
+    }
 }

crates/oxc_allocator/src/string.rs

@@ -130,6 +130,9 @@ impl<'alloc> String<'alloc> {
     /// Create a new [`String`] from a fixed-size array of `&str`s concatenated together,
     /// allocated in the given `allocator`.
     ///
+    /// If you're not altering the `String` after this call, and just converting it to an `Atom`,
+    /// `Atom::from_strs_array_in` may be slightly more efficient.
+    ///
     /// # Examples
     /// ```
     /// use oxc_allocator::{Allocator, String};

crates/oxc_ast/src/ast_builder_impl.rs

@@ -2,7 +2,7 @@
 
 use std::borrow::Cow;
 
-use oxc_allocator::{Allocator, AllocatorAccessor, Box, FromIn, IntoIn, String, Vec};
+use oxc_allocator::{Allocator, AllocatorAccessor, Box, FromIn, IntoIn, Vec};
 use oxc_span::{Atom, SPAN, Span};
 use oxc_syntax::{number::NumberBase, operator::UnaryOperator, scope::ScopeId};
 
@@ -90,9 +90,8 @@ impl<'a> AstBuilder<'a> {
 
     /// Allocate an [`Atom`] from an array of string slices.
     #[inline]
-    pub fn atom_from_strs_array<const N: usize>(self, array: [&str; N]) -> Atom<'a> {
-        let string = String::from_strs_array_in(array, self.allocator);
-        Atom::from(string)
+    pub fn atom_from_strs_array<const N: usize>(self, strings: [&str; N]) -> Atom<'a> {
+        Atom::from_strs_array_in(strings, self.allocator)
     }
 
     /// Convert a [`Cow<'a, str>`] to an [`Atom<'a>`].

crates/oxc_span/src/atom.rs

@@ -61,6 +61,33 @@ impl<'a> Atom<'a> {
     pub fn to_compact_str(self) -> CompactStr {
         CompactStr::new(self.as_str())
     }
+
+    /// Create new [`Atom`] from a fixed-size array of `&str`s concatenated together,
+    /// allocated in the given `allocator`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the sum of length of all strings exceeds `isize::MAX`.
+    ///
+    /// # Example
+    /// ```
+    /// use oxc_allocator::Allocator;
+    /// use oxc_span::Atom;
+    ///
+    /// let allocator = Allocator::new();
+    /// let s = Atom::from_strs_array_in(["hello", " ", "world", "!"], &allocator);
+    /// assert_eq!(s.as_str(), "hello world!");
+    /// ```
+    // `#[inline(always)]` because want compiler to be able to optimize where some of `strings`
+    // are statically known. See `Allocator::alloc_concat_strs_array`.
+    #[expect(clippy::inline_always)]
+    #[inline(always)]
+    pub fn from_strs_array_in<const N: usize>(
+        strings: [&str; N],
+        allocator: &'a Allocator,
+    ) -> Atom<'a> {
+        Self::from(allocator.alloc_concat_strs_array(strings))
+    }
 }
 
 impl<'new_alloc> CloneIn<'new_alloc> for Atom<'_> {

crates/oxc_transformer/src/typescript/rewrite_extensions.rs

@@ -5,7 +5,6 @@
 //!
 //! Based on Babel's [plugin-rewrite-ts-imports](https://github.com/babel/babel/blob/3bcfee232506a4cebe410f02042fb0f0adeeb0b1/packages/babel-preset-typescript/src/plugin-rewrite-ts-imports.ts)
 
-use oxc_allocator::String as ArenaString;
 use oxc_ast::ast::{
     ExportAllDeclaration, ExportNamedDeclaration, ImportDeclaration, StringLiteral,
 };
@@ -43,9 +42,7 @@ impl TypeScriptRewriteExtensions {
         source.value = if self.mode.is_remove() {
             Atom::from(without_extension)
         } else {
-            let new_value =
-                ArenaString::from_strs_array_in([without_extension, replace], ctx.ast.allocator);
-            Atom::from(new_value)
+            ctx.ast.atom_from_strs_array([without_extension, replace])
         };
         source.raw = None;
     }

crates/oxc_traverse/src/context/uid.rs

@@ -285,8 +285,8 @@ impl<'a> UidGenerator<'a> {
             let mut buffer = ItoaBuffer::new();
             let digits = buffer.format(uid_name.postfix);
 
-            let uid = if uid_name.underscore_count == 1 {
-                ArenaString::from_strs_array_in(["_", base, digits], self.allocator)
+            if uid_name.underscore_count == 1 {
+                Atom::from_strs_array_in(["_", base, digits], self.allocator)
             } else {
                 let mut uid = ArenaString::with_capacity_in(
                     uid_name.underscore_count as usize + base.len() + digits.len(),
@@ -295,12 +295,10 @@ impl<'a> UidGenerator<'a> {
                 uid.extend(iter::repeat_n("_", uid_name.underscore_count as usize));
                 uid.push_str(base);
                 uid.push_str(digits);
-                uid
-            };
-
-            Atom::from(uid)
+                Atom::from(uid)
+            }
         } else {
-            let uid = Atom::from(ArenaString::from_strs_array_in(["_", base], self.allocator));
+            let uid = Atom::from_strs_array_in(["_", base], self.allocator);
             // SAFETY: String starts with `_`, so trimming off that byte leaves a valid UTF-8 string
             let base = unsafe { uid.as_str().get_unchecked(1..) };
             self.names.insert(base, UidName { underscore_count: 1, postfix: 1 });