BREAKING CHANGE: String::at and StringView::at now return UInt16

- String::at and StringView::at now return UInt16 instead of Int
- These methods are now the primary interface for UTF-16 code unit access
- code_unit_at is now an alias for at (previously it was the other way around)
- The old Int-returning behavior is preserved in String::charcode_at (deprecated)

This change aligns the API with the actual UTF-16 representation of strings
and makes the type system more accurate.

Author: bobzhang

CHANGELOG.md

@@ -12,8 +12,13 @@ changelog should follow [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
 
 #### Changed
 
+- **BREAKING**: `String::at` and `StringView::at` now return `UInt16` instead of `Int` and are the primary methods for accessing UTF-16 code units
+- **BREAKING**: `String::code_unit_at` and `StringView::code_unit_at` are now aliases for `at` instead of being separate methods
+
 #### Deprecated
 
+- `String::charcode_at` (previously named `at`) is now deprecated, use `String::at` which returns `UInt16`
+
 #### Removed
 
 #### Fixed

builtin/intrinsics.mbt

@@ -1558,16 +1558,16 @@ pub fn String::length(self : String) -> Int = "%string_length"
 /// * `index` : The position in the string from which to retrieve the code unit.
 ///
 /// This method has O(1) complexity.
-#alias(charcode_at)
-#alias("_[_]")
 #deprecated("Use `code_unit_at` instead which returns UInt16")
-pub fn String::at(self : String, idx : Int) -> Int = "%string_get"
+pub fn String::charcode_at(self : String, idx : Int) -> Int = "%string_get"
 
 ///|
 /// Returns the UTF-16 code unit at the given index.
 /// 
 /// This method has O(1) complexity.
-pub fn String::code_unit_at(self : String, idx : Int) -> UInt16 = "%string_get"
+#alias("_[_]")
+#alias(code_unit_at)
+pub fn String::at(self : String, idx : Int) -> UInt16 = "%string_get"
 
 ///|
 /// Returns the UTF-16 code unit at a given position in the string without

builtin/pkg.generated.mbti

@@ -683,14 +683,14 @@ pub fn Double::until(Double, Double, step? : Double, inclusive? : Bool) -> Iter[
 pub fn Double::upto(Double, Double, inclusive? : Bool) -> Iter[Double]
 
 #alias("_[_]")
-#alias(charcode_at)
-#deprecated
-pub fn String::at(String, Int) -> Int
+#alias(code_unit_at)
+pub fn String::at(String, Int) -> UInt16
 #alias(codepoint_length, deprecated)
 pub fn String::char_length(String, start_offset? : Int, end_offset? : Int) -> Int
 pub fn String::char_length_eq(String, Int, start_offset? : Int, end_offset? : Int) -> Bool
 pub fn String::char_length_ge(String, Int, start_offset? : Int, end_offset? : Int) -> Bool
-pub fn String::code_unit_at(String, Int) -> UInt16
+#deprecated
+pub fn String::charcode_at(String, Int) -> Int
 #deprecated
 pub fn String::codepoint_at(String, Int) -> Char
 pub fn String::contains(String, StringView) -> Bool
@@ -955,12 +955,11 @@ pub fn BytesView::to_bytes(Self) -> Bytes
 pub fn BytesView::to_fixedarray(Self) -> FixedArray[Byte]
 
 #alias("_[_]")
-#deprecated
-pub fn StringView::at(Self, Int) -> Int
+#alias(code_unit_at)
+pub fn StringView::at(Self, Int) -> UInt16
 pub fn StringView::char_length(Self) -> Int
 pub fn StringView::char_length_eq(Self, Int) -> Bool
 pub fn StringView::char_length_ge(Self, Int) -> Bool
-pub fn StringView::code_unit_at(Self, Int) -> UInt16
 pub fn StringView::contains(Self, Self) -> Bool
 pub fn StringView::contains_any(Self, chars~ : Self) -> Bool
 pub fn StringView::contains_char(Self, Char) -> Bool

builtin/stringview.mbt

@@ -27,24 +27,13 @@ fn StringView::end(self : StringView) -> Int = "%stringview.end"
 ///|
 fn StringView::make_view(str : String, start : Int, end : Int) -> StringView = "%stringview.make"
 
-///|
-/// Returns the charcode(UTF-16 code unit) at the given index.
-/// 
-/// This method has O(1) complexity.
-#alias("_[_]")
-#deprecated("Use `code_unit_at` instead which returns UInt16")
-pub fn StringView::at(self : StringView, index : Int) -> Int {
-  guard index >= 0 && index < self.length() else {
-    abort("Index out of bounds")
-  }
-  self.str().unsafe_charcode_at(self.start() + index)
-}
-
 ///|
 /// Returns the UTF-16 code unit at the given index.
 /// 
 /// This method has O(1) complexity.
-pub fn StringView::code_unit_at(self : StringView, index : Int) -> UInt16 {
+#alias("_[_]")
+#alias(code_unit_at)
+pub fn StringView::at(self : StringView, index : Int) -> UInt16 {
   guard index >= 0 && index < self.length() else {
     abort("Index out of bounds")
   }