# Background
The KCL interpreter (written in Rust) reports errors, lints and other diagnostics as UTF-8 source range offsets. JavaScript, including CodeMirror, represents source code as UTF-16 strings.
# Problem
This means the UTF-8 source ranges sent from Rust don't correspond to the same text in the JS UTF-16 GUI. At best, this means the source ranges highlight the wrong thing if you use non-ASCII characters. At worst, it can cause exceptions by trying to highlight a range that doesn't even exist.
Here's the problem, on main:
<img width="178" alt="Screenshot 2025-06-20 at 11 52 03 AM" src="https://github.com/user-attachments/assets/9a4e75bf-965f-49d8-b238-8868b35912a1" />
# Solution
We define a KCL SourceRange as _always_ being UTF-8. This means if a source range is constructed in JS, it should be converted to UTF-8. When these ranges are converted into CodeMirror diagnostics, they should be converted to UTF-16.
Here's the same code as above, but on this branch:
<img width="170" alt="Screenshot 2025-06-20 at 11 50 55 AM" src="https://github.com/user-attachments/assets/a5b971c0-0b02-4acd-8fcf-5a133331682b" />
Closes https://github.com/KittyCAD/modeling-app/issues/4327
53d6613d0d