9e7302520e
## Problem
Zed was crashing with a UTF-8 character boundary error when rendering
text containing multi-byte characters (like emojis or CJK characters):
```
Thread "main" panicked with "byte index 49 is not a char boundary; it is inside '…' (bytes 48..51)"
```
## Root Cause Analysis
The PR reviewer correctly identified that the issue was not in the
DirectWrite boundary handling, but rather in the text run length
calculation in the text system. When text runs are split across lines in
`text_system.rs:426`, the calculation:
```rust
let run_len_within_line = cmp::min(line_end, run_start + run.len) - run_start;
```
This could result in `run_len_within_line` values that don't respect
UTF-8 character boundaries, especially when multi-byte characters (like
'…' which is 3 bytes) get split across lines. The resulting `FontRun`
objects would have lengths that don't align with character boundaries,
causing the panic when DirectWrite tries to slice the string.
## Solution
Fixed the issue by adding UTF-8 character boundary validation in the
text system where run lengths are calculated. The fix ensures that when
text runs are split across lines, the split always occurs at valid UTF-8
character boundaries:
```rust
// Ensure the run length respects UTF-8 character boundaries
if run_len_within_line > 0 {
let text_slice = &line_text[run_start - line_start..];
if run_len_within_line < text_slice.len() && !text_slice.is_char_boundary(run_len_within_line) {
// Find the previous character boundary using efficient bit-level checking
// UTF-8 characters are at most 4 bytes, so we only need to check up to 3 bytes back
let lower_bound = run_len_within_line.saturating_sub(3);
let search_range = &text_slice.as_bytes()[lower_bound..=run_len_within_line];
// SAFETY: A valid character boundary must exist in this range because:
// 1. run_len_within_line is a valid position in the string slice
// 2. UTF-8 characters are at most 4 bytes, so some boundary exists in [run_len_within_line-3..=run_len_within_line]
let pos_from_lower = unsafe {
search_range
.iter()
.rposition(|&b| (b as i8) >= -0x40)
.unwrap_unchecked()
};
run_len_within_line = lower_bound + pos_from_lower;
}
}
```
## Testing
- ✅ Builds successfully on all platforms
- ✅ Eliminates UTF-8 character boundary panics
- ✅ Maintains existing functionality for all text types
- ✅ Handles edge cases like very long multi-byte characters
## Benefits
1. **Root cause fix**: Addresses the issue at the source rather than
treating symptoms
2. **Performance optimal**: Uses the same efficient algorithm as the
standard library
3. **Minimal changes**: Only modifies the specific problematic code path
4. **Future compatible**: Can be easily replaced with
`str::floor_char_boundary()` when stabilized
## Alternative Approaches Considered
1. **DirectWrite boundary fixing**: Initially tried to fix in
DirectWrite, but this was treating symptoms rather than the root cause
2. **Helper function approach**: Considered extracting to a helper
function, but inlined implementation is more appropriate for this
specific use case
3. **Standard library methods**: `floor_char_boundary()` is not yet
stable, so implemented equivalent logic
The chosen approach provides the best balance of performance, safety,
and code maintainability.
---
Release Notes:
- N/A
Welcome to GPUI!
GPUI is a hybrid immediate and retained mode, GPU accelerated, UI framework for Rust, designed to support a wide variety of applications.
Getting Started
GPUI is still in active development as we work on the Zed code editor and isn't yet on crates.io. You'll also need to use the latest version of stable Rust and be on macOS or Linux. Add the following to your Cargo.toml:
gpui = { git = "https://github.com/zed-industries/zed" }
Everything in GPUI starts with an Application. You can create one with Application::new(), and kick off your application by passing a callback to Application::run(). Inside this callback, you can create a new window with App::open_window(), and register your first root view. See gpui.rs for a complete example.
Dependencies
GPUI has various system dependencies that it needs in order to work.
macOS
On macOS, GPUI uses Metal for rendering. In order to use Metal, you need to do the following:
- Install Xcode from the macOS App Store, or from the Apple Developer website. Note this requires a developer account.
Ensure you launch Xcode after installing, and install the macOS components, which is the default option. If you are on macOS 26 (Tahoe) you will need to use
--features gpui/runtime_shadersor add the feature in the rootCargo.toml
-
Install Xcode command line tools
xcode-select --install -
Ensure that the Xcode command line tools are using your newly installed copy of Xcode:
sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
The Big Picture
GPUI offers three different registers depending on your needs:
-
State management and communication with
Entity's. Whenever you need to store application state that communicates between different parts of your application, you'll want to use GPUI's entities. Entities are owned by GPUI and are only accessible through an owned smart pointer similar to anRc. See theapp::contextmodule for more information. -
High level, declarative UI with views. All UI in GPUI starts with a view. A view is simply an
Entitythat can be rendered, by implementing theRendertrait. At the start of each frame, GPUI will call this render method on the root view of a given window. Views build a tree ofelements, lay them out and style them with a tailwind-style API, and then give them to GPUI to turn into pixels. See thedivelement for an all purpose swiss-army knife of rendering. -
Low level, imperative UI with Elements. Elements are the building blocks of UI in GPUI, and they provide a nice wrapper around an imperative API that provides as much flexibility and control as you need. Elements have total control over how they and their child elements are rendered and can be used for making efficient views into large lists, implement custom layouting for a code editor, and anything else you can think of. See the
elementmodule for more information.
Each of these registers has one or more corresponding contexts that can be accessed from all GPUI services. This context is your main interface to GPUI, and is used extensively throughout the framework.
Other Resources
In addition to the systems above, GPUI provides a range of smaller services that are useful for building complex applications:
-
Actions are user-defined structs that are used for converting keystrokes into logical operations in your UI. Use this for implementing keyboard shortcuts, such as cmd-q. See the
actionmodule for more information. -
Platform services, such as
quit the apporopen a URLare available as methods on theapp::App. -
An async executor that is integrated with the platform's event loop. See the
executormodule for more information., -
The
[gpui::test]macro provides a convenient way to write tests for your GPUI applications. Tests also have their own kind of context, aTestAppContextwhich provides ways of simulating common platform input. Seeapp::test_contextandtestmodules for more details.
Currently, the best way to learn about these APIs is to read the Zed source code, ask us about it at a fireside hack, or drop a question in the Zed Discord. We're working on improving the documentation, creating more examples, and will be publishing more guides to GPUI on our blog.