slint-ui/slint · info

Not empty

Error message

Not empty

What it means

Internal invariant panic from `.expect("Not empty")` on `element_text_lines.first()` in tools/lsp/preview/drop_location.rs:1235. The enclosing `if element_text_lines.is_empty()` branch guarantees the vector has at least one element, so `first()` is `Some`; the expect documents that checked precondition while building the re-inserted element text with its pre-indent.

Solutions

1. Keep the `first()` access inside the branch guarded by `!element_text_lines.is_empty()`.
2. If the guard cannot be proven locally, switch to `element_text_lines.first().map(...).unwrap_or_default()`.
3. Prefer `if let Some(first) = element_text_lines.first()` to make the invariant explicit in code rather than in a panic message.

Example fix

// before
let mut tmp = format!("{}{}\n", drop_info.insert_info.pre_indent, element_text_lines.first().expect("Not empty"));
// after
let Some(first_line) = element_text_lines.first() else { return String::new(); };
let mut tmp = format!("{}{}\n", drop_info.insert_info.pre_indent, first_line);

if element_text_lines.is_empty() { return String::new(); }

fn first_line(lines: &[String]) -> Option<&str> { lines.first().map(String::as_str) }

// Rust: no catch; use let-else instead of expect
let Some(first) = element_text_lines.first() else { return String::new(); };

When it happens

Trigger: Not triggerable as written — the guard `if element_text_lines.is_empty() { String::new() } else { ... }` ensures `first()` succeeds. Would only panic if the code were restructured so the `.first()` call no longer sits inside the non-empty else branch (e.g. hoisting it out or converting the if to a match on something else).

Common situations: Encountered by maintainers after refactoring the workspace-edit builder in drop_location.rs so the emptiness check and the `first()` access become separated, or by anyone copying this pattern without the guard.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• There is always one line
• Global was just valid
• Length was checked
• an identifier
• EditorSession must have at least one preview

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/b4f1b589a43e998a. Report an issue: GitHub.

Appendix: source

Thrown at tools/lsp/preview/drop_location.rs:1235

}

pub fn create_swap_element_workspace_edit(
    drop_info: &DropInformation,
    element: &editor_preview::ElementRcNode,
    placeholder_text: String,
    format: editor_preview::ByteFormat,
) -> Option<(lsp_types::WorkspaceEdit, DropData)> {
    let component_type = element.component_type();
    let new_text = {
        let element_text_lines = extract_text_of_element(element, &["x", "y"]);

        if element_text_lines.is_empty() {
            String::new()
        } else {
            let mut tmp = format!(
                "{}{}\n",
                drop_info.insert_info.pre_indent,
                element_text_lines.first().expect("Not empty")
            );

            for l in element_text_lines.iter().take(element_text_lines.len() - 1).skip(1) {
                tmp.push_str(&format!("{}{l}\n", drop_info.insert_info.indent));
            }

            if element_text_lines.len() >= 2 {
                tmp.push_str(&format!(
                    "{}{}\n{}",
                    drop_info.insert_info.indent,
                    element_text_lines.last().expect("Length was checked"),
                    drop_info.insert_info.post_indent
                ));
            }

            tmp
        }
    };

View on GitHub (pinned to bb937076de)