rendering without a message highlighter cannot fail semantic

Error message

rendering without a message highlighter cannot fail semantic highlighting

What it means

render_miette always passes None as the message highlighter and asserts that the render call cannot fail semantic highlighting. Since no highlighter is supplied, the miette render of a diagnostic with highlights is expected to succeed; any Err trips this expect. It documents that the no-highlighter rendering path is infallible with respect to semantic highlighting.

Source

Thrown at baml_language/crates/baml_compiler_diagnostics/src/render.rs:307

    diagnostic: &Diagnostic,
    sources: &HashMap<FileId, String>,
    file_paths: &HashMap<FileId, PathBuf>,
    highlights: &SourceHighlights,
    color: bool,
    show_error_codes: bool,
) -> String {
    let highlighter = color.then(|| DiagnosticHighlighter::new(highlights, file_paths));
    let handler = miette_handler(highlighter, color, diagnostic.severity);
    render_miette_with_handler(
        diagnostic,
        sources,
        file_paths,
        &handler,
        color,
        show_error_codes,
        None,
    )
    .expect("rendering without a message highlighter cannot fail semantic highlighting")
}

fn miette_handler(
    highlighter: Option<DiagnosticHighlighter>,
    color: bool,
    severity: Severity,
) -> GraphicalReportHandler {
    let mut theme = if color {
        GraphicalTheme::unicode()
    } else {
        GraphicalTheme::unicode_nocolor()
    };
    if color {
        theme.styles.highlights = annotation_styles(severity);
    }
    let mut handler = GraphicalReportHandler::new_themed(theme)
        .with_links(false)
        .with_urls(false)

View on GitHub (pinned to bd85ce9dee)

Solutions

No action needed for library users.
If encountered in a modified build, propagate the error (return Result) instead of expect, or restore the None-highlighter configuration.
Verify the diagnostic's message highlights are valid byte ranges into the message text if you construct diagnostics manually.

Defensive patterns

Strategy: try-catch

Try / catch

let rendered = std::panic::catch_unwind(|| render_miette(&diag, &sources, &cfg));
if rendered.is_err() { /* fall back to agent-format rendering */ }

Prevention

Keep render_miette's highlighter argument as None as designed.
Ensure diagnostics carry highlight ranges within the message text bounds.
When changing the render pipeline, return Result instead of expect.

When it happens

Trigger: Calling render_miette (via render_diagnostic) only panics if the underlying miette render with highlighter=None returns Err — an unexpected internal condition, not a user input problem.

Common situations: Seen only when modifying the crate: e.g. wiring a highlighter into render_miette or changing miette's GraphicalReportHandler setup so highlighting can fail on normal diagnostics.

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

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/50473323f086c5cc. Report an issue: GitHub.