Function render_diagnostic 

pub fn render_diagnostic(source: &str, path: &str, err: &Diagnostic) -> String

Renders err against source (identified as path in the output) using miette’s graphical report handler, producing the source line, a caret under the exact span, and the help text if present.

The handler is fixed to unicode box-drawing, no color, and an explicit 80-column width, so the rendered string is byte-identical regardless of the terminal, OS, or CI runner it’s produced on (see the module doc comment).