Function render_error_in 

pub fn render_error_in(
    err: &CompileError,
    source: &str,
    filename: &str,
    homes: &[LabelHome<'_>],
) -> String

Render one error whose secondary labels may lie in other files, coloured as render_errors is: homes[i] says which file err.labels[i] lies in (see CompileError::report_in). A label in another file is drawn in a snippet of that file, under its own name, rather than against source.