Function render_mapping 

pub fn render_mapping(m: Mapping, tokens: &PathTokens) -> Mapping

Substitute {{namespace.key}} placeholders in a message template. The caller-supplied resolve closure returns the substituted value, or None to leave the placeholder literal.

Whitespace inside the braces ({{ ctx.primary }}) is ignored so users can format their messages for readability. Apply path-template substitution to every string inside a YAML mapping, recursively into nested mappings and sequences. Non-string values pass through unchanged. Used by nested-rule specs (e.g. for_each_dir) so that the {dir} in a nested rule’s paths, pattern, or partner field resolves to the iterated entry’s path at rule-build time.