Function apply_fields 

pub fn apply_fields<'a>(
    text: &str,
    fields: impl IntoIterator<Item = &'a Field>,
    f: impl Fn(&Field, &str) -> Option<String>,
) -> Result<String, RenderError>

Rewrite the spans of one text, returning the result.

f receives each field and the text it covers, and returns the replacement or None to leave it alone. Every field must index text — filter by Field::at before calling, since a range from another location means nothing here.

Nested spans are how the parser reports an address inside a channel name, so a replacement contained in another replacement is dropped: the outer rewrite already covers those bytes. Two replacements that overlap only partially have no such reading and are RenderError::OverlappingSpans.

Bounds and character boundaries are checked for every field, so this never panics on a hand-built span.