Function parse_icu_placeholder 

pub fn parse_icu_placeholder(
    inner: &str,
) -> Result<IcuPlaceholder<'_>, IcuParseError>

Parses one placeholder’s inner text (everything between the outer {/}, e.g. "count, plural, one {# item} other {# items}") — the precondition for calling this at all is that inner contains at least one top-level comma (split_template’s own trigger for treating a placeholder as ICU-dispatch rather than a plain {name}).