pub const SUB_PARTIAL_PREFIX: &str = "Sub:";Expand description
Prefixes of the ResolvedValue::Dynamic reason produced by a partially
resolved Fn::Sub or Fn::Join, and the placeholders standing in for values
the partial could not resolve. Defined once because the resolver writes them
and length estimation reads them back: a partial that still carries a
placeholder has no known length, and only a shared spelling keeps the reader
and the writer from drifting apart.