Function declares_length 

pub fn declares_length(name: &str) -> bool

Whether the builtin type name reports how many elements it holds.

This is the audience for the one diagnostic that teaches a spelling: a receiver that answers length() is a receiver a program might have written count() on, so Array, Vector, String, Range, Map, and Set are told what the spelling is and everything else is told it has no such method. Deriving the set from the table is the point — it used to be written out at both ends, and the two had drifted by two types.