Function canon 

fn canon(v: &Value) -> String

The text an attribute of this value would have been indexed under.

This is the contract between AttrPred and the block index, and both sides are written against it: Op::Eq on a stored string is defined as equality with this text, and crate::attrs::index writes exactly these bytes for the str, int and bool types. Doubles are not in the index at all — the numeric flag covers them — so a stored double is reached through HAS_DOUBLE rather than through this string.

Every value has a text form, including a fractional double: nothing forces a producer to send 0.5 as a double rather than as "0.5", and returning None here to mean “unindexable” made every fractional-double equality scan every block.