I think numeric columns should be handled in rust-postgres. Numeric columns are very common data types for high precision requirements.
Looking atdiesel-pg implementation, it seems to be converting the raw vec to bigdecimal and bigint.

https://github.com/diesel-rs/diesel/blob/master/diesel/src/pg/types/numeric.rs

bigdecimal
num_bigint

I couldn't wrap my head around the implementation, it's more complex that I though it would be.