i did search google,no clue
thank you, so what does "use_std" mean?
It's a feature defined in the project
Cargo.toml which you can select if you like
Without knowing where you found that cfg statement I would guess as follows:
Typically our Rust programs make use of the standard library "std::...".
But if you are building a program for an embedded system or such like it may well not have an operating system an cannot support the standard library.
Look like that crate has a configuration option to cause it to be dependent on the standard library or not, depending on where you want to us it.