What does "#[cfg(feature = "use_std")]" mean

i did search google,no clue

https://doc.rust-lang.org/rust-by-example/attribute/cfg.html

https://doc.rust-lang.org/reference/conditional-compilation.html

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

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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.

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