如何在`main.rs`中声明一个对整体可见的`use`？

Question

I have a rust project with main.rs , some other modules in the main.rs level and several submodules underneath. I use the anyhow crate in almost all of them.

Is there a clean way to declare use anyhow; only once (probably in main.rs ?) without the need to redclare its usage in every single mod/file?

Answer 1

Note that anyhow itself should be accessible without use (as of Edition 2018), because this is a crate name. If you mean to import something within that crate for your entire project ( use anyhow::Context; ), then no, there is no standard¹ way to have that.

Each module has its own scope for such symbols, which are imported via use . The one exception is in the standard preludes, defined by the compiler itself. These are what enable you to use Copy , Result , and others without having to import them explicitly. However, this is naturally out of your control.

There is also a common pattern for some libraries to provide their own prelude: a module that re-exports symbols that are likely to be useful. anyhow does not provide one at the time of writing.

See also:

• What is the prelude?

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¹Almost anything becomes possible with macros or another preprocessor, but it is hardly a good tradeoff for this case.

Answer 2

Rust doesn't support global imports.

However, for global macro imports, there is a way, as explained here . That won't solve your problem completely, but it will at least allow you to avoid importing the macros.

In your main.rs you can add this:

#[macro_use]
extern crate anyhow;

And then you can use macros like bail! and ensure! all around your project, without the need to explicitly import them.