CODEWITHSUNDEEP
rustrust-macros
C.M.
C.M.

Reputation: 3383

How to import required types inside of a custom derive macro?

I have a custom derive macro which generates some code for a struct it is applied to. That code uses few types (bytes::Buf, ect) that need to be imported.

#[derive(MyMacro)]
struct S { ... }

What is the correct way to handle it?

If I leave it with user to add required imports to a file where my macro is used -- every time macro is changed I run a risk of breaking user code (by introducing new import requirement).

If I add required import to the generated/emitted code -- compiler complain about duplicated imports every time macro is used more than once in the same file. Plus, I run the risk of breaking user code if (after I change the macro) I introduce an import user already used.

Upvotes: 0

Views: 677

Answers (1)

Finomnis
Finomnis

Reputation: 22818

Disclaimer: This is just my understanding of the best practices. I can not offer links to official guidelines backing up my claims. So take them with a grain of salt.

I think that macros in general should never do relative imports, or rely user provided external structs/imports. Macros should be self-contained.

If you do need imports, put your generated code in its own scope and add a use statement there. If you import inside of macros, always use the fully qualified path. That means, if you want to import HashMap, put a use ::std::collections::HashMap; inside of the macro. Again, make sure this import doesn't leak out.

Note the :: in front of it. That annotates that std is a top level module. Otherwise if the macro is used inside of a user::provided::std module, the std would reference this one. Adding the :: in front of it ensures that it is actually the built-in std module.

Upvotes: 2

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