What is the purpose of function-like procedural macros? Are there any function-like procedural macros in standard library?

help
1

In this section of documentation about procedural macros, the docs mention that there are mainly three types of macros, those are:

  • Function-like e.g. custom!()
  • Derive e.g. #[derive(Custom)]
  • Attribute e.g. #[custom]

I can understand attribute and procedural macros, but, at the same time, I wonder why function-like macros exist. Can't we have the same thing with declarative macros using macro_rules!?

So my question is:

  • What is the purpose of function-like macros?
  • Are there any function-like macros defined in the standard library? I think cfg!() is one of them.
2

Function-like macros can do arbitrary things. Unlike declarative macros.

The most famous ones are format_args! and include_bytes!.

3

No. For example, declarative macros can't conjure new identifiers.

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4

Strangely, the source code that the docs of format_args! refer to a declarative macro. :thinking:

Same goes for include_bytes! as well.

5

Those are not real macros (their "definition" is empty), they are compiler built-ins.

6

Correct me if I'm wrong. I believe that means, for a very simple example, the code below wouldn't compile with declarative macros:

macro_rules! new_struct_creator {
    () => {
        struct NewStruct {
            a: i32,
        }
    }
}

This wouldn't compile (haven't tried it actually) because it conjures a new identifier (creates a new struct)? Is that what you mean?

7

No. All of the identifiers that macro deals with are spelled out literally in the code, and it compiles. This doesn't create any new identifiers. It declares a new type, but that's uninteresting from the PoV of macros (they have nothing to do with type checking and don't care about types).

Macros are purely syntactic abstractions. When I mean a new identifier, I mean an identifier and nothing more (in particular, I don't mean any program construct such as a type or variable that identifiers may refer to).

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8

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