Unable to use self in macro because the macro expansion ignores token `self`

Question

I want to write a macro that prints "OK" then returns self in a method. It's my first macro, so I tried this, thinking it will just make something like a text replacement, but it fails:

macro_rules! print_ok_and_return_self { () => { println!("OK"); self } } fn main() { let a = A{}; a.a().a(); } struct A {} impl A { fn a(self) -> Self { print_ok_and_return_self!() } }

Error:

error: macro expansion ignores token `self` and any following --> src/main.rs:4:13 | 4 | self | ^^^^ | note: caused by the macro expansion here; the usage of `print_ok_and_return_self!` is likely invalid in expression context --> src/main.rs:17:13 | 17| print_ok_and_return_self!() | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

After a quick look at the documentation, I know it's not just text replacement, but I still don't know how to make it work.

Nurbol Alpysbayev · Accepted Answer · 2019-12-16 09:46:48Z

There are two errors in a row, let's fix the first one.

The syntax for a macro arm is:

(...) => { ... }

which means that what your macro expands to is:

println!("OK"); self

which is not OK (two statements).

Instead, it should expand to an expression (in this case), which you get by enclosing it within {}:

macro_rules! print_ok_and_return_self { () => { { println!("OK"); self } } }

This leads to the second error:

error[E0424]: `self` is not available in a static method --> <anon>:4:9 | 4 | self | ^^^^ not available in static method ... 17 | print_ok_and_return_self!() | --------------------------- in this macro invocation | = note: maybe a `self` argument is missing?

A macro cannot assume the existence of a variable in its scope, so you need to pass self as an argument:

macro_rules! print_ok_and_return_value { ($v:expr) => {{ println!("OK"); $v }} }

and the invocation becomes:

impl A { fn a(self) -> Self { print_ok_and_return_value!(self) } }

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