Equivalent of __func__ or __FUNCTION__ in Rust?

You can hack one together with std::any::type_name.

macro_rules! function {
    () => {{
        fn f() {}
        fn type_name_of<T>(_: T) -> &'static str {
            std::any::type_name::<T>()
        }
        let name = type_name_of(f);
        &name[..name.len() - 3]
    }}
}

Note that this gives a full pathname, so my::path::my_func instead of just my_func. A demo is available.

It appears that function_name crate will do this.

https://docs.rs/function_name/latest/function_name/

The example from the docs is

use ::function_name::named;

#[named]
fn my_super_duper_function ()
{
    assert_eq!(
        function_name!(),
        "my_super_duper_function",
    );
}

I am not involved with the project and have not actually tried it yet.

Adding to Veedrac's answer, you can get the function's name without its full path by adding this:

macro_rules! function {
    () => {{
        fn f() {}
        fn type_name_of<T>(_: T) -> &'static str {
            std::any::type_name::<T>()
        }
        let name = type_name_of(f);

        // Find and cut the rest of the path
        match &name[..name.len() - 3].rfind(':') {
            Some(pos) => &name[pos + 1..name.len() - 3],
            None => &name[..name.len() - 3],
        }
    }};
}

You will get my_func instead of my::path::my_func for example.

There was an RFC about this, but it was never agreed upon or implemented.

The rationale for its absence:

"In general I don't think any of us have given an inordinate amount of thought to these "debugging related" macros in terms of long term stability. Most of them seem fairly harmless, but committing to provide all of them for all Rust programs forever is a strong commitment to make. We may want to briefly consider the story of these macros in conjunction with considering adding this new macro."

Maybe Rust will have something comparable in the future,
but for now you will need to rely on your own tagging.

side note: __FUNCTION__ is non standard, __func__ exists in C99 / C++11.