Lib.rs

printer attribute prints the name of the function when the function is called.

Options for the attribute (separated by comma)

#[printer($a, ?$b, $3, test, prefix = "ABC ", suffix = " DEF")]
fn foo(a: u32, b: u32, c: u32) -> u32 {
    a + b + c
}

It adds #[cfg(test)] println!("ABC foo(a: {a}, b: {b:?}, c: {c}) DEF"); at the first line of foo. $a means it should print the value of a using Display trait. ? at ?$b means it uses Debug trait instead of Display. $3 points the third argument, which is c. If you want to print all the arguments simply write all or ?all.

test adds #[cfg(test)]. There are 3 profiles available: test, debug, and release. If no profile is given, it works on all profiles. Multiple profiles are also possible: #[printer(test, debug)] will add #[cfg(all(test, debug_assertions))] at the expanded result.

Don't use braces inside suffix and prefix.

If no option is given at all (#[printer]), it's equivalent to #[printer(?all)].

#[printer(dump)]
fn foo() {}

dump option dumps the expanded macro to stdout when compiled. It only shows the dump when compiled, not when run. It means you have to read the stdout after cargo build, not cargo run. cargo run works only when it's newly compiled.

#[printer(name = "foo")]
fn bar() {}

It uses the name foo instead of bar.

#[printer(eprintln)]
fn bar() {}

It uses eprintln instead of println. There're also print and eprint.

#[printer(cfg = "baz")]
fn foo() {}

It works only when #[cfg(feature = "baz")]. You can mix it with test, debug, and release. You can use at most one flag.

#[printer($a, cond(a > 3))]
fn foo(a: u32) -> u32 { a + 1 }

It works only when a > 3. It wraps the print macro with an if statement.