`macro_rules!` versions of `assert_tokens` (and `_ser_` and `_de_`) to preserve line number on failure

[rben01]

In serde_test, the family of assert_tokens functions, while useful, would be even more useful if converted to macros. During testing, the location of a failure is reported as the location where the panic occurred, which means assert_tokens failures get traced to the serde_test source file, e.g., /Users/rben01/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_test-1.0.130/src/ser.rs:111:9. If these functions were replaced with macros then the failure would get traced to the correct location in the user's source file. Example, with comments labeling the line they're on:

// Line 160
#[test]
fn test_fn() {
	let test_fn = || {
		assert!(false);
		//Line 165
	};
	test_fn();
}

// Line 170
#[test]
fn test_macro() {
	macro_rules! test_macro {
		//
		// Line 175
		() => {
			assert!(false);
		};
	}
	// Line 180
	test_macro!();
}

The failure in test_fn is reported as being on line 164, where the assert!(false) is, whereas the failure in test_macro is reported as being on line 181, where the failing use of the test_macro! occurred.

In general, converting functions to macros "just because" does not make sense. But when those functions are intended to be used in a testing context, then converting them to macros does make sense because macros preserve line numbers when they fail, whereas functions do not.

[dtolnay]

A track_caller attribute would be more appropriate I think. https://doc.rust-lang.org/1.56.0/reference/attributes/codegen.html#the-track_caller-attribute

[rben01]

@dtolnay TIL that exists. Would definitely be preferable to my (what I now see is a) hack.

[dtolnay]

A track_caller attribute was added in serde-rs/serde#1920.