unreachable!("{}") works on Rust 2021

[m-ou-se]

The behavior of unreachable does not depend the edition of the crate that invoked the macro. That's fine for todo!() and unimplemented!() due to the way they are implemented, but in #92068, it was discovered that unreachable!() behaves subtly different, due to the way it has a special one-argument case. Looks like missed this when working on the 2021 panic macros. :(

[m-ou-se]

todo!() and unimplemented!() are implemented like this:

rust/library/core/src/macros/mod.rs

Lines 738 to 741 in 23f6923

	macro_rules! todo {
	() => ($crate::panic!("not yet implemented"));
	($($arg:tt)+) => ($crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+)));
	}

They never accepted todo!("{}") or todo!(123) since their arguments are always directly passed to format_args!(). So, they didn't have to change for Rust 2021.

unreachable!() however, is implemented differently:

rust/library/core/src/macros/mod.rs

Lines 592 to 602 in 23f6923

	macro_rules! unreachable {
	() => ({
	$crate::panic!("internal error: entered unreachable code")
	});
	($msg:expr $(,)?) => ({
	$crate::unreachable!("{}", $msg)
	});
	($fmt:expr, $($arg:tt)*) => ({
	$crate::panic!($crate::concat!("internal error: entered unreachable code: ", $fmt), $($arg)*)
	});
	}

It doesn't just forward to panic!() or format_args!(), but instead it does its own concat shenanigans, and has special handling of the single argument case.

[m-ou-se]

So then the question is: Do we consider this a bug in Rust 2021 and fix it in Rust 1.59 for that edition? Or leave this in and only fix it in a later edition (or never)?

There's quite a few cases on crates.io of Rust 2015/2018 code depending on this behavior, but Rust 2021 is very new at this point. Adding this change/fix to Rust 2021 probably doesn't break anyone, yet.

[m-ou-se]

Behavior:

unreachable!();          // panic message: "internal error: entered unreachable code"
unreachable!(123);       // panic message: "internal error: entered unreachable code: 123"
unreachable!("{}");      // panic message: "internal error: entered unreachable code: {}"
unreachable!("{}", 123); // panic message: "internal error: entered unreachable code: 123"

[m-ou-se]

We can't fix unreachable without making it a builtin macro, like panic!(). That's because (unlike e.g. debug_assert), this macro doesn't just forward to assert/panic, but has its own problematic implementation with a special case that shouldn't exist.

[jyn514]

We can't fix unreachable without making it a builtin macro, like panic!(). That's because (unlike e.g. debug_assert), this macro doesn't just forward to assert/panic, but has its own problematic implementation with a special case that shouldn't exist.

I'm not sure I follow - can't we remove the concat special case and forward directly to panic, like todo does?

[m-ou-se]

I'm not sure I follow - can't we remove the concat special case and forward directly to panic, like todo does?

@jyn514 Then unreachable!(123) would stop working on Rust 2015/2018.

[m-ou-se]

From a quick grep through crates.io, it looks like there's about 15 crates on crates.io depending on the current behavior that would break if we made it work like todo!(). Here's a few examples from the grep results:

unreachable!(message);
unreachable!(&error)
unreachable!(format!("Cannot compute gcd of {:?} and {:?}", x, y))
Err(err) => unreachable!(err),
_ => unreachable!(format!("Unsupported socket family {}!", raw.family)),
l => unreachable!(format!("got unexpected length {0:?}", l)),
x => unreachable!(x),
Error { message } => unreachable!(message),
unreachable!(false);
unreachable!(msg)
unreachable!(&format!("n = {:?}", n)),
_ => unreachable!(ERROR_MSG),
_ => unreachable!(format!("{} is invalid month!", month)),
hash => { unreachable!(hash); }

[joshtriplett]

Making unreachable built-in to fix this on Rust 2021 seems reasonable to me.

[jhpratt]

IMO as long as there's a crater run fixing this is reasonable. It's certainly a bug.