`assert_eq!` has a terrible error message when given a trailing comma

[cramertj]

Trailing commas are an easy mistake to make, especially in a language like Rust where they are often accepted. Erroneously adding a trailing comma to assert_eq! triggers the error message "requires at least a format string argument", which is confusing and leads the user to think (incorrectly) that they need to add some kind of format string. Example.

[KalitaAlexey]

I think we should add a hint to remove the last comma.

[abhijeetbhagat]

er... aren't you supposed to provide one then? :)
assert_eq!(1, 2, "Nah, some other day maybe... {} {}", 1, 2,)

[cramertj]

@abhijeetbhagat You certainly can do that, yes, but you don't have to.

[zackmdavis]

This looks tricky. What's happening is that the comma puts us into the "match a 'token tree' of zero or more additional tokens after the $left and $right args" branch of assert_eq!, those tokens being passed to format_args!, which is what emits the "requires at least a format string argument" message.

What we would want is to only match one or more additional tokens (or emulate this by conditioning on the length of the token tree or something), but I don't immediately see how or whether this can be done with the macro system (my naïve hopes that + instead of * would work as in regular expressions seems to be unfounded).

[zackmdavis]

my naïve hopes that + instead of * would work as in regular expressions seems to be unfounded

Wait, it looks like I was wrong about this!