Rust Reference documents operator associativity incorrectly

[jameysharp]

In section 7.2.12.8 "Operator precedence", the Rust reference says, "Operators at the same precedence level are evaluated left-to-right." I assume this is intended to mean that the binary operators are all left-associative.

However, the comparison operators (== != < > <= >=) are apparently non-associative, requiring explicit parentheses if you want to apply one to the result of another, as in (2 < 3) == true. (That's a silly example, but I have in the past found utility in expressions like (a < b) != (c < d).)

I think the implemented behavior is both reasonable and a good idea, so this is just a documentation bug, but I'm not really sure how to fix the documentation to be clear.

[steveklabnik]

/cc @rust-lang/lang , is this intentional? if so, I will fix the docs

[eddyb]

Yes, it is. I believe it has to do with how a < b < c, even if it parsed, wouldn't be a < b && b < c like in some other languages (Python?) and would produce confusing type error messages in most cases.

[durka]

It actually does parse, even after the chained comparison error, and produces the expected type error :)

[Aatch]

@steveklabnik it's complicated, as the parser does technically parse a == b == c == d as ((a == b) == c) == d (I think, this is a based on the error messages I got). I think the fact that chained comparisons throws an error means we should probably document them as non-associative though.

Chances are that section was written before the chained comparisons error was implemented.

[durka]

And those error messages should be suppressed.

[Mark-Simulacrum]

This appears to be a documentation issue, closing in favor of an issue on the reference: https://github.com/rust-lang-nursery/reference.