allow attaching strings to assert messages

[bblum]

A great C idiom is:

assert(condition && "Failure message");

In rust, I keep writing:

if !condition { fail "Failure message"; }

I'd be pretty happy with an #assert macro that took a condition and a failure message and generated appropriate code (easy), or perhaps changing the grammar of assert itself (less feasible).

[brson]

I like the macro idea. The error message could be optional. At that point we could remove assert from the language. If we used bang for macros then we write assert ! condition or assert ! (condition, "message").

[bblum]

whoa, are you sure it's a good idea to have people write "!condition" to mean "condition must hold"? c.c

[brson]

I did think it was a good idea, but it is clearly not.

[eholk]

I think we can already write this with the existing macro system. Does this work?

#macro[
    [#assert[cond], if !(cond) { fail }],
    [#assert[cond, message], if !(cond) { fail message }]
];

[bblum]

It should still also print the file and line number, and ideally also the containing function, in either case (with message or not).

(this is me being demanding without knowing how to do macros on my own.)

Oh yeah, also the macro hygiene issue. It should be wrapped in whatever rust's equivalent of C's "do { .. } while (0)" is.

[eholk]

This does most of what you want. Unfortunately, it shows you the line number where the macro was defined, rather than where the macro was invoked. There's a case to be made that this is wrong behavior for #line, or maybe we need #invocation_line instead.

fn macros() {
    #macro[
        [#assert[cond],
         (#assert[cond, "%s", #stringify(cond)])],
        [#assert[cond, message, args, ...],
         {
             if !(cond) { fail #fmt("Assertion failed at %s:%u: %s",
                                    #file(), #line(),
                                    /* #stringify(cond) */
                                   #fmt(message, args, ...)) }
         }]
    ];
}

fn main() {
    #assert(true);
    #assert("123" == "4 5 6",
            "This was supposed to go wrong.");
    #assert("1" == "2");
    #assert("123" == "3 4 5", "This was supposed to go wrong");
}

[burg]

This macro has been added to Servo, btw. I was looking for an NS_ASSERT equivalent.