Convert `assert` to a macro/library function

[brson]

I don't think there are any obstacles to making this a function. The only difference is that it will require parens.

Remember to change the docs.

[nikomatsakis]

I would rather see a macro so that (1) we can still see the text of the expression which failed and (2) the line number is the line number that made the assertion, not some piece of code in core::util::assert.

[brson]

Yep, you're right.

[graydon]

Yeah, we'll need syntax extensions for #file and #line first, as well as #stringify or something. But we should have those anyways.

[catamorphism]

I opened an issue - #2245 - for the #file and #line macros that we need for this.

[Dretch]

If rust backtraces included line numbers and code fragments, like Python's:

Traceback (most recent call last):
  File "<doctest...>", line 10, in <module>
    lumberjack()
  File "<doctest...>", line 4, in lumberjack
    bright_side_of_death()

Then assert might be useable as a function.

[manuel-woelker]

I did some experiments on this and AFAICT there are the following open issues:

• file!, line! and col! (and possibly module!) do the Wrong Thing. #3066: reported file!/line! is in the macro declaration, not the macro use
• Macro import/export #3114 Macro import/export
• Getting the expression as a string: stringify! unhelpfully returns "an interpolated expression", while quote_expr! fails during compilation with the following message: rust: task failed at 'quote! with interpolated token', rust-0.5/src/libsyntax/ext/quote.rs:389

Regarding that last one, any suggestions for a course of action? Is there any good way to get the actual expression inside of a macro? Barring that would fixing quote.rs be a better solution than to_str() in token.rs?

For reference, here is the current POC:

macro_rules! Assert(
    ($inp:expr) => (
        if(!$inp) {
                io::println(fmt!("Assertion '%s' failed, %s:%u", stringify!($inp), file!(), line!()));
            }
    )
)

This gets me as far as: Assertion 'an interpolated expression' failed, assert-poc.rs:52
"Assert" is capitalized, since "assert" is a keyword. Any ideas on how to handle that in the future?

[sanxiyn]

For "assert is a keyword" issue, see how "fail" is handled in #4524.

[brson]

Completed by #5628