Add a way for macros to check and report errors

[eholk]

It'd be helpful to have something like macro_error! or static_assert!, that evaluates a condition and possible reports an error at compile time.

In my HTML parser macro, it'd allow the macro to make sure closing tags actually close what they should.

[bblum]

+1 for static_assert :)

[BrendanEich]

Yay static_assert!

[emberian]

See https://botbot.me/irc.mozilla.org/rust/msg/2999973/ and following conversation. static_assert, static_warn etc would be very useful when building for a platform lacking certain things.

[emberian]

static_assert added as attribute in #6670, but it doesn't fix this, this would require CTFE. Setting far future.

[eholk]

I don't actually think we need compile time function evaluation (CTFE) for what I originally had in mind. I was just thinking a macro like macro_error!(message) that when evaluated creates a compile error with the supplied message. By itself, it's not super useful, but it can be used as a piece of other macros. For example, say you had a macro that had to have an even number of macros. You could have the pattern that matches the single argument case expand into macro_error!, which would tell the programmer they misused the macro and what they should do instead.

[eholk]

Oh... I just read what my initial report said, and that wasn't what my most recent comment was. Still, I think the way to factor this is with a macro_error! macro and then some kind of conditional macro, and the combination would give my original request. The macro_error! part shouldn't be hard to implement at all.

[emberian]

@eholk well macros are a form of CTFE I think?

[eholk]

@cmr, I suppose so, given that they are Turing complete. I'd say Scheme's syntax-case macros count as CTFE, since they are really just arbitrary fragments of Scheme code that produce syntax, but syntax-rules macros (which are more like what Rust has) don't have that same feel to me since they are just pattern matching and substitution, but you can't evaluate arbitrary Rust code as part of the expansion.

[catamorphism]

(bug triage) Milestone looks right.

[steveklabnik]

https://github.com/huonw/compile_msg looks like it might be a solution to this issue?

[emberian]

Yup!