Remove official ndebug variable support

[Aatch]

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[huonw]

I'm in favour of movement along these lines; I find it weird to think that a --cfg flag will be used by the compiler for internal codegen choices, rather than just source-level stripping.

[SimonSapin]

It can be useful to run tests with both debug_assert! and optimizations enabled (if running tests take a lot of time).

[huonw]

@SimonSapin I think the suggestion is to allow -C debug-assertions=true to override the opt-level-based default settings.

[SimonSapin]

Ok, that sounds fine.

[nikomatsakis]

Yes, this makes sense. ndebug in my head has kind of been what you are actually proposing -- that is, a way to enable/disable expensive assertions.

[nikomatsakis]

Thinking a bit more, I think it would be useful to have the ability to narrow down the list of modules for which debug-assertions are enabled, as well. I've wanted this in C++ from time to time (and achieved it, by playing with #undef NDEBUG and so on). I'd start by just having this controlled via the command-line, but eventually we may want in-code scoped annotations, much like what was proposed for integer overflow initially. My opinion on that, though, is that I'd prefer to take it slow and start simple; I feel we don't have enough experience to judge just how much control is really desired, and I want to avoid over-designing things. To that end, even this proposal of scoping by module is probably overboard: I think all I mean is that I'd like to leave room for specifying a list of modules or other information in the future (categories of debug-assertions and so on).

[brson]

In favor of the general idea.

[brson]

For context ndebug started out as an in-tree thing and sorta seeped into the outside world.

[sfackler]

The debug! logging macro also disables itself if ndebug is set at the moment, but my restructuring of the log crate does away with that in favor of a more configurable system: https://github.com/sfackler/log-ng/blob/master/src/macros.rs

[alexcrichton]

Just clarifying, but by variable do you mean cfg directive? For example, I would be able to do:

if cfg!(debug_assertions) { /* super-expensive code */ }

[brson]

I have a minor concern that -C debug-assertions might not be the right place for this command line flag - it doesn't really affect code generation, at least in the current codebase (also --cfg debug_assertions has the same effect).

What other things might debug_assertions control? Does it affect integer overflow?

[huonw]

It seems like the flag could be more than just a boolean, but rather take a list of what to enable to allow fine-grained control, e.g. none, overflow-checks, debug_cfg,overflow-checks, all. (Where -C debug-assertions=debug_cfg acts like --cfg debug.)

[nikomatsakis]

Just re-read the RFC again. I like it. I definitely like this part of it:

• -O0 enables all optional debugging stuff
• -O1 and higher disables all optional debugging stuff, but you can opt back into it

The only question then is how fine-grained to make the "opting back in". That is, do we want to distinguish debug-assertions from integer-overflow etc? I think we certainly want a simple way to say "put back in all the optional debugging stuff". Having more control also seems good but I'd be happy for that to come later too.

[huonw]

Sorry for bikeshedding... if we want this to apply to more than just debug_assert do we want to use a name other than -C debug-assertions?

[nikomatsakis]

@huonw I don't have a strong opinion. I'm going to merge the RFC in any case, I think, and we can bikeshed independently and update the RFC/impl as we go. I think the important part is that we plan on integrating this more deeply in the compiler and that we plan on having the defaults be connected to the optimization level.

[nikomatsakis]

The core team has decided to merge this RFC. There was no significant opposition, seems like a popular proposal all around. Some [minor] questions as to the precise command-line flags remain to be resolved [1, 2, 3], but the high-level idea of integrating ndebug more deeply into the compiler and tying it to the optimization flags is uncontroversial.

[nikomatsakis]

Tracking issue: rust-lang/rust#22492