- Start Date: 2015-01-08
- RFC PR: [rust-lang/rfcs#563](https://github.com/rust-lang/rfcs/pull/563)
- Rust Issue: [rust-lang/rust#22492](https://github.com/rust-lang/rust/issues/22492)

## Summary

Remove official support for the `ndebug` config variable, replace the current usage of it with a
more appropriate `debug_assertions` compiler-provided config variable.

## Motivation

The usage of 'ndebug' to indicate a release build is a strange holdover from C/C++. It is not used
much and is easy to forget about. Since it used like any other value passed to the `cfg` flag, it
does not interact with other flags such as `-g` or `-O`.

The only current users of `ndebug` are the implementations of the `debug_assert!` macro. At the
time of this writing integer overflow checking is will also be controlled by this variable. Since
the optimisation setting does not influence `ndebug`, this means that code that the user expects to
be optimised will still contain the overflow checking logic. Similarly, `debug_assert!` invocations
are not removed, contrary to what intuition should expect. Enabling optimisations should been seen
as a request to make the user's code faster, removing `debug_assert!` and other checks seems like
a natural consequence.

## Detailed design

The `debug_assertions` configuration variable, the replacement for the `ndebug` variable, will be
compiler provided based on the value of the `opt-level` codegen flag, including the implied value
from `-O`.  Any value higher than 0 will disable the variable.

Another codegen flag `debug-assertions` will override this, forcing it on or off based on the value
passed to it.

## Drawbacks

Technically backwards incompatible change. However the only usage of the `ndebug` variable in the
rust tree is in the implementation of `debug_assert!`, so it's unlikely that any external code is
using it.

## Alternatives

No real alternatives beyond different names and defaults.

## Unresolved questions

From the RFC discussion there remain some unresolved details:

* brson
  [writes](https://github.com/rust-lang/rfcs/pull/563#issuecomment-72549694),
  "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).".
* huonw
  [writes](https://github.com/rust-lang/rfcs/pull/563#issuecomment-72550619),
  "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.)".
* huonw
  [writes](https://github.com/rust-lang/rfcs/pull/563#issuecomment-74762795),
  "if we want this to apply to more than just debug_assert do we want
  to use a name other than -C debug-assertions?".

