compiler (cfg)
Remove official support for the ndebug config variable, replace the current usage of it with a
more appropriate debug_assertions compiler-provided config variable.
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.
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.
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.
No real alternatives beyond different names and defaults.
From the RFC discussion there remain some unresolved details:
-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).".