- Start Date: 2015-01-08
- RFC PR: rust-lang/rfcs#563
- Rust Issue: rust-lang/rust#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,
“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, “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, “if we want this to apply to more than just debug_assert do we want to use a name other than -C debug-assertions?”.