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Assert that some code is not present in the final binary, at compile or link time.

I'd like to assert that some code can be optimized out, and is not present in the final binary object.
```c
#define CONSTANT 0
#if (!CONSTANT)
[[landmine_A]]
#endif
static int foo(void);
void bar(void)
{
if (CONSTANT) {
foo();
}
}
static int foo(void)
{
if (!CONSTANT)
landmine_B();
}
```
Attributes, builtins, expressions, ..., everything is fair play, as long as it _guarantees_ that the program is not built with `foo()`, unless `#define CONSTANT 1`.
I'd like the compiler (or linker, but preferably the compiler) to warn/error if `foo()` is used, but not if it's inside an `if (0)`.
~~In the bast, this could probably be achieved by `__builtin_unreachable();`, and the corresponding warning, but it's ignored nowadays...~~
An option (the one in use, which I'm trying to improve), is to build conditionally foo, but then I also need to use preprocessor stuff at call site, which I don't entirely like, because it hides code to the compiler, so I need to test multiple configurations.

I'd like to assert that some code can be optimized out, and is not present in the final binary object.
```c
#define CONSTANT 0
#if (!CONSTANT)
[[landmine_A]]
#endif
static int foo(void);
void bar(void)
{
if (CONSTANT) {
foo();
}
}
static int foo(void)
{
if (!CONSTANT)
landmine_B();
}
```
Attributes, builtins, expressions, ..., everything is fair play, as long as it _guarantees_ that the program is not built with `foo()`, unless `#define CONSTANT 1`.
I'd like the compiler (or linker, but preferably the compiler) to warn/error if `foo()` is used, but not if it's inside an `if (0)`.
In the past, this could probably be achieved by `__builtin_unreachable();`, and the corresponding warning, but it's ignored nowadays...
An option (the one in use, which I'm trying to improve), is to build conditionally `foo`, but then I also need to use preprocessor stuff at call site, which I don't entirely like, because it hides code to the compiler, so I need to test multiple configurations.

c compiler-errors gcc compiler-warnings clang

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