I also want to write code that is reasonably generic. Thus, it will have checks and branches that cover important corner cases; they are required for completeness and correctness. But very often, all of these checks turn out to be redundant in a specific context, and an optimizing compiler can figure it out, and eliminate these checks for me.

So I don't manually need to go and write two or three versions of each function like do_foo and assume_x_is_not_null_and_do_foo and assume_y_is_less_than_int_max_minus_sizeof_z_and_do_foo and make damn sure not to call the wrong one.

I just write one version, with the right checks in place, and if after macro expansion, inlining, range analysis, common subexpression elimination, and other inference from context, with C's semantics at hand, the compiler can figure out that some of these checks are redundant, then it will optimize them out.

I ask for it, and I'm glad compiler developers deliver it. You don't need to ask for it. Just turn off these optimizations (or, rather, don't enable them) if you prefer slow and redundant code.