const fn allows unreachable calls to non-const fns

[Centril]

E.g. the following compiles on stable:

const fn f() {
    return;
    fn f() {}
    f();
}

@eddyb notes that this is because we do a "semantic check" and that f(); is not linked in the CFG.

However, it seems to me that the basic blocks should be there (tho I haven't checked yet...) so you can visit the call to f(); and ban it.

cc @oli-obk

[eddyb]

Everything is allowed in dead code - control-flow, loops, pointer ops, etc.
@rust-lang/lang might want to discuss it further but IMO this is okay.

[Centril]

cc #62272 #62274

[Enselic]

@rustbot label A-const-fn