#[no_core] loses overflow checks with optimizations

[hanna-kruppe]

The following file, when compiled without optimizations, correctly generates an overflow check in add_one:

#![feature(no_core, lang_items, rustc_attrs)]
#![no_core]
#![crate_type="lib"]

#[lang="sized"]
trait Sized {}

#[lang="add"]
trait Add<RHS=Self> {
 type Output;
 fn add(self, rhs: RHS) -> Self::Output;
}

impl Add for i32 {
 type Output = i32;

 #[inline]
 #[rustc_inherit_overflow_checks]
 fn add(self, other: i32) -> i32 { self + other }
}

#[cold] #[inline(never)] // this is the slow path, always
#[lang = "panic"]
pub fn panic(_expr_file_line: &(&'static str, &'static str, u32)) -> ! {
	loop {}
}

#[lang="copy"]
trait Copy {}

pub fn add_one(x: i32) -> i32 {
 x + 1
}

However, compiling with optimizations plus -C debug-assertions=on or -Z force-overflow-checks=on generates code without the overflow check.

However, if I move add_one to a separate no_core crate like this, the issue disappears and the overflow check is generated:

#![feature(no_core)]
#![no_core]
#![crate_type="lib"]
extern crate minimal;

pub fn add_one(x: i32) -> i32 {
 x + 1
}

[hanna-kruppe]

@eddyb noticed that my panic implementation (loop {}) is UB according to LLVM (see #28728), so it optimizes the overflow check out on the assumption that it can never be triggered (since that would be UB). Replacing it with e.g. intrinsics::abort() fixes the issue.