The introduction of a locally known loop breaks manual assertions

[c410-f3r]

use std::ptr;

pub struct BytesVector {
  vec: Vec<u8>,
}

impl BytesVector {
  #[inline(always)] // Doesn't work without 'always'
  fn reserve(&mut self, additional: usize) {
    self.vec.reserve(additional);
    unsafe {
      if self.vec.len().unchecked_add(additional) > self.vec.capacity() {
        std::hint::unreachable_unchecked();
      }
    }
  }

  fn push(&mut self, value: u8) {
    if self.vec.len() >= self.vec.capacity() {
      panic!();
    }
    unsafe {
      ptr::write(self.vec.as_mut_ptr().add(self.vec.len()), value);
      let new_len = self.vec.len().unchecked_add(1);
      self.vec.set_len(new_len);
    }
  }
}

pub fn push(v: &mut BytesVector) {
    v.reserve(4);
    v.push(0); // No branch
    v.push(1); // No branch
    v.push(2); // No branch
    v.push(3); // No branch
}

The above snippet successfully removes panicking branches due to unreachable_unchecked but the same thing can not be said when working with loops that contain a locally known stopping criteria.

pub fn push(v: &mut BytesVector) {
    v.reserve(4);
    for idx in 0..4 {
       v.push(idx);  // Branches everywhere
    }
}

https://godbolt.org/z/b7hTb9ahG

[c410-f3r]

Pinging the members that are more involved with codegen to create awareness. Let me known if I can somehow help in this matter.

@nikic @bjorn3