Add fixme comment to revert change once const_panic is stable

Author: detrumi

compiler/rustc_index/src/vec.rs

@@ -124,6 +124,7 @@ macro_rules! newtype_index {
 
             #[inline]
             $v const fn from_usize(value: usize) -> Self {
+                // FIXME: replace with `assert!(value <= ($max as usize));` once `const_panic` is stable
                 [()][(value > ($max as usize)) as usize];
                 unsafe {
                     Self::from_u32_unchecked(value as u32)
@@ -132,6 +133,7 @@ macro_rules! newtype_index {
 
             #[inline]
             $v const fn from_u32(value: u32) -> Self {
+                // FIXME: replace with `assert!(value <= $max);` once `const_panic` is stable
                 [()][(value > $max) as usize];
                 unsafe {
                     Self::from_u32_unchecked(value)