message: Add comment about deprecation
diff: @@ -101,8 +101,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let num_fields = dest.layout.layout.fields.count();
  if !(4..=5).contains(&num_fields) {
  - // Always mention 5 fields, since the 4-field struct is only supported
  - // for backwards compatiblity. New code should declare 5 fields.
  + // Always mention 5 fields, since the 4-field struct
  + // is deprecated and slated for removal.
  throw_ub_format!("bad declaration of miri_resolve_frame - should return a struct with 5 fields");
  }
  @@ -133,6 +133,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  this.write_scalar(lineno_alloc, this.mplace_field(dest, 2)?.into())?;
  this.write_scalar(colno_alloc, this.mplace_field(dest, 3)?.into())?;
  + // Support a 4-field struct for now - this is deprecated
  + // and slated for removal.
  if num_fields == 5 {
  this.write_scalar(fn_ptr, this.mplace_field(dest, 4)?.into())?;
  }
  