Function mir_to_const 

pub fn mir_to_const<'tcx>(
    tcx: TyCtxt<'tcx>,
    val: ConstValue,
    ty: Ty<'tcx>,
) -> Option<Constant>

Converts a [ConstValue] to a Constant, if possible.

• If ty is an Adt describing a struct, returns a Constant::Adt containing val.
• If val is a Int, ty determines the variant of the returned constant:
  • Bool → Constant::Bool
  • Uint/Int → Constant::Int
  • Float → Constant::F16/Constant::F32/Constant::F64/Constant::F128, depending on width
  • RawPtr → Constant::RawPtr
  • any other ty → None
• If ty is a Ref referring to a Str (i.e. a &str), returns a Constant::Str.
• If val is a [ConstValue::Indirect] and ty is an Array of Float, returns a Constant::Vec

Otherwise, returns None.