src/librustc_mir_build/hair/constant.rs

use rustc::mir::interpret::{
    truncate, Allocation, ConstValue, LitToConstError, LitToConstInput, Scalar,
};
use rustc::ty::{self, layout::Size, ParamEnv, TyCtxt};
use rustc_span::symbol::Symbol;
use syntax::ast;

crate fn lit_to_const<'tcx>(
    tcx: TyCtxt<'tcx>,
    lit_input: LitToConstInput<'tcx>,
) -> Result<&'tcx ty::Const<'tcx>, LitToConstError> {
    let LitToConstInput { lit, ty, neg } = lit_input;

    let trunc = |n| {
        let param_ty = ParamEnv::reveal_all().and(ty);
        let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
        trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
        let result = truncate(n, width);
        trace!("trunc result: {}", result);
        Ok(ConstValue::Scalar(Scalar::from_uint(result, width)))
    };

    let lit = match *lit {
        ast::LitKind::Str(ref s, _) => {
            let s = s.as_str();
            let allocation = Allocation::from_byte_aligned_bytes(s.as_bytes());
            let allocation = tcx.intern_const_alloc(allocation);
            ConstValue::Slice { data: allocation, start: 0, end: s.len() }
        }
        ast::LitKind::ByteStr(ref data) => {
            if let ty::Ref(_, ref_ty, _) = ty.kind {
                match ref_ty.kind {
                    ty::Slice(_) => {
                        let allocation = Allocation::from_byte_aligned_bytes(data as &Vec<u8>);
                        let allocation = tcx.intern_const_alloc(allocation);
                        ConstValue::Slice { data: allocation, start: 0, end: data.len() }
                    }
                    ty::Array(_, _) => {
                        let id = tcx.allocate_bytes(data);
                        ConstValue::Scalar(Scalar::Ptr(id.into()))
                    }
                    _ => {
                        bug!("bytestring should have type of either &[u8] or &[u8; _], not {}", ty)
                    }
                }
            } else {
                bug!("bytestring should have type of either &[u8] or &[u8; _], not {}", ty)
            }
        }
        ast::LitKind::Byte(n) => ConstValue::Scalar(Scalar::from_uint(n, Size::from_bytes(1))),
        ast::LitKind::Int(n, _) if neg => {
            let n = n as i128;
            let n = n.overflowing_neg().0;
            trunc(n as u128)?
        }
        ast::LitKind::Int(n, _) => trunc(n)?,
        ast::LitKind::Float(n, _) => {
            let fty = match ty.kind {
                ty::Float(fty) => fty,
                _ => bug!(),
            };
            parse_float(n, fty, neg).map_err(|_| LitToConstError::UnparseableFloat)?
        }
        ast::LitKind::Bool(b) => ConstValue::Scalar(Scalar::from_bool(b)),
        ast::LitKind::Char(c) => ConstValue::Scalar(Scalar::from_char(c)),
        ast::LitKind::Err(_) => return Err(LitToConstError::Reported),
    };
    Ok(tcx.mk_const(ty::Const { val: ty::ConstKind::Value(lit), ty }))
}

fn parse_float<'tcx>(num: Symbol, fty: ast::FloatTy, neg: bool) -> Result<ConstValue<'tcx>, ()> {
    let num = num.as_str();
    use rustc_apfloat::ieee::{Double, Single};
    let scalar = match fty {
        ast::FloatTy::F32 => {
            num.parse::<f32>().map_err(|_| ())?;
            let mut f = num.parse::<Single>().unwrap_or_else(|e| {
                panic!("apfloat::ieee::Single failed to parse `{}`: {:?}", num, e)
            });
            if neg {
                f = -f;
            }
            Scalar::from_f32(f)
        }
        ast::FloatTy::F64 => {
            num.parse::<f64>().map_err(|_| ())?;
            let mut f = num.parse::<Double>().unwrap_or_else(|e| {
                panic!("apfloat::ieee::Double failed to parse `{}`: {:?}", num, e)
            });
            if neg {
                f = -f;
            }
            Scalar::from_f64(f)
        }
    };

    Ok(ConstValue::Scalar(scalar))
}