compiler/rustc_mir_build/src/build/expr/as_constant.rs

   1 //! See docs in build/expr/mod.rs
   2
   3 use crate::build::{parse_float_into_constval, Builder};
   4 use rustc_ast as ast;
   5 use rustc_middle::mir;
   6 use rustc_middle::mir::interpret::{
   7     Allocation, ConstValue, LitToConstError, LitToConstInput, Scalar,
   8 };
   9 use rustc_middle::mir::*;
  10 use rustc_middle::thir::*;
  11 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, TyCtxt};
  12 use rustc_target::abi::Size;
  13
  14 impl<'a, 'tcx> Builder<'a, 'tcx> {
  15     /// Compile `expr`, yielding a compile-time constant. Assumes that
  16     /// `expr` is a valid compile-time constant!
  17     pub(crate) fn as_constant(&mut self, expr: &Expr<'tcx>) -> Constant<'tcx> {
  18         let this = self;
  19         let tcx = this.tcx;
  20         let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
  21         match *kind {
  22             ExprKind::Scope { region_scope: _, lint_level: _, value } => {
  23                 this.as_constant(&this.thir[value])
  24             }
  25             ExprKind::Literal { lit, neg } => {
  26                 let literal =
  27                     match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
  28                         Ok(c) => c,
  29                         Err(LitToConstError::Reported) => ConstantKind::Ty(tcx.const_error(ty)),
  30                         Err(LitToConstError::TypeError) => {
  31                             bug!("encountered type error in `lit_to_mir_constant")
  32                         }
  33                     };
  34
  35                 Constant { span, user_ty: None, literal }
  36             }
  37             ExprKind::NonHirLiteral { lit, ref user_ty } => {
  38                 let user_ty = user_ty.as_ref().map(|user_ty| {
  39                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
  40                         span,
  41                         user_ty: user_ty.clone(),
  42                         inferred_ty: ty,
  43                     })
  44                 });
  45                 let literal = ConstantKind::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);
  46
  47                 Constant { span, user_ty: user_ty, literal }
  48             }
  49             ExprKind::ZstLiteral { ref user_ty } => {
  50                 let user_ty = user_ty.as_ref().map(|user_ty| {
  51                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
  52                         span,
  53                         user_ty: user_ty.clone(),
  54                         inferred_ty: ty,
  55                     })
  56                 });
  57                 let literal = ConstantKind::Val(ConstValue::ZeroSized, ty);
  58
  59                 Constant { span, user_ty: user_ty, literal }
  60             }
  61             ExprKind::NamedConst { def_id, substs, ref user_ty } => {
  62                 let user_ty = user_ty.as_ref().map(|user_ty| {
  63                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
  64                         span,
  65                         user_ty: user_ty.clone(),
  66                         inferred_ty: ty,
  67                     })
  68                 });
  69
  70                 let uneval =
  71                     mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
  72                 let literal = ConstantKind::Unevaluated(uneval, ty);
  73
  74                 Constant { user_ty, span, literal }
  75             }
  76             ExprKind::ConstParam { param, def_id: _ } => {
  77                 let const_param = tcx.mk_const(ty::ConstKind::Param(param), expr.ty);
  78                 let literal = ConstantKind::Ty(const_param);
  79
  80                 Constant { user_ty: None, span, literal }
  81             }
  82             ExprKind::ConstBlock { did: def_id, substs } => {
  83                 let uneval =
  84                     mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
  85                 let literal = ConstantKind::Unevaluated(uneval, ty);
  86
  87                 Constant { user_ty: None, span, literal }
  88             }
  89             ExprKind::StaticRef { alloc_id, ty, .. } => {
  90                 let const_val = ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &tcx));
  91                 let literal = ConstantKind::Val(const_val, ty);
  92
  93                 Constant { span, user_ty: None, literal }
  94             }
  95             _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
  96         }
  97     }
  98 }
  99
 100 #[instrument(skip(tcx, lit_input))]
 101 pub(crate) fn lit_to_mir_constant<'tcx>(
 102     tcx: TyCtxt<'tcx>,
 103     lit_input: LitToConstInput<'tcx>,
 104 ) -> Result<ConstantKind<'tcx>, LitToConstError> {
 105     let LitToConstInput { lit, ty, neg } = lit_input;
 106     let trunc = |n| {
 107         let param_ty = ty::ParamEnv::reveal_all().and(ty);
 108         let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
 109         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
 110         let result = width.truncate(n);
 111         trace!("trunc result: {}", result);
 112         Ok(ConstValue::Scalar(Scalar::from_uint(result, width)))
 113     };
 114
 115     let value = match (lit, &ty.kind()) {
 116         (ast::LitKind::Str(s, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_str() => {
 117             let s = s.as_str();
 118             let allocation = Allocation::from_bytes_byte_aligned_immutable(s.as_bytes());
 119             let allocation = tcx.intern_const_alloc(allocation);
 120             ConstValue::Slice { data: allocation, start: 0, end: s.len() }
 121         }
 122         (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _))
 123             if matches!(inner_ty.kind(), ty::Slice(_)) =>
 124         {
 125             let allocation = Allocation::from_bytes_byte_aligned_immutable(data as &[u8]);
 126             let allocation = tcx.intern_const_alloc(allocation);
 127             ConstValue::Slice { data: allocation, start: 0, end: data.len() }
 128         }
 129         (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
 130             let id = tcx.allocate_bytes(data);
 131             ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
 132         }
 133         (ast::LitKind::Byte(n), ty::Uint(ty::UintTy::U8)) => {
 134             ConstValue::Scalar(Scalar::from_uint(*n, Size::from_bytes(1)))
 135         }
 136         (ast::LitKind::Int(n, _), ty::Uint(_)) | (ast::LitKind::Int(n, _), ty::Int(_)) => {
 137             trunc(if neg { (*n as i128).overflowing_neg().0 as u128 } else { *n })?
 138         }
 139         (ast::LitKind::Float(n, _), ty::Float(fty)) => {
 140             parse_float_into_constval(*n, *fty, neg).ok_or(LitToConstError::Reported)?
 141         }
 142         (ast::LitKind::Bool(b), ty::Bool) => ConstValue::Scalar(Scalar::from_bool(*b)),
 143         (ast::LitKind::Char(c), ty::Char) => ConstValue::Scalar(Scalar::from_char(*c)),
 144         (ast::LitKind::Err, _) => return Err(LitToConstError::Reported),
 145         _ => return Err(LitToConstError::TypeError),
 146     };
 147
 148     Ok(ConstantKind::Val(value, ty))
 149 }