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