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::{
  12     self, CanonicalUserType, CanonicalUserTypeAnnotation, TyCtxt, UserTypeAnnotationIndex,
  13 };
  14 use rustc_span::DUMMY_SP;
  15 use rustc_target::abi::Size;
  16
  17 impl<'a, 'tcx> Builder<'a, 'tcx> {
  18     /// Compile `expr`, yielding a compile-time constant. Assumes that
  19     /// `expr` is a valid compile-time constant!
  20     pub(crate) fn as_constant(&mut self, expr: &Expr<'tcx>) -> Constant<'tcx> {
  21         let this = self;
  22         let tcx = this.tcx;
  23         let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
  24         match kind {
  25             ExprKind::Scope { region_scope: _, lint_level: _, value } => {
  26                 this.as_constant(&this.thir[*value])
  27             }
  28             _ => as_constant_inner(
  29                 expr,
  30                 |user_ty| {
  31                     Some(this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
  32                         span,
  33                         user_ty: user_ty.clone(),
  34                         inferred_ty: ty,
  35                     }))
  36                 },
  37                 tcx,
  38             ),
  39         }
  40     }
  41 }
  42
  43 pub fn as_constant_inner<'tcx>(
  44     expr: &Expr<'tcx>,
  45     push_cuta: impl FnMut(&Box<CanonicalUserType<'tcx>>) -> Option<UserTypeAnnotationIndex>,
  46     tcx: TyCtxt<'tcx>,
  47 ) -> Constant<'tcx> {
  48     let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
  49     match *kind {
  50         ExprKind::Literal { lit, neg } => {
  51             let literal =
  52                 match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
  53                     Ok(c) => c,
  54                     Err(LitToConstError::Reported(guar)) => {
  55                         ConstantKind::Ty(tcx.const_error_with_guaranteed(ty, guar))
  56                     }
  57                     Err(LitToConstError::TypeError) => {
  58                         bug!("encountered type error in `lit_to_mir_constant")
  59                     }
  60                 };
  61
  62             Constant { span, user_ty: None, literal }
  63         }
  64         ExprKind::NonHirLiteral { lit, ref user_ty } => {
  65             let user_ty = user_ty.as_ref().map(push_cuta).flatten();
  66
  67             let literal = ConstantKind::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);
  68
  69             Constant { span, user_ty: user_ty, literal }
  70         }
  71         ExprKind::ZstLiteral { ref user_ty } => {
  72             let user_ty = user_ty.as_ref().map(push_cuta).flatten();
  73
  74             let literal = ConstantKind::Val(ConstValue::ZeroSized, ty);
  75
  76             Constant { span, user_ty: user_ty, literal }
  77         }
  78         ExprKind::NamedConst { def_id, substs, ref user_ty } => {
  79             let user_ty = user_ty.as_ref().map(push_cuta).flatten();
  80
  81             let uneval = mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
  82             let literal = ConstantKind::Unevaluated(uneval, ty);
  83
  84             Constant { user_ty, span, literal }
  85         }
  86         ExprKind::ConstParam { param, def_id: _ } => {
  87             let const_param = tcx.mk_const(ty::ConstKind::Param(param), expr.ty);
  88             let literal = ConstantKind::Ty(const_param);
  89
  90             Constant { user_ty: None, span, literal }
  91         }
  92         ExprKind::ConstBlock { did: def_id, substs } => {
  93             let uneval = mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
  94             let literal = ConstantKind::Unevaluated(uneval, ty);
  95
  96             Constant { user_ty: None, span, literal }
  97         }
  98         ExprKind::StaticRef { alloc_id, ty, .. } => {
  99             let const_val = ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &tcx));
 100             let literal = ConstantKind::Val(const_val, ty);
 101
 102             Constant { span, user_ty: None, literal }
 103         }
 104         _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
 105     }
 106 }
 107
 108 #[instrument(skip(tcx, lit_input))]
 109 pub(crate) fn lit_to_mir_constant<'tcx>(
 110     tcx: TyCtxt<'tcx>,
 111     lit_input: LitToConstInput<'tcx>,
 112 ) -> Result<ConstantKind<'tcx>, LitToConstError> {
 113     let LitToConstInput { lit, ty, neg } = lit_input;
 114     let trunc = |n| {
 115         let param_ty = ty::ParamEnv::reveal_all().and(ty);
 116         let width = tcx
 117             .layout_of(param_ty)
 118             .map_err(|_| {
 119                 LitToConstError::Reported(tcx.sess.delay_span_bug(
 120                     DUMMY_SP,
 121                     format!("couldn't compute width of literal: {:?}", lit_input.lit),
 122                 ))
 123             })?
 124             .size;
 125         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
 126         let result = width.truncate(n);
 127         trace!("trunc result: {}", result);
 128         Ok(ConstValue::Scalar(Scalar::from_uint(result, width)))
 129     };
 130
 131     let value = match (lit, &ty.kind()) {
 132         (ast::LitKind::Str(s, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_str() => {
 133             let s = s.as_str();
 134             let allocation = Allocation::from_bytes_byte_aligned_immutable(s.as_bytes());
 135             let allocation = tcx.intern_const_alloc(allocation);
 136             ConstValue::Slice { data: allocation, start: 0, end: s.len() }
 137         }
 138         (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _))
 139             if matches!(inner_ty.kind(), ty::Slice(_)) =>
 140         {
 141             let allocation = Allocation::from_bytes_byte_aligned_immutable(data as &[u8]);
 142             let allocation = tcx.intern_const_alloc(allocation);
 143             ConstValue::Slice { data: allocation, start: 0, end: data.len() }
 144         }
 145         (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
 146             let id = tcx.allocate_bytes(data);
 147             ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
 148         }
 149         (ast::LitKind::Byte(n), ty::Uint(ty::UintTy::U8)) => {
 150             ConstValue::Scalar(Scalar::from_uint(*n, Size::from_bytes(1)))
 151         }
 152         (ast::LitKind::Int(n, _), ty::Uint(_)) | (ast::LitKind::Int(n, _), ty::Int(_)) => {
 153             trunc(if neg { (*n as i128).overflowing_neg().0 as u128 } else { *n })?
 154         }
 155         (ast::LitKind::Float(n, _), ty::Float(fty)) => parse_float_into_constval(*n, *fty, neg)
 156             .ok_or_else(|| {
 157                 LitToConstError::Reported(tcx.sess.delay_span_bug(
 158                     DUMMY_SP,
 159                     format!("couldn't parse float literal: {:?}", lit_input.lit),
 160                 ))
 161             })?,
 162         (ast::LitKind::Bool(b), ty::Bool) => ConstValue::Scalar(Scalar::from_bool(*b)),
 163         (ast::LitKind::Char(c), ty::Char) => ConstValue::Scalar(Scalar::from_char(*c)),
 164         (ast::LitKind::Err, _) => {
 165             return Err(LitToConstError::Reported(
 166                 tcx.sess.delay_span_bug(DUMMY_SP, "encountered LitKind::Err during mir build"),
 167             ));
 168         }
 169         _ => return Err(LitToConstError::TypeError),
 170     };
 171
 172     Ok(ConstantKind::Val(value, ty))
 173 }