compiler/rustc_middle/src/ty/consts/kind.rs

   1 use crate::mir::interpret::ConstValue;
   2 use crate::mir::interpret::Scalar;
   3 use crate::mir::Promoted;
   4 use crate::ty::subst::{InternalSubsts, SubstsRef};
   5 use crate::ty::ParamEnv;
   6 use crate::ty::{self, TyCtxt, TypeFoldable};
   7 use rustc_errors::ErrorReported;
   8 use rustc_hir::def_id::DefId;
   9 use rustc_macros::HashStable;
  10 use rustc_target::abi::Size;
  11
  12 /// Represents a constant in Rust.
  13 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Hash)]
  14 #[derive(HashStable)]
  15 pub enum ConstKind<'tcx> {
  16     /// A const generic parameter.
  17     Param(ty::ParamConst),
  18
  19     /// Infer the value of the const.
  20     Infer(InferConst<'tcx>),
  21
  22     /// Bound const variable, used only when preparing a trait query.
  23     Bound(ty::DebruijnIndex, ty::BoundVar),
  24
  25     /// A placeholder const - universally quantified higher-ranked const.
  26     Placeholder(ty::PlaceholderConst<'tcx>),
  27
  28     /// Used in the HIR by using `Unevaluated` everywhere and later normalizing to one of the other
  29     /// variants when the code is monomorphic enough for that.
  30     Unevaluated(ty::WithOptConstParam<DefId>, SubstsRef<'tcx>, Option<Promoted>),
  31
  32     /// Used to hold computed value.
  33     Value(ConstValue<'tcx>),
  34
  35     /// A placeholder for a const which could not be computed; this is
  36     /// propagated to avoid useless error messages.
  37     Error(ty::DelaySpanBugEmitted),
  38 }
  39
  40 #[cfg(target_arch = "x86_64")]
  41 static_assert_size!(ConstKind<'_>, 40);
  42
  43 impl<'tcx> ConstKind<'tcx> {
  44     #[inline]
  45     pub fn try_to_value(self) -> Option<ConstValue<'tcx>> {
  46         if let ConstKind::Value(val) = self { Some(val) } else { None }
  47     }
  48
  49     #[inline]
  50     pub fn try_to_scalar(self) -> Option<Scalar> {
  51         self.try_to_value()?.try_to_scalar()
  52     }
  53
  54     #[inline]
  55     pub fn try_to_bits(self, size: Size) -> Option<u128> {
  56         self.try_to_value()?.try_to_bits(size)
  57     }
  58
  59     #[inline]
  60     pub fn try_to_bool(self) -> Option<bool> {
  61         self.try_to_value()?.try_to_bool()
  62     }
  63
  64     #[inline]
  65     pub fn try_to_machine_usize(self, tcx: TyCtxt<'tcx>) -> Option<u64> {
  66         self.try_to_value()?.try_to_machine_usize(tcx)
  67     }
  68 }
  69
  70 /// An inference variable for a const, for use in const generics.
  71 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Hash)]
  72 #[derive(HashStable)]
  73 pub enum InferConst<'tcx> {
  74     /// Infer the value of the const.
  75     Var(ty::ConstVid<'tcx>),
  76     /// A fresh const variable. See `infer::freshen` for more details.
  77     Fresh(u32),
  78 }
  79
  80 impl<'tcx> ConstKind<'tcx> {
  81     #[inline]
  82     /// Tries to evaluate the constant if it is `Unevaluated`. If that doesn't succeed, return the
  83     /// unevaluated constant.
  84     pub fn eval(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Self {
  85         self.try_eval(tcx, param_env).and_then(Result::ok).map(ConstKind::Value).unwrap_or(self)
  86     }
  87
  88     #[inline]
  89     /// Tries to evaluate the constant if it is `Unevaluated`. If that isn't possible or necessary
  90     /// return `None`.
  91     pub(super) fn try_eval(
  92         self,
  93         tcx: TyCtxt<'tcx>,
  94         param_env: ParamEnv<'tcx>,
  95     ) -> Option<Result<ConstValue<'tcx>, ErrorReported>> {
  96         if let ConstKind::Unevaluated(def, substs, promoted) = self {
  97             use crate::mir::interpret::ErrorHandled;
  98
  99             // HACK(eddyb) this erases lifetimes even though `const_eval_resolve`
 100             // also does later, but we want to do it before checking for
 101             // inference variables.
 102             // Note that we erase regions *before* calling `with_reveal_all_normalized`,
 103             // so that we don't try to invoke this query with
 104             // any region variables.
 105             let param_env_and_substs = tcx
 106                 .erase_regions(param_env)
 107                 .with_reveal_all_normalized(tcx)
 108                 .and(tcx.erase_regions(substs));
 109
 110             // HACK(eddyb) when the query key would contain inference variables,
 111             // attempt using identity substs and `ParamEnv` instead, that will succeed
 112             // when the expression doesn't depend on any parameters.
 113             // FIXME(eddyb, skinny121) pass `InferCtxt` into here when it's available, so that
 114             // we can call `infcx.const_eval_resolve` which handles inference variables.
 115             let param_env_and_substs = if param_env_and_substs.needs_infer() {
 116                 tcx.param_env(def.did).and(InternalSubsts::identity_for_item(tcx, def.did))
 117             } else {
 118                 param_env_and_substs
 119             };
 120
 121             // FIXME(eddyb) maybe the `const_eval_*` methods should take
 122             // `ty::ParamEnvAnd<SubstsRef>` instead of having them separate.
 123             let (param_env, substs) = param_env_and_substs.into_parts();
 124             // try to resolve e.g. associated constants to their definition on an impl, and then
 125             // evaluate the const.
 126             match tcx.const_eval_resolve(param_env, def, substs, promoted, None) {
 127                 // NOTE(eddyb) `val` contains no lifetimes/types/consts,
 128                 // and we use the original type, so nothing from `substs`
 129                 // (which may be identity substs, see above),
 130                 // can leak through `val` into the const we return.
 131                 Ok(val) => Some(Ok(val)),
 132                 Err(ErrorHandled::TooGeneric | ErrorHandled::Linted) => None,
 133                 Err(ErrorHandled::Reported(e)) => Some(Err(e)),
 134             }
 135         } else {
 136             None
 137         }
 138     }
 139 }