src/librustc/ty/_match.rs

use crate::ty::{self, Ty, TyCtxt, InferConst};
use crate::ty::error::TypeError;
use crate::ty::relate::{self, Relate, TypeRelation, RelateResult};

/// A type "A" *matches* "B" if the fresh types in B could be
/// substituted with values so as to make it equal to A. Matching is
/// intended to be used only on freshened types, and it basically
/// indicates if the non-freshened versions of A and B could have been
/// unified.
///
/// It is only an approximation. If it yields false, unification would
/// definitely fail, but a true result doesn't mean unification would
/// succeed. This is because we don't track the "side-constraints" on
/// type variables, nor do we track if the same freshened type appears
/// more than once. To some extent these approximations could be
/// fixed, given effort.
///
/// Like subtyping, matching is really a binary relation, so the only
/// important thing about the result is Ok/Err. Also, matching never
/// affects any type variables or unification state.
pub struct Match<'tcx> {
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
}

impl Match<'tcx> {
    pub fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Match<'tcx> {
        Match { tcx, param_env }
    }
}

impl TypeRelation<'tcx> for Match<'tcx> {
    fn tag(&self) -> &'static str { "Match" }
    fn tcx(&self) -> TyCtxt<'tcx> { self.tcx }
    fn param_env(&self) -> ty::ParamEnv<'tcx> { self.param_env }
    fn a_is_expected(&self) -> bool { true } // irrelevant

    fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                             _: ty::Variance,
                                             a: &T,
                                             b: &T)
                                             -> RelateResult<'tcx, T>
    {
        self.relate(a, b)
    }

    fn regions(&mut self, a: ty::Region<'tcx>, b: ty::Region<'tcx>)
               -> RelateResult<'tcx, ty::Region<'tcx>> {
        debug!("{}.regions({:?}, {:?})",
               self.tag(),
               a,
               b);
        Ok(a)
    }

    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
        debug!("{}.tys({:?}, {:?})", self.tag(),
               a, b);
        if a == b { return Ok(a); }

        match (&a.kind, &b.kind) {
            (_, &ty::Infer(ty::FreshTy(_))) |
            (_, &ty::Infer(ty::FreshIntTy(_))) |
            (_, &ty::Infer(ty::FreshFloatTy(_))) => {
                Ok(a)
            }

            (&ty::Infer(_), _) |
            (_, &ty::Infer(_)) => {
                Err(TypeError::Sorts(relate::expected_found(self, &a, &b)))
            }

            (&ty::Error, _) | (_, &ty::Error) => {
                Ok(self.tcx().types.err)
            }

            _ => {
                relate::super_relate_tys(self, a, b)
            }
        }
    }

    fn consts(
        &mut self,
        a: &'tcx ty::Const<'tcx>,
        b: &'tcx ty::Const<'tcx>,
    ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
        debug!("{}.consts({:?}, {:?})", self.tag(), a, b);
        if a == b {
            return Ok(a);
        }

        match (a.val, b.val) {
            (_, ty::ConstKind::Infer(InferConst::Fresh(_))) => {
                return Ok(a);
            }

            (ty::ConstKind::Infer(_), _) | (_, ty::ConstKind::Infer(_)) => {
                return Err(TypeError::ConstMismatch(relate::expected_found(self, &a, &b)));
            }

            _ => {}
        }

        relate::super_relate_consts(self, a, b)
    }

    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
                  -> RelateResult<'tcx, ty::Binder<T>>
        where T: Relate<'tcx>
    {
        Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
    }
}