src/librustc/infer/unify_key.rs

use crate::ty::{self, FloatVarValue, IntVarValue, Ty, TyCtxt, InferConst};
use rustc_data_structures::unify::{NoError, EqUnifyValue, UnifyKey, UnifyValue, UnificationTable};
use rustc_data_structures::unify::InPlace;
use syntax_pos::{Span, DUMMY_SP};
use syntax::symbol::Symbol;

use std::cmp;
use std::marker::PhantomData;
use std::cell::RefMut;

pub trait ToType {
    fn to_type<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
}

impl UnifyKey for ty::IntVid {
    type Value = Option<IntVarValue>;
    fn index(&self) -> u32 { self.index }
    fn from_index(i: u32) -> ty::IntVid { ty::IntVid { index: i } }
    fn tag() -> &'static str { "IntVid" }
}

impl EqUnifyValue for IntVarValue {}

#[derive(PartialEq, Copy, Clone, Debug)]
pub struct RegionVidKey {
    /// The minimum region vid in the unification set. This is needed
    /// to have a canonical name for a type to prevent infinite
    /// recursion.
    pub min_vid: ty::RegionVid
}

impl UnifyValue for RegionVidKey {
    type Error = NoError;

    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
        let min_vid = if value1.min_vid.index() < value2.min_vid.index() {
            value1.min_vid
        } else {
            value2.min_vid
        };

        Ok(RegionVidKey { min_vid })
    }
}

impl UnifyKey for ty::RegionVid {
    type Value = RegionVidKey;
    fn index(&self) -> u32 { u32::from(*self) }
    fn from_index(i: u32) -> ty::RegionVid { ty::RegionVid::from(i) }
    fn tag() -> &'static str { "RegionVid" }
}

impl ToType for IntVarValue {
    fn to_type<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
        match *self {
            ty::IntType(i) => tcx.mk_mach_int(i),
            ty::UintType(i) => tcx.mk_mach_uint(i),
        }
    }
}

// Floating point type keys

impl UnifyKey for ty::FloatVid {
    type Value = Option<FloatVarValue>;
    fn index(&self) -> u32 { self.index }
    fn from_index(i: u32) -> ty::FloatVid { ty::FloatVid { index: i } }
    fn tag() -> &'static str { "FloatVid" }
}

impl EqUnifyValue for FloatVarValue {}

impl ToType for FloatVarValue {
    fn to_type<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
        tcx.mk_mach_float(self.0)
    }
}

// Generic consts.

#[derive(Copy, Clone, Debug)]
pub struct ConstVariableOrigin {
    pub kind: ConstVariableOriginKind,
    pub span: Span,
}

/// Reasons to create a const inference variable
#[derive(Copy, Clone, Debug)]
pub enum ConstVariableOriginKind {
    MiscVariable,
    ConstInference,
    ConstParameterDefinition(Symbol),
    SubstitutionPlaceholder,
}

#[derive(Copy, Clone, Debug)]
pub enum ConstVariableValue<'tcx> {
    Known { value: &'tcx ty::Const<'tcx> },
    Unknown { universe: ty::UniverseIndex },
}

impl<'tcx> ConstVariableValue<'tcx> {
    /// If this value is known, returns the const it is known to be.
    /// Otherwise, `None`.
    pub fn known(&self) -> Option<&'tcx ty::Const<'tcx>> {
        match *self {
            ConstVariableValue::Unknown { .. } => None,
            ConstVariableValue::Known { value } => Some(value),
        }
    }

    pub fn is_unknown(&self) -> bool {
        match *self {
            ConstVariableValue::Unknown { .. } => true,
            ConstVariableValue::Known { .. } => false,
        }
    }
}

#[derive(Copy, Clone, Debug)]
pub struct ConstVarValue<'tcx> {
    pub origin: ConstVariableOrigin,
    pub val: ConstVariableValue<'tcx>,
}

impl<'tcx> UnifyKey for ty::ConstVid<'tcx> {
    type Value = ConstVarValue<'tcx>;
    fn index(&self) -> u32 { self.index }
    fn from_index(i: u32) -> Self { ty::ConstVid { index: i, phantom: PhantomData } }
    fn tag() -> &'static str { "ConstVid" }
}

impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
    type Error = (&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>);

    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error> {
        let val = match (value1.val, value2.val) {
            (
                ConstVariableValue::Known { .. },
                ConstVariableValue::Known { .. }
            ) => {
                bug!("equating two const variables, both of which have known values")
            }

            // If one side is known, prefer that one.
            (ConstVariableValue::Known { .. }, ConstVariableValue::Unknown { .. }) => {
                Ok(value1.val)
            }
            (ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => {
                Ok(value2.val)
            }

            // If both sides are *unknown*, it hardly matters, does it?
            (ConstVariableValue::Unknown { universe: universe1 },
             ConstVariableValue::Unknown { universe: universe2 }) =>  {
                // If we unify two unbound variables, ?T and ?U, then whatever
                // value they wind up taking (which must be the same value) must
                // be nameable by both universes. Therefore, the resulting
                // universe is the minimum of the two universes, because that is
                // the one which contains the fewest names in scope.
                let universe = cmp::min(universe1, universe2);
                Ok(ConstVariableValue::Unknown { universe })
            }
        }?;

        Ok(ConstVarValue {
            origin: ConstVariableOrigin {
                kind: ConstVariableOriginKind::ConstInference,
                span: DUMMY_SP,
            },
            val,
        })
    }
}

impl<'tcx> EqUnifyValue for &'tcx ty::Const<'tcx> {}

pub fn replace_if_possible(
    mut table: RefMut<'_, UnificationTable<InPlace<ty::ConstVid<'tcx>>>>,
    c: &'tcx ty::Const<'tcx>
) -> &'tcx ty::Const<'tcx> {
    if let ty::Const { val: ty::ConstKind::Infer(InferConst::Var(vid)), .. } = c {
        match table.probe_value(*vid).val.known() {
            Some(c) => c,
            None => c,
        }
    } else {
        c
    }
}