fudge.rs

use crate::infer::type_variable::TypeVariableMap;
use crate::ty::{self, Ty, TyCtxt};
use crate::ty::fold::{TypeFoldable, TypeFolder};

use super::InferCtxt;
use super::RegionVariableOrigin;

impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
    /// This rather funky routine is used while processing expected
    /// types. What happens here is that we want to propagate a
    /// coercion through the return type of a fn to its
    /// argument. Consider the type of `Option::Some`, which is
    /// basically `for<T> fn(T) -> Option<T>`. So if we have an
    /// expression `Some(&[1, 2, 3])`, and that has the expected type
    /// `Option<&[u32]>`, we would like to type check `&[1, 2, 3]`
    /// with the expectation of `&[u32]`. This will cause us to coerce
    /// from `&[u32; 3]` to `&[u32]` and make the users life more
    /// pleasant.
    ///
    /// The way we do this is using `fudge_regions_if_ok`. What the
    /// routine actually does is to start a snapshot and execute the
    /// closure `f`. In our example above, what this closure will do
    /// is to unify the expectation (`Option<&[u32]>`) with the actual
    /// return type (`Option<?T>`, where `?T` represents the variable
    /// instantiated for `T`). This will cause `?T` to be unified
    /// with `&?a [u32]`, where `?a` is a fresh lifetime variable. The
    /// input type (`?T`) is then returned by `f()`.
    ///
    /// At this point, `fudge_regions_if_ok` will normalize all type
    /// variables, converting `?T` to `&?a [u32]` and end the
    /// snapshot. The problem is that we can't just return this type
    /// out, because it references the region variable `?a`, and that
    /// region variable was popped when we popped the snapshot.
    ///
    /// So what we do is to keep a list (`region_vars`, in the code below)
    /// of region variables created during the snapshot (here, `?a`). We
    /// fold the return value and replace any such regions with a *new*
    /// region variable (e.g., `?b`) and return the result (`&?b [u32]`).
    /// This can then be used as the expectation for the fn argument.
    ///
    /// The important point here is that, for soundness purposes, the
    /// regions in question are not particularly important. We will
    /// use the expected types to guide coercions, but we will still
    /// type-check the resulting types from those coercions against
    /// the actual types (`?T`, `Option<?T`) -- and remember that
    /// after the snapshot is popped, the variable `?T` is no longer
    /// unified.
    pub fn fudge_regions_if_ok<T, E, F>(&self,
                                        origin: &RegionVariableOrigin,
                                        f: F) -> Result<T, E> where
        F: FnOnce() -> Result<T, E>,
        T: TypeFoldable<'tcx>,
    {
        debug!("fudge_regions_if_ok(origin={:?})", origin);

        let (type_variables, region_vars, value) = self.probe(|snapshot| {
            match f() {
                Ok(value) => {
                    let value = self.resolve_type_vars_if_possible(&value);

                    // At this point, `value` could in principle refer
                    // to types/regions that have been created during
                    // the snapshot. Once we exit `probe()`, those are
                    // going to be popped, so we will have to
                    // eliminate any references to them.

                    let type_variables =
                        self.type_variables.borrow_mut().types_created_since_snapshot(
                            &snapshot.type_snapshot);
                    let region_vars =
                        self.borrow_region_constraints().vars_created_since_snapshot(
                            &snapshot.region_constraints_snapshot);

                    Ok((type_variables, region_vars, value))
                }
                Err(e) => Err(e),
            }
        })?;

        // At this point, we need to replace any of the now-popped
        // type/region variables that appear in `value` with a fresh
        // variable of the appropriate kind. We can't do this during
        // the probe because they would just get popped then too. =)

        // Micro-optimization: if no variables have been created, then
        // `value` can't refer to any of them. =) So we can just return it.
        if type_variables.is_empty() && region_vars.is_empty() {
            return Ok(value);
        }

        let mut fudger = RegionFudger {
            infcx: self,
            type_variables: &type_variables,
            region_vars: &region_vars,
            origin,
        };

        Ok(value.fold_with(&mut fudger))
    }
}

pub struct RegionFudger<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
    infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
    type_variables: &'a TypeVariableMap,
    region_vars: &'a Vec<ty::RegionVid>,
    origin: &'a RegionVariableOrigin,
}

impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionFudger<'a, 'gcx, 'tcx> {
    fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> {
        self.infcx.tcx
    }

    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
        match ty.sty {
            ty::Infer(ty::InferTy::TyVar(vid)) => {
                match self.type_variables.get(&vid) {
                    None => {
                        // This variable was created before the
                        // "fudging".  Since we refresh all type
                        // variables to their binding anyhow, we know
                        // that it is unbound, so we can just return
                        // it.
                        debug_assert!(self.infcx.type_variables.borrow_mut()
                                      .probe(vid)
                                      .is_unknown());
                        ty
                    }

                    Some(&origin) => {
                        // This variable was created during the
                        // fudging. Recreate it with a fresh variable
                        // here.
                        self.infcx.next_ty_var(origin)
                    }
                }
            }
            _ => ty.super_fold_with(self),
        }
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        match *r {
            ty::ReVar(v) if self.region_vars.contains(&v) => {
                self.infcx.next_region_var(self.origin.clone())
            }
            _ => {
                r
            }
        }
    }
}