rust/src/librustc/middle/infer/coercion.rs

// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! # Type Coercion
//!
//! Under certain circumstances we will coerce from one type to another,
//! for example by auto-borrowing.  This occurs in situations where the
//! compiler has a firm 'expected type' that was supplied from the user,
//! and where the actual type is similar to that expected type in purpose
//! but not in representation (so actual subtyping is inappropriate).
//!
//! ## Reborrowing
//!
//! Note that if we are expecting a reference, we will *reborrow*
//! even if the argument provided was already a reference.  This is
//! useful for freezing mut/const things (that is, when the expected is &T
//! but you have &const T or &mut T) and also for avoiding the linearity
//! of mut things (when the expected is &mut T and you have &mut T).  See
//! the various `src/test/run-pass/coerce-reborrow-*.rs` tests for
//! examples of where this is useful.
//!
//! ## Subtle note
//!
//! When deciding what type coercions to consider, we do not attempt to
//! resolve any type variables we may encounter.  This is because `b`
//! represents the expected type "as the user wrote it", meaning that if
//! the user defined a generic function like
//!
//!    fn foo<A>(a: A, b: A) { ... }
//!
//! and then we wrote `foo(&1, @2)`, we will not auto-borrow
//! either argument.  In older code we went to some lengths to
//! resolve the `b` variable, which could mean that we'd
//! auto-borrow later arguments but not earlier ones, which
//! seems very confusing.
//!
//! ## Subtler note
//!
//! However, right now, if the user manually specifies the
//! values for the type variables, as so:
//!
//!    foo::<&int>(@1, @2)
//!
//! then we *will* auto-borrow, because we can't distinguish this from a
//! function that declared `&int`.  This is inconsistent but it's easiest
//! at the moment. The right thing to do, I think, is to consider the
//! *unsubstituted* type when deciding whether to auto-borrow, but the
//! *substituted* type when considering the bounds and so forth. But most
//! of our methods don't give access to the unsubstituted type, and
//! rightly so because they'd be error-prone.  So maybe the thing to do is
//! to actually determine the kind of coercions that should occur
//! separately and pass them in.  Or maybe it's ok as is.  Anyway, it's
//! sort of a minor point so I've opted to leave it for later---after all
//! we may want to adjust precisely when coercions occur.

use super::{CoerceResult, Coercion};
use super::combine::{CombineFields, Combine};
use super::sub::Sub;

use middle::subst;
use middle::ty::{AutoPtr, AutoDerefRef, AdjustDerefRef, AutoUnsize, AutoUnsafe};
use middle::ty::{mt};
use middle::ty::{mod, Ty};
use util::ppaux;
use util::ppaux::Repr;

use syntax::abi;
use syntax::ast;

// Note: Coerce is not actually a combiner, in that it does not
// conform to the same interface, though it performs a similar
// function.
pub struct Coerce<'f, 'tcx: 'f>(pub CombineFields<'f, 'tcx>);

impl<'f, 'tcx> Coerce<'f, 'tcx> {
    pub fn get_ref<'a>(&'a self) -> &'a CombineFields<'f, 'tcx> {
        let Coerce(ref v) = *self; v
    }

    fn tcx(&self) -> &ty::ctxt<'tcx> {
        self.get_ref().infcx.tcx
    }

    pub fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        debug!("Coerce.tys({} => {})",
               a.repr(self.tcx()),
               b.repr(self.tcx()));

        // Consider coercing the subtype to a DST
        let unsize = self.unpack_actual_value(a, |a| {
            self.coerce_unsized(a, b)
        });
        if unsize.is_ok() {
            return unsize;
        }

        // Examine the supertype and consider auto-borrowing.
        //
        // Note: does not attempt to resolve type variables we encounter.
        // See above for details.
        match b.sty {
            ty::ty_ptr(mt_b) => {
                match mt_b.ty.sty {
                    ty::ty_str => {
                        return self.unpack_actual_value(a, |a| {
                            self.coerce_unsafe_ptr(a, b, ast::MutImmutable)
                        });
                    }

                    ty::ty_trait(..) => {
                        let result = self.unpack_actual_value(a, |a| {
                            self.coerce_unsafe_object(a, b, mt_b.mutbl)
                        });

                        match result {
                            Ok(t) => return Ok(t),
                            Err(..) => {}
                        }
                    }

                    _ => {
                        return self.unpack_actual_value(a, |a| {
                            self.coerce_unsafe_ptr(a, b, mt_b.mutbl)
                        });
                    }
                };
            }

            ty::ty_rptr(_, mt_b) => {
                match mt_b.ty.sty {
                    ty::ty_str => {
                        return self.unpack_actual_value(a, |a| {
                            self.coerce_borrowed_pointer(a, b, ast::MutImmutable)
                        });
                    }

                    ty::ty_trait(..) => {
                        let result = self.unpack_actual_value(a, |a| {
                            self.coerce_borrowed_object(a, b, mt_b.mutbl)
                        });

                        match result {
                            Ok(t) => return Ok(t),
                            Err(..) => {}
                        }
                    }

                    _ => {
                        return self.unpack_actual_value(a, |a| {
                            self.coerce_borrowed_pointer(a, b, mt_b.mutbl)
                        });
                    }
                };
            }

            ty::ty_closure(box ty::ClosureTy {
                    store: ty::RegionTraitStore(..),
                    ..
                }) => {
                return self.unpack_actual_value(a, |a| {
                    self.coerce_borrowed_fn(a, b)
                });
            }

            _ => {}
        }

        self.unpack_actual_value(a, |a| {
            match a.sty {
                ty::ty_bare_fn(Some(a_def_id), a_f) => {
                    // Function items are coercible to any closure
                    // type; function pointers are not (that would
                    // require double indirection).
                    self.coerce_from_fn_item(a, a_def_id, a_f, b)
                }
                _ => {
                    // Otherwise, just use subtyping rules.
                    self.subtype(a, b)
                }
            }
        })
    }

    pub fn subtype(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        match Sub(self.get_ref().clone()).tys(a, b) {
            Ok(_) => Ok(None),         // No coercion required.
            Err(ref e) => Err(*e)
        }
    }

    pub fn unpack_actual_value<T, F>(&self, a: Ty<'tcx>, f: F) -> T where
        F: FnOnce(Ty<'tcx>) -> T,
    {
        f(self.get_ref().infcx.shallow_resolve(a))
    }

    // ~T -> &T or &mut T -> &T (including where T = [U] or str)
    pub fn coerce_borrowed_pointer(&self,
                                   a: Ty<'tcx>,
                                   b: Ty<'tcx>,
                                   mutbl_b: ast::Mutability)
                                   -> CoerceResult<'tcx> {
        debug!("coerce_borrowed_pointer(a={}, b={})",
               a.repr(self.tcx()),
               b.repr(self.tcx()));

        // If we have a parameter of type `&M T_a` and the value
        // provided is `expr`, we will be adding an implicit borrow,
        // meaning that we convert `f(expr)` to `f(&M *expr)`.  Therefore,
        // to type check, we will construct the type that `&M*expr` would
        // yield.

        let sub = Sub(self.get_ref().clone());
        let coercion = Coercion(self.get_ref().trace.clone());
        let r_borrow = self.get_ref().infcx.next_region_var(coercion);

        let inner_ty = match a.sty {
            ty::ty_uniq(_) => return Err(ty::terr_mismatch),
            ty::ty_rptr(_, mt_a) => mt_a.ty,
            _ => {
                return self.subtype(a, b);
            }
        };

        let a_borrowed = ty::mk_rptr(self.tcx(),
                                     self.tcx().mk_region(r_borrow),
                                     mt {ty: inner_ty, mutbl: mutbl_b});
        try!(sub.tys(a_borrowed, b));

        Ok(Some(AdjustDerefRef(AutoDerefRef {
            autoderefs: 1,
            autoref: Some(AutoPtr(r_borrow, mutbl_b, None))
        })))
    }


    // &[T, ..n] or &mut [T, ..n] -> &[T]
    // or &mut [T, ..n] -> &mut [T]
    // or &Concrete -> &Trait, etc.
    fn coerce_unsized(&self,
                      a: Ty<'tcx>,
                      b: Ty<'tcx>)
                      -> CoerceResult<'tcx> {
        debug!("coerce_unsized(a={}, b={})",
               a.repr(self.tcx()),
               b.repr(self.tcx()));

        // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
        // a DST unless we have to. This currently comes out in the wash since
        // we can't unify [T] with U. But to properly support DST, we need to allow
        // that, at which point we will need extra checks on b here.

        let sub = Sub(self.get_ref().clone());

        match (&a.sty, &b.sty) {
            (&ty::ty_rptr(_, ty::mt{ty: t_a, mutbl: mutbl_a}), &ty::ty_rptr(_, mt_b)) => {
                self.unpack_actual_value(t_a, |a| {
                    match self.unsize_ty(t_a, a, mt_b.ty) {
                        Some((ty, kind)) => {
                            if !can_coerce_mutbls(mutbl_a, mt_b.mutbl) {
                                return Err(ty::terr_mutability);
                            }

                            let coercion = Coercion(self.get_ref().trace.clone());
                            let r_borrow = self.get_ref().infcx.next_region_var(coercion);
                            let ty = ty::mk_rptr(self.tcx(),
                                                 self.tcx().mk_region(r_borrow),
                                                 ty::mt{ty: ty, mutbl: mt_b.mutbl});
                            try!(self.get_ref().infcx.try(|_| sub.tys(ty, b)));
                            debug!("Success, coerced with AutoDerefRef(1, \
                                    AutoPtr(AutoUnsize({})))", kind);
                            Ok(Some(AdjustDerefRef(AutoDerefRef {
                                autoderefs: 1,
                                autoref: Some(ty::AutoPtr(r_borrow, mt_b.mutbl,
                                                          Some(box AutoUnsize(kind))))
                            })))
                        }
                        _ => Err(ty::terr_mismatch)
                    }
                })
            }
            (&ty::ty_rptr(_, ty::mt{ty: t_a, mutbl: mutbl_a}), &ty::ty_ptr(mt_b)) => {
                self.unpack_actual_value(t_a, |a| {
                    match self.unsize_ty(t_a, a, mt_b.ty) {
                        Some((ty, kind)) => {
                            if !can_coerce_mutbls(mutbl_a, mt_b.mutbl) {
                                return Err(ty::terr_mutability);
                            }

                            let ty = ty::mk_ptr(self.tcx(),
                                                 ty::mt{ty: ty, mutbl: mt_b.mutbl});
                            try!(self.get_ref().infcx.try(|_| sub.tys(ty, b)));
                            debug!("Success, coerced with AutoDerefRef(1, \
                                    AutoPtr(AutoUnsize({})))", kind);
                            Ok(Some(AdjustDerefRef(AutoDerefRef {
                                autoderefs: 1,
                                autoref: Some(ty::AutoUnsafe(mt_b.mutbl,
                                                             Some(box AutoUnsize(kind))))
                            })))
                        }
                        _ => Err(ty::terr_mismatch)
                    }
                })
            }
            (&ty::ty_uniq(t_a), &ty::ty_uniq(t_b)) => {
                self.unpack_actual_value(t_a, |a| {
                    match self.unsize_ty(t_a, a, t_b) {
                        Some((ty, kind)) => {
                            let ty = ty::mk_uniq(self.tcx(), ty);
                            try!(self.get_ref().infcx.try(|_| sub.tys(ty, b)));
                            debug!("Success, coerced with AutoDerefRef(1, \
                                    AutoUnsizeUniq({}))", kind);
                            Ok(Some(AdjustDerefRef(AutoDerefRef {
                                autoderefs: 1,
                                autoref: Some(ty::AutoUnsizeUniq(kind))
                            })))
                        }
                        _ => Err(ty::terr_mismatch)
                    }
                })
            }
            _ => Err(ty::terr_mismatch)
        }
    }

    // Takes a type and returns an unsized version along with the adjustment
    // performed to unsize it.
    // E.g., `[T, ..n]` -> `([T], UnsizeLength(n))`
    fn unsize_ty(&self,
                 ty_a: Ty<'tcx>,
                 a: Ty<'tcx>,
                 ty_b: Ty<'tcx>)
                 -> Option<(Ty<'tcx>, ty::UnsizeKind<'tcx>)> {
        debug!("unsize_ty(a={}, ty_b={})", a, ty_b.repr(self.tcx()));

        let tcx = self.tcx();

        self.unpack_actual_value(ty_b, |b|
            match (&a.sty, &b.sty) {
                (&ty::ty_vec(t_a, Some(len)), &ty::ty_vec(_, None)) => {
                    let ty = ty::mk_vec(tcx, t_a, None);
                    Some((ty, ty::UnsizeLength(len)))
                }
                (&ty::ty_trait(..), &ty::ty_trait(..)) => {
                    None
                }
                (_, &ty::ty_trait(box ty::TyTrait { ref principal, ref bounds })) => {
                    // FIXME what is the purpose of `ty`?
                    let ty = ty::mk_trait(tcx, principal.clone(), bounds.clone());
                    Some((ty, ty::UnsizeVtable(ty::TyTrait { principal: principal.clone(),
                                                             bounds: bounds.clone() },
                                               ty_a)))
                }
                (&ty::ty_struct(did_a, substs_a), &ty::ty_struct(did_b, substs_b))
                  if did_a == did_b => {
                    debug!("unsizing a struct");
                    // Try unsizing each type param in turn to see if we end up with ty_b.
                    let ty_substs_a = substs_a.types.get_slice(subst::TypeSpace);
                    let ty_substs_b = substs_b.types.get_slice(subst::TypeSpace);
                    assert!(ty_substs_a.len() == ty_substs_b.len());

                    let sub = Sub(self.get_ref().clone());

                    let mut result = None;
                    let mut tps = ty_substs_a.iter().zip(ty_substs_b.iter()).enumerate();
                    for (i, (tp_a, tp_b)) in tps {
                        if self.get_ref().infcx.try(|_| sub.tys(*tp_a, *tp_b)).is_ok() {
                            continue;
                        }
                        match
                            self.unpack_actual_value(
                                *tp_a,
                                |tp| self.unsize_ty(*tp_a, tp, *tp_b))
                        {
                            Some((new_tp, k)) => {
                                // Check that the whole types match.
                                let mut new_substs = substs_a.clone();
                                new_substs.types.get_mut_slice(subst::TypeSpace)[i] = new_tp;
                                let ty = ty::mk_struct(tcx, did_a, tcx.mk_substs(new_substs));
                                if self.get_ref().infcx.try(|_| sub.tys(ty, ty_b)).is_err() {
                                    debug!("Unsized type parameter '{}', but still \
                                            could not match types {} and {}",
                                           ppaux::ty_to_string(tcx, *tp_a),
                                           ppaux::ty_to_string(tcx, ty),
                                           ppaux::ty_to_string(tcx, ty_b));
                                    // We can only unsize a single type parameter, so
                                    // if we unsize one and it doesn't give us the
                                    // type we want, then we won't succeed later.
                                    break;
                                }

                                result = Some((ty, ty::UnsizeStruct(box k, i)));
                                break;
                            }
                            None => {}
                        }
                    }
                    result
                }
                _ => None
            }
        )
    }

    fn coerce_borrowed_object(&self,
                              a: Ty<'tcx>,
                              b: Ty<'tcx>,
                              b_mutbl: ast::Mutability) -> CoerceResult<'tcx>
    {
        let tcx = self.tcx();

        debug!("coerce_borrowed_object(a={}, b={}, b_mutbl={})",
               a.repr(tcx),
               b.repr(tcx), b_mutbl);

        let coercion = Coercion(self.get_ref().trace.clone());
        let r_a = self.get_ref().infcx.next_region_var(coercion);

        self.coerce_object(a, b, b_mutbl,
                           |tr| ty::mk_rptr(tcx, tcx.mk_region(r_a),
                                            ty::mt{ mutbl: b_mutbl, ty: tr }),
                           || AutoPtr(r_a, b_mutbl, None))
    }

    fn coerce_unsafe_object(&self,
                            a: Ty<'tcx>,
                            b: Ty<'tcx>,
                            b_mutbl: ast::Mutability) -> CoerceResult<'tcx>
    {
        let tcx = self.tcx();

        debug!("coerce_unsafe_object(a={}, b={}, b_mutbl={})",
               a.repr(tcx),
               b.repr(tcx), b_mutbl);

        self.coerce_object(a, b, b_mutbl,
                           |tr| ty::mk_ptr(tcx, ty::mt{ mutbl: b_mutbl, ty: tr }),
                           || AutoUnsafe(b_mutbl, None))
    }

    fn coerce_object<F, G>(&self,
                           a: Ty<'tcx>,
                           b: Ty<'tcx>,
                           b_mutbl: ast::Mutability,
                           mk_ty: F,
                           mk_adjust: G) -> CoerceResult<'tcx> where
        F: FnOnce(Ty<'tcx>) -> Ty<'tcx>,
        G: FnOnce() -> ty::AutoRef<'tcx>,
    {
        let tcx = self.tcx();

        match a.sty {
            ty::ty_rptr(_, ty::mt{ty, mutbl}) => match ty.sty {
                ty::ty_trait(box ty::TyTrait { ref principal, ref bounds }) => {
                    debug!("mutbl={} b_mutbl={}", mutbl, b_mutbl);
                    let tr = ty::mk_trait(tcx, principal.clone(), bounds.clone());
                    try!(self.subtype(mk_ty(tr), b));
                    Ok(Some(AdjustDerefRef(AutoDerefRef {
                        autoderefs: 1,
                        autoref: Some(mk_adjust())
                    })))
                }
                _ => {
                    self.subtype(a, b)
                }
            },
            _ => {
                self.subtype(a, b)
            }
        }
    }

    pub fn coerce_borrowed_fn(&self,
                              a: Ty<'tcx>,
                              b: Ty<'tcx>)
                              -> CoerceResult<'tcx> {
        debug!("coerce_borrowed_fn(a={}, b={})",
               a.repr(self.tcx()),
               b.repr(self.tcx()));

        match a.sty {
            ty::ty_bare_fn(Some(a_def_id), f) => {
                self.coerce_from_fn_item(a, a_def_id, f, b)
            }
            _ => {
                self.subtype(a, b)
            }
        }
    }

    fn coerce_from_fn_item(&self,
                           a: Ty<'tcx>,
                           fn_def_id_a: ast::DefId,
                           fn_ty_a: &'tcx ty::BareFnTy<'tcx>,
                           b: Ty<'tcx>)
                           -> CoerceResult<'tcx> {
        /*!
         * Attempts to coerce from the type of a Rust function item
         * into a closure or a `proc`.
         */

        self.unpack_actual_value(b, |b| {
            debug!("coerce_from_fn_item(a={}, b={})",
                   a.repr(self.tcx()), b.repr(self.tcx()));

            match b.sty {
                ty::ty_closure(ref f) => {
                    if fn_ty_a.abi != abi::Rust || fn_ty_a.unsafety != ast::Unsafety::Normal {
                        return self.subtype(a, b);
                    }

                    let fn_ty_b = (*f).clone();
                    let adj = ty::AdjustAddEnv(fn_def_id_a, fn_ty_b.store);
                    let a_closure = ty::mk_closure(self.tcx(),
                                                   ty::ClosureTy {
                                                       sig: fn_ty_a.sig.clone(),
                                                       .. *fn_ty_b
                                                   });
                    try!(self.subtype(a_closure, b));
                    Ok(Some(adj))
                }
                ty::ty_bare_fn(None, _) => {
                    let a_fn_pointer = ty::mk_bare_fn(self.tcx(), None, fn_ty_a);
                    try!(self.subtype(a_fn_pointer, b));
                    Ok(Some(ty::AdjustReifyFnPointer(fn_def_id_a)))
                }
                _ => {
                    return self.subtype(a, b)
                }
            }
        })
    }

    pub fn coerce_unsafe_ptr(&self,
                             a: Ty<'tcx>,
                             b: Ty<'tcx>,
                             mutbl_b: ast::Mutability)
                             -> CoerceResult<'tcx> {
        debug!("coerce_unsafe_ptr(a={}, b={})",
               a.repr(self.tcx()),
               b.repr(self.tcx()));

        let mt_a = match a.sty {
            ty::ty_rptr(_, mt) | ty::ty_ptr(mt) => mt,
            _ => {
                return self.subtype(a, b);
            }
        };

        // Check that the types which they point at are compatible.
        let a_unsafe = ty::mk_ptr(self.tcx(), ty::mt{ mutbl: mutbl_b, ty: mt_a.ty });
        try!(self.subtype(a_unsafe, b));
        if !can_coerce_mutbls(mt_a.mutbl, mutbl_b) {
            return Err(ty::terr_mutability);
        }

        // Although references and unsafe ptrs have the same
        // representation, we still register an AutoDerefRef so that
        // regionck knows that the region for `a` must be valid here.
        Ok(Some(AdjustDerefRef(AutoDerefRef {
            autoderefs: 1,
            autoref: Some(ty::AutoUnsafe(mutbl_b, None))
        })))
    }
}

fn can_coerce_mutbls(from_mutbl: ast::Mutability,
                     to_mutbl: ast::Mutability)
                     -> bool {
    match (from_mutbl, to_mutbl) {
        (ast::MutMutable, ast::MutMutable) => true,
        (ast::MutImmutable, ast::MutImmutable) => true,
        (ast::MutMutable, ast::MutImmutable) => true,
        (ast::MutImmutable, ast::MutMutable) => false,
    }
}