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 check::{autoderef, FnCtxt, UnresolvedTypeAction};

use middle::infer::{Coercion, TypeOrigin, TypeTrace};
use middle::traits::{self, ObligationCause};
use middle::traits::{predicate_for_trait_def, report_selection_error};
use middle::ty::adjustment::{AutoAdjustment, AutoDerefRef, AdjustDerefRef};
use middle::ty::adjustment::{AutoPtr, AutoUnsafe, AdjustReifyFnPointer};
use middle::ty::adjustment::{AdjustUnsafeFnPointer, AdjustMutToConstPointer};
use middle::ty::{self, LvaluePreference, TypeAndMut, Ty, TyCtxt};
use middle::ty::fold::TypeFoldable;
use middle::ty::error::TypeError;
use middle::ty::relate::{relate_substs, RelateResult, TypeRelation};
use util::common::indent;

use std::cell::RefCell;
use std::collections::VecDeque;
use rustc_front::hir;

struct Coerce<'a, 'tcx: 'a> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    origin: TypeOrigin,
    use_lub: bool,
    unsizing_obligations: RefCell<Vec<traits::PredicateObligation<'tcx>>>,
}

type CoerceResult<'tcx> = RelateResult<'tcx, (Ty<'tcx>, AutoAdjustment<'tcx>)>;

fn coerce_mutbls<'tcx>(from_mutbl: hir::Mutability,
                       to_mutbl: hir::Mutability)
                       -> RelateResult<'tcx, ()> {
    match (from_mutbl, to_mutbl) {
        (hir::MutMutable, hir::MutMutable) |
        (hir::MutImmutable, hir::MutImmutable) |
        (hir::MutMutable, hir::MutImmutable) => Ok(()),
        (hir::MutImmutable, hir::MutMutable) => Err(TypeError::Mutability)
    }
}

impl<'f, 'tcx> Coerce<'f, 'tcx> {
    fn new(fcx: &'f FnCtxt<'f, 'tcx>, origin: TypeOrigin) -> Self {
        Coerce {
            fcx: fcx,
            origin: origin,
            use_lub: false,
            unsizing_obligations: RefCell::new(vec![])
        }
    }

    fn tcx(&self) -> &TyCtxt<'tcx> {
        self.fcx.tcx()
    }

    /// Unify two types (using sub or lub) and produce a noop coercion.
    fn unify_and_identity(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        let infcx = self.fcx.infcx();
        infcx.commit_if_ok(|_| {
            let trace = TypeTrace::types(self.origin, false, a, b);
            if self.use_lub {
                infcx.lub(false, trace).relate(&a, &b)
            } else {
                infcx.sub(false, trace).relate(&a, &b)
            }
        }).and_then(|ty| self.identity(ty))
    }

    /// Synthesize an identity adjustment.
    fn identity(&self, ty: Ty<'tcx>) -> CoerceResult<'tcx> {
        Ok((ty, AdjustDerefRef(AutoDerefRef {
            autoderefs: 0,
            autoref: None,
            unsize: None
        })))
    }

    fn coerce<'a, E, I>(&self,
                        exprs: &E,
                        a: Ty<'tcx>,
                        b: Ty<'tcx>)
                        -> CoerceResult<'tcx>
        // FIXME(eddyb) use copyable iterators when that becomes ergonomic.
        where E: Fn() -> I,
              I: IntoIterator<Item=&'a hir::Expr> {

        let a = self.fcx.infcx().shallow_resolve(a);
        debug!("Coerce.tys({:?} => {:?})", a, b);

        // Just ignore error types.
        if a.references_error() || b.references_error() {
            return self.identity(b);
        }

        // Consider coercing the subtype to a DST
        let unsize = 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::TyRawPtr(mt_b) => {
                return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
            }

            ty::TyRef(r_b, mt_b) => {
                return self.coerce_borrowed_pointer(exprs, a, b, r_b, mt_b.mutbl);
            }

            _ => {}
        }

        match a.sty {
            ty::TyFnDef(_, _, 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_f, b)
            }
            ty::TyFnPtr(a_f) => {
                // We permit coercion of fn pointers to drop the
                // unsafe qualifier.
                self.coerce_from_fn_pointer(a, a_f, b)
            }
            _ => {
                // Otherwise, just use unification rules.
                self.unify_and_identity(a, b)
            }
        }
    }

    /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
    /// To match `A` with `B`, autoderef will be performed,
    /// calling `deref`/`deref_mut` where necessary.
    fn coerce_borrowed_pointer<'a, E, I>(&self,
                                         exprs: &E,
                                         a: Ty<'tcx>,
                                         b: Ty<'tcx>,
                                         r_b: &'tcx ty::Region,
                                         mutbl_b: hir::Mutability)
                                         -> CoerceResult<'tcx>
        // FIXME(eddyb) use copyable iterators when that becomes ergonomic.
        where E: Fn() -> I,
              I: IntoIterator<Item=&'a hir::Expr> {

        debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);

        // 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 (_r_a, _mutbl_a) = match a.sty {
            ty::TyRef(r_a, mt_a) => {
                try!(coerce_mutbls(mt_a.mutbl, mutbl_b));
                (r_a, mt_a.mutbl)
            }
            _ => return self.unify_and_identity(a, b)
        };

        let span = self.origin.span();
        let coercion = Coercion(span);
        let r_borrow = {
            // If are coercing from `&'a T` to `&'b U`, then we want to
            // reborrow the contents of `'a` for the lifetime `'b`
            // (which ought to be a sublifetime of `'a`).
            if !self.use_lub {
                r_b
            } else {
                // With LUB, we need more flexibility.
                let r_borrow = self.fcx.infcx().next_region_var(coercion);
                self.tcx().mk_region(r_borrow)
            }
        };
        let autoref = Some(AutoPtr(r_borrow, mutbl_b));

        let lvalue_pref = LvaluePreference::from_mutbl(mutbl_b);
        let mut first_error = None;
        let (_, autoderefs, success) = autoderef(self.fcx, span, a, exprs,
                                                 UnresolvedTypeAction::Ignore,
                                                 lvalue_pref,
                                                 |inner_ty, autoderef| {
            if autoderef == 0 {
                // Don't let this pass, otherwise it would cause
                // &T to autoref to &&T.
                return None;
            }
            let ty = self.tcx().mk_ref(r_borrow,
                                        TypeAndMut {ty: inner_ty, mutbl: mutbl_b});
            match self.unify_and_identity(ty, b) {
                Err(err) => {
                    if first_error.is_none() {
                        first_error = Some(err);
                    }
                    None
                }
                Ok((ty, _)) => Some(ty)
            }
        });

        match success {
            Some(ty) => {
                Ok((ty, AdjustDerefRef(AutoDerefRef {
                    autoderefs: autoderefs,
                    autoref: autoref,
                    unsize: None
                })))
            }
            None => {
                // Return original error as if overloaded deref was never
                // attempted, to avoid irrelevant/confusing error messages.
                Err(first_error.expect("coerce_borrowed_pointer failed with no error?"))
            }
        }
    }


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

        let traits = (self.tcx().lang_items.unsize_trait(),
                      self.tcx().lang_items.coerce_unsized_trait());
        let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
            (u, cu)
        } else {
            debug!("Missing Unsize or CoerceUnsized traits");
            return Err(TypeError::Mismatch);
        };

        // 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 the target here.

        // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
        let (source, reborrow) = match (&source.sty, &target.sty) {
            (&ty::TyRef(_, mt_a), &ty::TyRef(_, mt_b)) => {
                try!(coerce_mutbls(mt_a.mutbl, mt_b.mutbl));

                let coercion = Coercion(self.origin.span());
                let r_borrow = self.fcx.infcx().next_region_var(coercion);
                let region = self.tcx().mk_region(r_borrow);
                (mt_a.ty, Some(AutoPtr(region, mt_b.mutbl)))
            }
            (&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) => {
                try!(coerce_mutbls(mt_a.mutbl, mt_b.mutbl));
                (mt_a.ty, Some(AutoUnsafe(mt_b.mutbl)))
            }
            _ => (source, None)
        };
        let source = source.adjust_for_autoref(self.tcx(), reborrow);

        let mut selcx = traits::SelectionContext::new(self.fcx.infcx());

        // Use a FIFO queue for this custom fulfillment procedure.
        let mut queue = VecDeque::new();
        let mut leftover_predicates = vec![];

        // Create an obligation for `Source: CoerceUnsized<Target>`.
        let cause = ObligationCause::misc(self.origin.span(), self.fcx.body_id);
        queue.push_back(predicate_for_trait_def(self.tcx(),
                                                cause,
                                                coerce_unsized_did,
                                                0,
                                                source,
                                                vec![target]));

        // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
        // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
        // inference might unify those two inner type variables later.
        let traits = [coerce_unsized_did, unsize_did];
        while let Some(obligation) = queue.pop_front() {
            debug!("coerce_unsized resolve step: {:?}", obligation);
            let trait_ref =  match obligation.predicate {
                ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
                    tr.clone()
                }
                _ => {
                    leftover_predicates.push(obligation);
                    continue;
                }
            };
            match selcx.select(&obligation.with(trait_ref)) {
                // Uncertain or unimplemented.
                Ok(None) | Err(traits::Unimplemented) => {
                    debug!("coerce_unsized: early return - can't prove obligation");
                    return Err(TypeError::Mismatch);
                }

                // Object safety violations or miscellaneous.
                Err(err) => {
                    report_selection_error(self.fcx.infcx(), &obligation, &err);
                    // Treat this like an obligation and follow through
                    // with the unsizing - the lack of a coercion should
                    // be silent, as it causes a type mismatch later.
                }

                Ok(Some(vtable)) => {
                    for obligation in vtable.nested_obligations() {
                        queue.push_back(obligation);
                    }
                }
            }
        }

        *self.unsizing_obligations.borrow_mut() = leftover_predicates;

        let adjustment = AutoDerefRef {
            autoderefs: if reborrow.is_some() { 1 } else { 0 },
            autoref: reborrow,
            unsize: Some(target)
        };
        debug!("Success, coerced with {:?}", adjustment);
        Ok((target, AdjustDerefRef(adjustment)))
    }

    fn coerce_from_fn_pointer(&self,
                           a: Ty<'tcx>,
                           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`.
         */

        let b = self.fcx.infcx().shallow_resolve(b);
        debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);

        if let ty::TyFnPtr(fn_ty_b) = b.sty {
            match (fn_ty_a.unsafety, fn_ty_b.unsafety) {
                (hir::Unsafety::Normal, hir::Unsafety::Unsafe) => {
                    let unsafe_a = self.tcx().safe_to_unsafe_fn_ty(fn_ty_a);
                    return self.unify_and_identity(unsafe_a, b).map(|(ty, _)| {
                        (ty, AdjustUnsafeFnPointer)
                    });
                }
                _ => {}
            }
        }
        self.unify_and_identity(a, b)
    }

    fn coerce_from_fn_item(&self,
                           a: Ty<'tcx>,
                           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`.
         */

        let b = self.fcx.infcx().shallow_resolve(b);
        debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);

        match b.sty {
            ty::TyFnPtr(_) => {
                let a_fn_pointer = self.tcx().mk_ty(ty::TyFnPtr(fn_ty_a));
                self.unify_and_identity(a_fn_pointer, b).map(|(ty, _)| {
                    (ty, AdjustReifyFnPointer)
                })
            }
            _ => self.unify_and_identity(a, b)
        }
    }

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

        let (is_ref, mt_a) = match a.sty {
            ty::TyRef(_, mt) => (true, mt),
            ty::TyRawPtr(mt) => (false, mt),
            _ => {
                return self.unify_and_identity(a, b);
            }
        };

        // Check that the types which they point at are compatible.
        let a_unsafe = self.tcx().mk_ptr(ty::TypeAndMut{ mutbl: mutbl_b, ty: mt_a.ty });
        let (ty, noop) = try!(self.unify_and_identity(a_unsafe, b));
        try!(coerce_mutbls(mt_a.mutbl, mutbl_b));

        // 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((ty, if is_ref {
            AdjustDerefRef(AutoDerefRef {
                autoderefs: 1,
                autoref: Some(AutoUnsafe(mutbl_b)),
                unsize: None
            })
        } else if mt_a.mutbl != mutbl_b {
            AdjustMutToConstPointer
        } else {
            noop
        }))
    }
}

fn apply<'a, 'b, 'tcx, E, I>(coerce: &mut Coerce<'a, 'tcx>,
                             exprs: &E,
                             a: Ty<'tcx>,
                             b: Ty<'tcx>)
                             -> CoerceResult<'tcx>
    where E: Fn() -> I,
          I: IntoIterator<Item=&'b hir::Expr> {

    let (ty, adjustment) = try!(indent(|| coerce.coerce(exprs, a, b)));

    let fcx = coerce.fcx;
    if let AdjustDerefRef(auto) = adjustment {
        if auto.unsize.is_some() {
            let mut obligations = coerce.unsizing_obligations.borrow_mut();
            for obligation in obligations.drain(..) {
                fcx.register_predicate(obligation);
            }
        }
    }

    Ok((ty, adjustment))
}

/// Attempt to coerce an expression to a type, and return the
/// adjusted type of the expression, if successful.
/// Adjustments are only recorded if the coercion succeeded.
/// The expressions *must not* have any pre-existing adjustments.
pub fn try<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                     expr: &hir::Expr,
                     target: Ty<'tcx>)
                     -> RelateResult<'tcx, Ty<'tcx>> {
    let source = fcx.resolve_type_vars_if_possible(fcx.expr_ty(expr));
    debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);

    let mut coerce = Coerce::new(fcx, TypeOrigin::ExprAssignable(expr.span));
    fcx.infcx().commit_if_ok(|_| {
        let (ty, adjustment) =
            try!(apply(&mut coerce, &|| Some(expr), source, target));
        if !adjustment.is_identity() {
            debug!("Success, coerced with {:?}", adjustment);
            assert!(!fcx.inh.tables.borrow().adjustments.contains_key(&expr.id));
            fcx.write_adjustment(expr.id, adjustment);
        }
        Ok(ty)
    })
}

/// Given some expressions, their known unified type and another expression,
/// tries to unify the types, potentially inserting coercions on any of the
/// provided expressions and returns their LUB (aka "common supertype").
pub fn try_find_lub<'a, 'b, 'tcx, E, I>(fcx: &FnCtxt<'a, 'tcx>,
                                        origin: TypeOrigin,
                                        exprs: E,
                                        prev_ty: Ty<'tcx>,
                                        new: &'b hir::Expr)
                                        -> RelateResult<'tcx, Ty<'tcx>>
    // FIXME(eddyb) use copyable iterators when that becomes ergonomic.
    where E: Fn() -> I,
          I: IntoIterator<Item=&'b hir::Expr> {

    let prev_ty = fcx.resolve_type_vars_if_possible(prev_ty);
    let new_ty = fcx.resolve_type_vars_if_possible(fcx.expr_ty(new));
    debug!("coercion::try_find_lub({:?}, {:?})", prev_ty, new_ty);

    let trace = TypeTrace::types(origin, true, prev_ty, new_ty);
    let mut lub = fcx.infcx().lub(true, trace);

    // Special-case that coercion alone cannot handle:
    // Two function item types of differing IDs or Substs.
    match (&prev_ty.sty, &new_ty.sty) {
        (&ty::TyFnDef(a_def_id, a_substs, a_fty),
         &ty::TyFnDef(b_def_id, b_substs, b_fty)) => {
            // The signature must always match.
            let fty = try!(lub.relate(a_fty, b_fty));

            if a_def_id == b_def_id {
                // Same function, maybe the parameters match.
                let substs = fcx.infcx().commit_if_ok(|_| {
                    relate_substs(&mut lub, None, a_substs, b_substs)
                }).map(|s| fcx.tcx().mk_substs(s));

                if let Ok(substs) = substs {
                    // We have a LUB of prev_ty and new_ty, just return it.
                    return Ok(fcx.tcx().mk_fn_def(a_def_id, substs, fty));
                }
            }

            // Reify both sides and return the reified fn pointer type.
            for expr in exprs().into_iter().chain(Some(new)) {
                // No adjustments can produce a fn item, so this should never trip.
                assert!(!fcx.inh.tables.borrow().adjustments.contains_key(&expr.id));
                fcx.write_adjustment(expr.id, AdjustReifyFnPointer);
            }
            return Ok(fcx.tcx().mk_fn_ptr(fty));
        }
        _ => {}
    }

    let mut coerce = Coerce::new(fcx, origin);
    coerce.use_lub = true;

    // First try to coerce the new expression to the type of the previous ones,
    // but only if the new expression has no coercion already applied to it.
    let mut first_error = None;
    if !fcx.inh.tables.borrow().adjustments.contains_key(&new.id) {
        let result = fcx.infcx().commit_if_ok(|_| {
            apply(&mut coerce, &|| Some(new), new_ty, prev_ty)
        });
        match result {
            Ok((ty, adjustment)) => {
                if !adjustment.is_identity() {
                    fcx.write_adjustment(new.id, adjustment);
                }
                return Ok(ty);
            }
            Err(e) => first_error = Some(e)
        }
    }

    // Then try to coerce the previous expressions to the type of the new one.
    // This requires ensuring there are no coercions applied to *any* of the
    // previous expressions, other than noop reborrows (ignoring lifetimes).
    for expr in exprs() {
        let noop = match fcx.inh.tables.borrow().adjustments.get(&expr.id) {
            Some(&AdjustDerefRef(AutoDerefRef {
                autoderefs: 1,
                autoref: Some(AutoPtr(_, mutbl_adj)),
                unsize: None
            })) => match fcx.expr_ty(expr).sty {
                ty::TyRef(_, mt_orig) => {
                    // Reborrow that we can safely ignore.
                    mutbl_adj == mt_orig.mutbl
                }
                _ => false
            },
            Some(_) => false,
            None => true
        };

        if !noop {
            return fcx.infcx().commit_if_ok(|_| lub.relate(&prev_ty, &new_ty));
        }
    }

    match fcx.infcx().commit_if_ok(|_| apply(&mut coerce, &exprs, prev_ty, new_ty)) {
        Err(_) => {
            // Avoid giving strange errors on failed attempts.
            if let Some(e) = first_error {
                Err(e)
            } else {
                fcx.infcx().commit_if_ok(|_| lub.relate(&prev_ty, &new_ty))
            }
        }
        Ok((ty, adjustment)) => {
            if !adjustment.is_identity() {
                for expr in exprs() {
                    fcx.write_adjustment(expr.id, adjustment);
                }
            }
            Ok(ty)
        }
    }
}