src/librustc_typeck/check/callee.rs

// Copyright 2014 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.

use super::{DeferredCallResolution, Expectation, FnCtxt,
            TupleArgumentsFlag};

use CrateCtxt;
use middle::cstore::LOCAL_CRATE;
use hir::def::Def;
use hir::def_id::DefId;
use rustc::infer;
use rustc::ty::{self, LvaluePreference, Ty};
use syntax::parse::token;
use syntax::ptr::P;
use syntax_pos::Span;

use rustc::hir;

/// Check that it is legal to call methods of the trait corresponding
/// to `trait_id` (this only cares about the trait, not the specific
/// method that is called)
pub fn check_legal_trait_for_method_call(ccx: &CrateCtxt, span: Span, trait_id: DefId) {
    let tcx = ccx.tcx;
    let did = Some(trait_id);
    let li = &tcx.lang_items;

    if did == li.drop_trait() {
        span_err!(tcx.sess, span, E0040, "explicit use of destructor method");
    } else if !tcx.sess.features.borrow().unboxed_closures {
        // the #[feature(unboxed_closures)] feature isn't
        // activated so we need to enforce the closure
        // restrictions.

        let method = if did == li.fn_trait() {
            "call"
        } else if did == li.fn_mut_trait() {
            "call_mut"
        } else if did == li.fn_once_trait() {
            "call_once"
        } else {
            return // not a closure method, everything is OK.
        };

        struct_span_err!(tcx.sess, span, E0174,
                         "explicit use of unboxed closure method `{}` is experimental",
                         method)
            .help("add `#![feature(unboxed_closures)]` to the crate \
                  attributes to enable")
            .emit();
    }
}

enum CallStep<'tcx> {
    Builtin,
    DeferredClosure(ty::FnSig<'tcx>),
    Overloaded(ty::MethodCallee<'tcx>)
}

impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
    pub fn check_call(&self,
                      call_expr: &'gcx hir::Expr,
                      callee_expr: &'gcx hir::Expr,
                      arg_exprs: &'gcx [P<hir::Expr>],
                      expected: Expectation<'tcx>)
    {
        self.check_expr(callee_expr);
        let original_callee_ty = self.expr_ty(callee_expr);

        let mut autoderef = self.autoderef(callee_expr.span, original_callee_ty);
        let result = autoderef.by_ref().flat_map(|(adj_ty, idx)| {
            self.try_overloaded_call_step(call_expr, callee_expr, adj_ty, idx)
        }).next();
        let callee_ty = autoderef.unambiguous_final_ty();
        autoderef.finalize(LvaluePreference::NoPreference, Some(callee_expr));

        match result {
            None => {
                // this will report an error since original_callee_ty is not a fn
                self.confirm_builtin_call(call_expr, original_callee_ty, arg_exprs, expected);
            }

            Some(CallStep::Builtin) => {
                self.confirm_builtin_call(call_expr, callee_ty, arg_exprs, expected);
            }

            Some(CallStep::DeferredClosure(fn_sig)) => {
                self.confirm_deferred_closure_call(call_expr, arg_exprs, expected, fn_sig);
            }

            Some(CallStep::Overloaded(method_callee)) => {
                self.confirm_overloaded_call(call_expr, callee_expr,
                                             arg_exprs, expected, method_callee);
            }
        }
    }

    fn try_overloaded_call_step(&self,
                                call_expr: &'gcx hir::Expr,
                                callee_expr: &'gcx hir::Expr,
                                adjusted_ty: Ty<'tcx>,
                                autoderefs: usize)
                                -> Option<CallStep<'tcx>>
    {
        debug!("try_overloaded_call_step(call_expr={:?}, adjusted_ty={:?}, autoderefs={})",
               call_expr,
               adjusted_ty,
               autoderefs);

        // If the callee is a bare function or a closure, then we're all set.
        match self.structurally_resolved_type(callee_expr.span, adjusted_ty).sty {
            ty::TyFnDef(..) | ty::TyFnPtr(_) => {
                self.write_autoderef_adjustment(callee_expr.id, autoderefs);
                return Some(CallStep::Builtin);
            }

            ty::TyClosure(def_id, substs) => {
                assert_eq!(def_id.krate, LOCAL_CRATE);

                // Check whether this is a call to a closure where we
                // haven't yet decided on whether the closure is fn vs
                // fnmut vs fnonce. If so, we have to defer further processing.
                if self.closure_kind(def_id).is_none() {
                    let closure_ty =
                        self.closure_type(def_id, substs);
                    let fn_sig =
                        self.replace_late_bound_regions_with_fresh_var(call_expr.span,
                                                                       infer::FnCall,
                                                                       &closure_ty.sig).0;
                    self.record_deferred_call_resolution(def_id, Box::new(CallResolution {
                        call_expr: call_expr,
                        callee_expr: callee_expr,
                        adjusted_ty: adjusted_ty,
                        autoderefs: autoderefs,
                        fn_sig: fn_sig.clone(),
                        closure_def_id: def_id
                    }));
                    return Some(CallStep::DeferredClosure(fn_sig));
                }
            }

            // Hack: we know that there are traits implementing Fn for &F
            // where F:Fn and so forth. In the particular case of types
            // like `x: &mut FnMut()`, if there is a call `x()`, we would
            // normally translate to `FnMut::call_mut(&mut x, ())`, but
            // that winds up requiring `mut x: &mut FnMut()`. A little
            // over the top. The simplest fix by far is to just ignore
            // this case and deref again, so we wind up with
            // `FnMut::call_mut(&mut *x, ())`.
            ty::TyRef(..) if autoderefs == 0 => {
                return None;
            }

            _ => {}
        }

        self.try_overloaded_call_traits(call_expr, callee_expr, adjusted_ty, autoderefs)
            .map(|method_callee| CallStep::Overloaded(method_callee))
    }

    fn try_overloaded_call_traits(&self,
                                  call_expr: &hir::Expr,
                                  callee_expr: &hir::Expr,
                                  adjusted_ty: Ty<'tcx>,
                                  autoderefs: usize)
                                  -> Option<ty::MethodCallee<'tcx>>
    {
        // Try the options that are least restrictive on the caller first.
        for &(opt_trait_def_id, method_name) in &[
            (self.tcx.lang_items.fn_trait(), token::intern("call")),
            (self.tcx.lang_items.fn_mut_trait(), token::intern("call_mut")),
            (self.tcx.lang_items.fn_once_trait(), token::intern("call_once")),
        ] {
            let trait_def_id = match opt_trait_def_id {
                Some(def_id) => def_id,
                None => continue,
            };

            match self.lookup_method_in_trait_adjusted(call_expr.span,
                                                       Some(&callee_expr),
                                                       method_name,
                                                       trait_def_id,
                                                       autoderefs,
                                                       false,
                                                       adjusted_ty,
                                                       None) {
                None => continue,
                Some(method_callee) => {
                    return Some(method_callee);
                }
            }
        }

        None
    }

    fn confirm_builtin_call(&self,
                            call_expr: &hir::Expr,
                            callee_ty: Ty<'tcx>,
                            arg_exprs: &'gcx [P<hir::Expr>],
                            expected: Expectation<'tcx>)
    {
        let error_fn_sig;

        let fn_sig = match callee_ty.sty {
            ty::TyFnDef(_, _, &ty::BareFnTy {ref sig, ..}) |
            ty::TyFnPtr(&ty::BareFnTy {ref sig, ..}) => {
                sig
            }
            _ => {
                let mut err = self.type_error_struct(call_expr.span, |actual| {
                    format!("expected function, found `{}`", actual)
                }, callee_ty, None);

                if let hir::ExprCall(ref expr, _) = call_expr.node {
                    let tcx = self.tcx;
                    if let Some(pr) = tcx.def_map.borrow().get(&expr.id) {
                        if pr.depth == 0 && pr.base_def != Def::Err {
                            if let Some(span) = tcx.map.span_if_local(pr.base_def.def_id()) {
                                err.span_note(span, "defined here");
                            }
                        }
                    }
                }

                err.emit();

                // This is the "default" function signature, used in case of error.
                // In that case, we check each argument against "error" in order to
                // set up all the node type bindings.
                error_fn_sig = ty::Binder(ty::FnSig {
                    inputs: self.err_args(arg_exprs.len()),
                    output: ty::FnConverging(self.tcx.types.err),
                    variadic: false
                });

                &error_fn_sig
            }
        };

        // Replace any late-bound regions that appear in the function
        // signature with region variables. We also have to
        // renormalize the associated types at this point, since they
        // previously appeared within a `Binder<>` and hence would not
        // have been normalized before.
        let fn_sig =
            self.replace_late_bound_regions_with_fresh_var(call_expr.span,
                                                           infer::FnCall,
                                                           fn_sig).0;
        let fn_sig =
            self.normalize_associated_types_in(call_expr.span, &fn_sig);

        // Call the generic checker.
        let expected_arg_tys = self.expected_types_for_fn_args(call_expr.span,
                                                               expected,
                                                               fn_sig.output,
                                                               &fn_sig.inputs);
        self.check_argument_types(call_expr.span,
                                  &fn_sig.inputs,
                                  &expected_arg_tys[..],
                                  arg_exprs,
                                  fn_sig.variadic,
                                  TupleArgumentsFlag::DontTupleArguments);

        self.write_call(call_expr, fn_sig.output);
    }

    fn confirm_deferred_closure_call(&self,
                                     call_expr: &hir::Expr,
                                     arg_exprs: &'gcx [P<hir::Expr>],
                                     expected: Expectation<'tcx>,
                                     fn_sig: ty::FnSig<'tcx>)
    {
        // `fn_sig` is the *signature* of the cosure being called. We
        // don't know the full details yet (`Fn` vs `FnMut` etc), but we
        // do know the types expected for each argument and the return
        // type.

        let expected_arg_tys =
            self.expected_types_for_fn_args(call_expr.span,
                                            expected,
                                            fn_sig.output.clone(),
                                            &fn_sig.inputs);

        self.check_argument_types(call_expr.span,
                                  &fn_sig.inputs,
                                  &expected_arg_tys,
                                  arg_exprs,
                                  fn_sig.variadic,
                                  TupleArgumentsFlag::TupleArguments);

        self.write_call(call_expr, fn_sig.output);
    }

    fn confirm_overloaded_call(&self,
                               call_expr: &hir::Expr,
                               callee_expr: &'gcx hir::Expr,
                               arg_exprs: &'gcx [P<hir::Expr>],
                               expected: Expectation<'tcx>,
                               method_callee: ty::MethodCallee<'tcx>)
    {
        let output_type =
            self.check_method_argument_types(call_expr.span,
                                             method_callee.ty,
                                             callee_expr,
                                             arg_exprs,
                                             TupleArgumentsFlag::TupleArguments,
                                             expected);
        self.write_call(call_expr, output_type);

        self.write_overloaded_call_method_map(call_expr, method_callee);
    }

    fn write_overloaded_call_method_map(&self,
                                        call_expr: &hir::Expr,
                                        method_callee: ty::MethodCallee<'tcx>) {
        let method_call = ty::MethodCall::expr(call_expr.id);
        self.tables.borrow_mut().method_map.insert(method_call, method_callee);
    }
}

#[derive(Debug)]
struct CallResolution<'gcx: 'tcx, 'tcx> {
    call_expr: &'gcx hir::Expr,
    callee_expr: &'gcx hir::Expr,
    adjusted_ty: Ty<'tcx>,
    autoderefs: usize,
    fn_sig: ty::FnSig<'tcx>,
    closure_def_id: DefId,
}

impl<'gcx, 'tcx> DeferredCallResolution<'gcx, 'tcx> for CallResolution<'gcx, 'tcx> {
    fn resolve<'a>(&mut self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) {
        debug!("DeferredCallResolution::resolve() {:?}",
               self);

        // we should not be invoked until the closure kind has been
        // determined by upvar inference
        assert!(fcx.closure_kind(self.closure_def_id).is_some());

        // We may now know enough to figure out fn vs fnmut etc.
        match fcx.try_overloaded_call_traits(self.call_expr, self.callee_expr,
                                             self.adjusted_ty, self.autoderefs) {
            Some(method_callee) => {
                // One problem is that when we get here, we are going
                // to have a newly instantiated function signature
                // from the call trait. This has to be reconciled with
                // the older function signature we had before. In
                // principle we *should* be able to fn_sigs(), but we
                // can't because of the annoying need for a TypeTrace.
                // (This always bites me, should find a way to
                // refactor it.)
                let method_sig = fcx.tcx.no_late_bound_regions(method_callee.ty.fn_sig())
                                        .unwrap();

                debug!("attempt_resolution: method_callee={:?}",
                       method_callee);

                for (&method_arg_ty, &self_arg_ty) in
                    method_sig.inputs[1..].iter().zip(&self.fn_sig.inputs)
                {
                    fcx.demand_eqtype(self.call_expr.span, self_arg_ty, method_arg_ty);
                }

                let nilty = fcx.tcx.mk_nil();
                fcx.demand_eqtype(self.call_expr.span,
                                  method_sig.output.unwrap_or(nilty),
                                  self.fn_sig.output.unwrap_or(nilty));

                fcx.write_overloaded_call_method_map(self.call_expr, method_callee);
            }
            None => {
                span_bug!(
                    self.call_expr.span,
                    "failed to find an overloaded call trait for closure call");
            }
        }
    }
}