src/librustc_typeck/check/closure.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Code for type-checking closure expressions.
  12
  13 use super::{check_fn, Expectation, FnCtxt};
  14
  15 use astconv;
  16 use middle::region;
  17 use middle::subst;
  18 use middle::ty::{self, ToPolyTraitRef, Ty};
  19 use std::cmp;
  20 use syntax::abi;
  21 use syntax::ast;
  22 use syntax::ast_util;
  23
  24 pub fn check_expr_closure<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
  25                                    expr: &ast::Expr,
  26                                    _capture: ast::CaptureClause,
  27                                    decl: &'tcx ast::FnDecl,
  28                                    body: &'tcx ast::Block,
  29                                    expected: Expectation<'tcx>) {
  30     debug!("check_expr_closure(expr={:?},expected={:?})",
  31            expr,
  32            expected);
  33
  34     // It's always helpful for inference if we know the kind of
  35     // closure sooner rather than later, so first examine the expected
  36     // type, and see if can glean a closure kind from there.
  37     let (expected_sig,expected_kind) = match expected.to_option(fcx) {
  38         Some(ty) => deduce_expectations_from_expected_type(fcx, ty),
  39         None => (None, None)
  40     };
  41     check_closure(fcx, expr, expected_kind, decl, body, expected_sig)
  42 }
  43
  44 fn check_closure<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
  45                           expr: &ast::Expr,
  46                           opt_kind: Option<ty::ClosureKind>,
  47                           decl: &'tcx ast::FnDecl,
  48                           body: &'tcx ast::Block,
  49                           expected_sig: Option<ty::FnSig<'tcx>>) {
  50     let expr_def_id = ast_util::local_def(expr.id);
  51
  52     debug!("check_closure opt_kind={:?} expected_sig={:?}",
  53            opt_kind,
  54            expected_sig);
  55
  56     let mut fn_ty = astconv::ty_of_closure(fcx,
  57                                            ast::Unsafety::Normal,
  58                                            decl,
  59                                            abi::RustCall,
  60                                            expected_sig);
  61
  62     // Create type variables (for now) to represent the transformed
  63     // types of upvars. These will be unified during the upvar
  64     // inference phase (`upvar.rs`).
  65     let num_upvars = fcx.tcx().with_freevars(expr.id, |fv| fv.len());
  66     let upvar_tys = fcx.infcx().next_ty_vars(num_upvars);
  67
  68     debug!("check_closure: expr.id={:?} upvar_tys={:?}",
  69            expr.id, upvar_tys);
  70
  71     let closure_type =
  72         fcx.ccx.tcx.mk_closure(
  73             expr_def_id,
  74             fcx.ccx.tcx.mk_substs(fcx.inh.infcx.parameter_environment.free_substs.clone()),
  75             upvar_tys);
  76
  77     fcx.write_ty(expr.id, closure_type);
  78
  79     let fn_sig = fcx.tcx().liberate_late_bound_regions(
  80         region::DestructionScopeData::new(body.id), &fn_ty.sig);
  81
  82     check_fn(fcx.ccx,
  83              ast::Unsafety::Normal,
  84              expr.id,
  85              &fn_sig,
  86              decl,
  87              expr.id,
  88              &*body,
  89              fcx.inh);
  90
  91     // Tuple up the arguments and insert the resulting function type into
  92     // the `closures` table.
  93     fn_ty.sig.0.inputs = vec![fcx.tcx().mk_tup(fn_ty.sig.0.inputs)];
  94
  95     debug!("closure for {:?} --> sig={:?} opt_kind={:?}",
  96            expr_def_id,
  97            fn_ty.sig,
  98            opt_kind);
  99
 100     fcx.inh.tables.borrow_mut().closure_tys.insert(expr_def_id, fn_ty);
 101     match opt_kind {
 102         Some(kind) => { fcx.inh.tables.borrow_mut().closure_kinds.insert(expr_def_id, kind); }
 103         None => { }
 104     }
 105 }
 106
 107 fn deduce_expectations_from_expected_type<'a,'tcx>(
 108     fcx: &FnCtxt<'a,'tcx>,
 109     expected_ty: Ty<'tcx>)
 110     -> (Option<ty::FnSig<'tcx>>,Option<ty::ClosureKind>)
 111 {
 112     debug!("deduce_expectations_from_expected_type(expected_ty={:?})",
 113            expected_ty);
 114
 115     match expected_ty.sty {
 116         ty::TyTrait(ref object_type) => {
 117             let proj_bounds = object_type.projection_bounds_with_self_ty(fcx.tcx(),
 118                                                                          fcx.tcx().types.err);
 119             let sig = proj_bounds.iter()
 120                                  .filter_map(|pb| deduce_sig_from_projection(fcx, pb))
 121                                  .next();
 122             let kind = fcx.tcx().lang_items.fn_trait_kind(object_type.principal_def_id());
 123             (sig, kind)
 124         }
 125         ty::TyInfer(ty::TyVar(vid)) => {
 126             deduce_expectations_from_obligations(fcx, vid)
 127         }
 128         _ => {
 129             (None, None)
 130         }
 131     }
 132 }
 133
 134 fn deduce_expectations_from_obligations<'a,'tcx>(
 135     fcx: &FnCtxt<'a,'tcx>,
 136     expected_vid: ty::TyVid)
 137     -> (Option<ty::FnSig<'tcx>>, Option<ty::ClosureKind>)
 138 {
 139     let fulfillment_cx = fcx.inh.infcx.fulfillment_cx.borrow();
 140     // Here `expected_ty` is known to be a type inference variable.
 141
 142     let expected_sig =
 143         fulfillment_cx
 144         .pending_obligations()
 145         .iter()
 146         .filter_map(|obligation| {
 147             debug!("deduce_expectations_from_obligations: obligation.predicate={:?}",
 148                    obligation.predicate);
 149
 150             match obligation.predicate {
 151                 // Given a Projection predicate, we can potentially infer
 152                 // the complete signature.
 153                 ty::Predicate::Projection(ref proj_predicate) => {
 154                     let trait_ref = proj_predicate.to_poly_trait_ref();
 155                     self_type_matches_expected_vid(fcx, trait_ref, expected_vid)
 156                         .and_then(|_| deduce_sig_from_projection(fcx, proj_predicate))
 157                 }
 158                 _ => {
 159                     None
 160                 }
 161             }
 162         })
 163         .next();
 164
 165     // Even if we can't infer the full signature, we may be able to
 166     // infer the kind. This can occur if there is a trait-reference
 167     // like `F : Fn<A>`. Note that due to subtyping we could encounter
 168     // many viable options, so pick the most restrictive.
 169     let expected_kind =
 170         fulfillment_cx
 171         .pending_obligations()
 172         .iter()
 173         .filter_map(|obligation| {
 174             let opt_trait_ref = match obligation.predicate {
 175                 ty::Predicate::Projection(ref data) => Some(data.to_poly_trait_ref()),
 176                 ty::Predicate::Trait(ref data) => Some(data.to_poly_trait_ref()),
 177                 ty::Predicate::Equate(..) => None,
 178                 ty::Predicate::RegionOutlives(..) => None,
 179                 ty::Predicate::TypeOutlives(..) => None,
 180                 ty::Predicate::WellFormed(..) => None,
 181                 ty::Predicate::ObjectSafe(..) => None,
 182             };
 183             opt_trait_ref
 184                 .and_then(|trait_ref| self_type_matches_expected_vid(fcx, trait_ref, expected_vid))
 185                 .and_then(|trait_ref| fcx.tcx().lang_items.fn_trait_kind(trait_ref.def_id()))
 186         })
 187         .fold(None, pick_most_restrictive_closure_kind);
 188
 189     (expected_sig, expected_kind)
 190 }
 191
 192 fn pick_most_restrictive_closure_kind(best: Option<ty::ClosureKind>,
 193                                       cur: ty::ClosureKind)
 194                                       -> Option<ty::ClosureKind>
 195 {
 196     match best {
 197         None => Some(cur),
 198         Some(best) => Some(cmp::min(best, cur))
 199     }
 200 }
 201
 202 /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 203 /// everything we need to know about a closure.
 204 fn deduce_sig_from_projection<'a,'tcx>(
 205     fcx: &FnCtxt<'a,'tcx>,
 206     projection: &ty::PolyProjectionPredicate<'tcx>)
 207     -> Option<ty::FnSig<'tcx>>
 208 {
 209     let tcx = fcx.tcx();
 210
 211     debug!("deduce_sig_from_projection({:?})",
 212            projection);
 213
 214     let trait_ref = projection.to_poly_trait_ref();
 215
 216     if tcx.lang_items.fn_trait_kind(trait_ref.def_id()).is_none() {
 217         return None;
 218     }
 219
 220     let arg_param_ty = *trait_ref.substs().types.get(subst::TypeSpace, 0);
 221     let arg_param_ty = fcx.infcx().resolve_type_vars_if_possible(&arg_param_ty);
 222     debug!("deduce_sig_from_projection: arg_param_ty {:?}", arg_param_ty);
 223
 224     let input_tys = match arg_param_ty.sty {
 225         ty::TyTuple(ref tys) => { (*tys).clone() }
 226         _ => { return None; }
 227     };
 228     debug!("deduce_sig_from_projection: input_tys {:?}", input_tys);
 229
 230     let ret_param_ty = projection.0.ty;
 231     let ret_param_ty = fcx.infcx().resolve_type_vars_if_possible(&ret_param_ty);
 232     debug!("deduce_sig_from_projection: ret_param_ty {:?}", ret_param_ty);
 233
 234     let fn_sig = ty::FnSig {
 235         inputs: input_tys,
 236         output: ty::FnConverging(ret_param_ty),
 237         variadic: false
 238     };
 239     debug!("deduce_sig_from_projection: fn_sig {:?}", fn_sig);
 240
 241     Some(fn_sig)
 242 }
 243
 244 fn self_type_matches_expected_vid<'a,'tcx>(
 245     fcx: &FnCtxt<'a,'tcx>,
 246     trait_ref: ty::PolyTraitRef<'tcx>,
 247     expected_vid: ty::TyVid)
 248     -> Option<ty::PolyTraitRef<'tcx>>
 249 {
 250     let self_ty = fcx.infcx().shallow_resolve(trait_ref.self_ty());
 251     debug!("self_type_matches_expected_vid(trait_ref={:?}, self_ty={:?})",
 252            trait_ref,
 253            self_ty);
 254     match self_ty.sty {
 255         ty::TyInfer(ty::TyVar(v)) if expected_vid == v => Some(trait_ref),
 256         _ => None,
 257     }
 258 }