src/librustc_typeck/check/autoderef.rs

   1 // Copyright 2016 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 use astconv::AstConv;
  12
  13 use super::{FnCtxt, LvalueOp};
  14 use super::method::MethodCallee;
  15
  16 use rustc::infer::InferOk;
  17 use rustc::traits;
  18 use rustc::ty::{self, Ty, TraitRef};
  19 use rustc::ty::{ToPredicate, TypeFoldable};
  20 use rustc::ty::{LvaluePreference, NoPreference};
  21 use rustc::ty::adjustment::{Adjustment, Adjust, OverloadedDeref};
  22
  23 use syntax_pos::Span;
  24 use syntax::symbol::Symbol;
  25
  26 use std::iter;
  27
  28 #[derive(Copy, Clone, Debug)]
  29 enum AutoderefKind {
  30     Builtin,
  31     Overloaded,
  32 }
  33
  34 pub struct Autoderef<'a, 'gcx: 'tcx, 'tcx: 'a> {
  35     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
  36     steps: Vec<(Ty<'tcx>, AutoderefKind)>,
  37     cur_ty: Ty<'tcx>,
  38     obligations: Vec<traits::PredicateObligation<'tcx>>,
  39     at_start: bool,
  40     span: Span,
  41 }
  42
  43 impl<'a, 'gcx, 'tcx> Iterator for Autoderef<'a, 'gcx, 'tcx> {
  44     type Item = (Ty<'tcx>, usize);
  45
  46     fn next(&mut self) -> Option<Self::Item> {
  47         let tcx = self.fcx.tcx;
  48
  49         debug!("autoderef: steps={:?}, cur_ty={:?}",
  50                self.steps,
  51                self.cur_ty);
  52         if self.at_start {
  53             self.at_start = false;
  54             debug!("autoderef stage #0 is {:?}", self.cur_ty);
  55             return Some((self.cur_ty, 0));
  56         }
  57
  58         if self.steps.len() == tcx.sess.recursion_limit.get() {
  59             // We've reached the recursion limit, error gracefully.
  60             let suggested_limit = tcx.sess.recursion_limit.get() * 2;
  61             struct_span_err!(tcx.sess,
  62                              self.span,
  63                              E0055,
  64                              "reached the recursion limit while auto-dereferencing {:?}",
  65                              self.cur_ty)
  66                 .span_label(self.span, "deref recursion limit reached")
  67                 .help(&format!(
  68                         "consider adding a `#[recursion_limit=\"{}\"]` attribute to your crate",
  69                         suggested_limit))
  70                 .emit();
  71             return None;
  72         }
  73
  74         if self.cur_ty.is_ty_var() {
  75             return None;
  76         }
  77
  78         // Otherwise, deref if type is derefable:
  79         let (kind, new_ty) = if let Some(mt) = self.cur_ty.builtin_deref(false, NoPreference) {
  80             (AutoderefKind::Builtin, mt.ty)
  81         } else {
  82             match self.overloaded_deref_ty(self.cur_ty) {
  83                 Some(ty) => (AutoderefKind::Overloaded, ty),
  84                 _ => return None,
  85             }
  86         };
  87
  88         if new_ty.references_error() {
  89             return None;
  90         }
  91
  92         self.steps.push((self.cur_ty, kind));
  93         debug!("autoderef stage #{:?} is {:?} from {:?}",
  94                self.steps.len(),
  95                new_ty,
  96                (self.cur_ty, kind));
  97         self.cur_ty = new_ty;
  98
  99         Some((self.cur_ty, self.steps.len()))
 100     }
 101 }
 102
 103 impl<'a, 'gcx, 'tcx> Autoderef<'a, 'gcx, 'tcx> {
 104     fn overloaded_deref_ty(&mut self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 105         debug!("overloaded_deref_ty({:?})", ty);
 106
 107         let tcx = self.fcx.tcx();
 108
 109         // <cur_ty as Deref>
 110         let trait_ref = TraitRef {
 111             def_id: match tcx.lang_items.deref_trait() {
 112                 Some(f) => f,
 113                 None => return None,
 114             },
 115             substs: tcx.mk_substs_trait(self.cur_ty, &[]),
 116         };
 117
 118         let cause = traits::ObligationCause::misc(self.span, self.fcx.body_id);
 119
 120         let mut selcx = traits::SelectionContext::new(self.fcx);
 121         let obligation = traits::Obligation::new(cause.clone(), trait_ref.to_predicate());
 122         if !selcx.evaluate_obligation(&obligation) {
 123             debug!("overloaded_deref_ty: cannot match obligation");
 124             return None;
 125         }
 126
 127         let normalized = traits::normalize_projection_type(&mut selcx,
 128                                                            ty::ProjectionTy::from_ref_and_name(
 129                                                                tcx,
 130                                                                trait_ref,
 131                                                                Symbol::intern("Target"),
 132                                                            ),
 133                                                            cause,
 134                                                            0);
 135
 136         debug!("overloaded_deref_ty({:?}) = {:?}", ty, normalized);
 137         self.obligations.extend(normalized.obligations);
 138
 139         Some(self.fcx.resolve_type_vars_if_possible(&normalized.value))
 140     }
 141
 142     /// Returns the final type, generating an error if it is an
 143     /// unresolved inference variable.
 144     pub fn unambiguous_final_ty(&self) -> Ty<'tcx> {
 145         self.fcx.structurally_resolved_type(self.span, self.cur_ty)
 146     }
 147
 148     /// Returns the final type we ended up with, which may well be an
 149     /// inference variable (we will resolve it first, if possible).
 150     pub fn maybe_ambiguous_final_ty(&self) -> Ty<'tcx> {
 151         self.fcx.resolve_type_vars_if_possible(&self.cur_ty)
 152     }
 153
 154     pub fn step_count(&self) -> usize {
 155         self.steps.len()
 156     }
 157
 158     /// Returns the adjustment steps.
 159     pub fn adjust_steps(&self, pref: LvaluePreference)
 160                         -> Vec<Adjustment<'tcx>> {
 161         self.fcx.register_infer_ok_obligations(self.adjust_steps_as_infer_ok(pref))
 162     }
 163
 164     pub fn adjust_steps_as_infer_ok(&self, pref: LvaluePreference)
 165                                     -> InferOk<'tcx, Vec<Adjustment<'tcx>>> {
 166         let mut obligations = vec![];
 167         let targets = self.steps.iter().skip(1).map(|&(ty, _)| ty)
 168             .chain(iter::once(self.cur_ty));
 169         let steps: Vec<_> = self.steps.iter().map(|&(source, kind)| {
 170             if let AutoderefKind::Overloaded = kind {
 171                 self.fcx.try_overloaded_deref(self.span, source, pref)
 172                     .and_then(|InferOk { value: method, obligations: o }| {
 173                         obligations.extend(o);
 174                         if let ty::TyRef(region, mt) = method.sig.output().sty {
 175                             Some(OverloadedDeref {
 176                                 region,
 177                                 mutbl: mt.mutbl,
 178                             })
 179                         } else {
 180                             None
 181                         }
 182                     })
 183             } else {
 184                 None
 185             }
 186         }).zip(targets).map(|(autoderef, target)| {
 187             Adjustment {
 188                 kind: Adjust::Deref(autoderef),
 189                 target
 190             }
 191         }).collect();
 192
 193         InferOk {
 194             obligations,
 195             value: steps
 196         }
 197     }
 198
 199     pub fn finalize(self) {
 200         let fcx = self.fcx;
 201         fcx.register_predicates(self.into_obligations());
 202     }
 203
 204     pub fn into_obligations(self) -> Vec<traits::PredicateObligation<'tcx>> {
 205         self.obligations
 206     }
 207 }
 208
 209 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 210     pub fn autoderef(&'a self, span: Span, base_ty: Ty<'tcx>) -> Autoderef<'a, 'gcx, 'tcx> {
 211         Autoderef {
 212             fcx: self,
 213             steps: vec![],
 214             cur_ty: self.resolve_type_vars_if_possible(&base_ty),
 215             obligations: vec![],
 216             at_start: true,
 217             span: span,
 218         }
 219     }
 220
 221     pub fn try_overloaded_deref(&self,
 222                                 span: Span,
 223                                 base_ty: Ty<'tcx>,
 224                                 pref: LvaluePreference)
 225                                 -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 226         self.try_overloaded_lvalue_op(span, base_ty, &[], pref, LvalueOp::Deref)
 227     }
 228 }