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     include_raw_pointers: bool,
  41     span: Span,
  42 }
  43
  44 impl<'a, 'gcx, 'tcx> Iterator for Autoderef<'a, 'gcx, 'tcx> {
  45     type Item = (Ty<'tcx>, usize);
  46
  47     fn next(&mut self) -> Option<Self::Item> {
  48         let tcx = self.fcx.tcx;
  49
  50         debug!("autoderef: steps={:?}, cur_ty={:?}",
  51                self.steps,
  52                self.cur_ty);
  53         if self.at_start {
  54             self.at_start = false;
  55             debug!("autoderef stage #0 is {:?}", self.cur_ty);
  56             return Some((self.cur_ty, 0));
  57         }
  58
  59         if self.steps.len() == tcx.sess.recursion_limit.get() {
  60             // We've reached the recursion limit, error gracefully.
  61             let suggested_limit = tcx.sess.recursion_limit.get() * 2;
  62             struct_span_err!(tcx.sess,
  63                              self.span,
  64                              E0055,
  65                              "reached the recursion limit while auto-dereferencing {:?}",
  66                              self.cur_ty)
  67                 .span_label(self.span, "deref recursion limit reached")
  68                 .help(&format!(
  69                         "consider adding a `#[recursion_limit=\"{}\"]` attribute to your crate",
  70                         suggested_limit))
  71                 .emit();
  72             return None;
  73         }
  74
  75         if self.cur_ty.is_ty_var() {
  76             return None;
  77         }
  78
  79         // Otherwise, deref if type is derefable:
  80         let (kind, new_ty) =
  81             if let Some(mt) = self.cur_ty.builtin_deref(self.include_raw_pointers, NoPreference) {
  82                 (AutoderefKind::Builtin, mt.ty)
  83             } else {
  84                 let ty = self.overloaded_deref_ty(self.cur_ty)?;
  85                 (AutoderefKind::Overloaded, ty)
  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: tcx.lang_items().deref_trait()?,
 112             substs: tcx.mk_substs_trait(self.cur_ty, &[]),
 113         };
 114
 115         let cause = traits::ObligationCause::misc(self.span, self.fcx.body_id);
 116
 117         let mut selcx = traits::SelectionContext::new(self.fcx);
 118         let obligation = traits::Obligation::new(cause.clone(),
 119                                                  self.fcx.param_env,
 120                                                  trait_ref.to_predicate());
 121         if !selcx.evaluate_obligation(&obligation) {
 122             debug!("overloaded_deref_ty: cannot match obligation");
 123             return None;
 124         }
 125
 126         let normalized = traits::normalize_projection_type(&mut selcx,
 127                                                            self.fcx.param_env,
 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     /// also dereference through raw pointer types
 200     /// e.g. assuming ptr_to_Foo is the type `*const Foo`
 201     /// fcx.autoderef(span, ptr_to_Foo)  => [*const Foo]
 202     /// fcx.autoderef(span, ptr_to_Foo).include_raw_ptrs() => [*const Foo, Foo]
 203     pub fn include_raw_pointers(mut self) -> Self {
 204         self.include_raw_pointers = true;
 205         self
 206     }
 207
 208     pub fn finalize(self) {
 209         let fcx = self.fcx;
 210         fcx.register_predicates(self.into_obligations());
 211     }
 212
 213     pub fn into_obligations(self) -> Vec<traits::PredicateObligation<'tcx>> {
 214         self.obligations
 215     }
 216 }
 217
 218 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 219     pub fn autoderef(&'a self, span: Span, base_ty: Ty<'tcx>) -> Autoderef<'a, 'gcx, 'tcx> {
 220         Autoderef {
 221             fcx: self,
 222             steps: vec![],
 223             cur_ty: self.resolve_type_vars_if_possible(&base_ty),
 224             obligations: vec![],
 225             at_start: true,
 226             include_raw_pointers: false,
 227             span,
 228         }
 229     }
 230
 231     pub fn try_overloaded_deref(&self,
 232                                 span: Span,
 233                                 base_ty: Ty<'tcx>,
 234                                 pref: LvaluePreference)
 235                                 -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 236         self.try_overloaded_lvalue_op(span, base_ty, &[], pref, LvalueOp::Deref)
 237     }
 238 }