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
  15 use check::coercion::AsCoercionSite;
  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::{MethodCall, MethodCallee};
  21 use rustc::ty::{LvaluePreference, NoPreference};
  22 use rustc::hir;
  23
  24 use syntax_pos::Span;
  25 use syntax::symbol::Symbol;
  26
  27 #[derive(Copy, Clone, Debug)]
  28 enum AutoderefKind {
  29     Builtin,
  30     Overloaded,
  31 }
  32
  33 pub struct Autoderef<'a, 'gcx: 'tcx, 'tcx: 'a> {
  34     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
  35     steps: Vec<(Ty<'tcx>, AutoderefKind)>,
  36     cur_ty: Ty<'tcx>,
  37     obligations: Vec<traits::PredicateObligation<'tcx>>,
  38     at_start: bool,
  39     span: Span,
  40 }
  41
  42 impl<'a, 'gcx, 'tcx> Iterator for Autoderef<'a, 'gcx, 'tcx> {
  43     type Item = (Ty<'tcx>, usize);
  44
  45     fn next(&mut self) -> Option<Self::Item> {
  46         let tcx = self.fcx.tcx;
  47
  48         debug!("autoderef: steps={:?}, cur_ty={:?}",
  49                self.steps,
  50                self.cur_ty);
  51         if self.at_start {
  52             self.at_start = false;
  53             debug!("autoderef stage #0 is {:?}", self.cur_ty);
  54             return Some((self.cur_ty, 0));
  55         }
  56
  57         if self.steps.len() == tcx.sess.recursion_limit.get() {
  58             // We've reached the recursion limit, error gracefully.
  59             let suggested_limit = tcx.sess.recursion_limit.get() * 2;
  60             struct_span_err!(tcx.sess,
  61                              self.span,
  62                              E0055,
  63                              "reached the recursion limit while auto-dereferencing {:?}",
  64                              self.cur_ty)
  65                 .span_label(self.span, "deref recursion limit reached")
  66                 .help(&format!(
  67                         "consider adding a `#[recursion_limit=\"{}\"]` attribute to your crate",
  68                         suggested_limit))
  69                 .emit();
  70             return None;
  71         }
  72
  73         if self.cur_ty.is_ty_var() {
  74             return None;
  75         }
  76
  77         // Otherwise, deref if type is derefable:
  78         let (kind, new_ty) = if let Some(mt) = self.cur_ty.builtin_deref(false, NoPreference) {
  79             (AutoderefKind::Builtin, mt.ty)
  80         } else {
  81             match self.overloaded_deref_ty(self.cur_ty) {
  82                 Some(ty) => (AutoderefKind::Overloaded, ty),
  83                 _ => return None,
  84             }
  85         };
  86
  87         if new_ty.references_error() {
  88             return None;
  89         }
  90
  91         self.steps.push((self.cur_ty, kind));
  92         debug!("autoderef stage #{:?} is {:?} from {:?}",
  93                self.steps.len(),
  94                new_ty,
  95                (self.cur_ty, kind));
  96         self.cur_ty = new_ty;
  97
  98         Some((self.cur_ty, self.steps.len()))
  99     }
 100 }
 101
 102 impl<'a, 'gcx, 'tcx> Autoderef<'a, 'gcx, 'tcx> {
 103     fn overloaded_deref_ty(&mut self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 104         debug!("overloaded_deref_ty({:?})", ty);
 105
 106         let tcx = self.fcx.tcx();
 107
 108         // <cur_ty as Deref>
 109         let trait_ref = TraitRef {
 110             def_id: match tcx.lang_items.deref_trait() {
 111                 Some(f) => f,
 112                 None => return None,
 113             },
 114             substs: tcx.mk_substs_trait(self.cur_ty, &[]),
 115         };
 116
 117         let cause = traits::ObligationCause::misc(self.span, self.fcx.body_id);
 118
 119         let mut selcx = traits::SelectionContext::new(self.fcx);
 120         let obligation = traits::Obligation::new(cause.clone(), 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                                                            ty::ProjectionTy {
 128                                                                trait_ref: trait_ref,
 129                                                                item_name: Symbol::intern("Target"),
 130                                                            },
 131                                                            cause,
 132                                                            0);
 133
 134         debug!("overloaded_deref_ty({:?}) = {:?}", ty, normalized);
 135         self.obligations.extend(normalized.obligations);
 136
 137         Some(self.fcx.resolve_type_vars_if_possible(&normalized.value))
 138     }
 139
 140     /// Returns the final type, generating an error if it is an
 141     /// unresolved inference variable.
 142     pub fn unambiguous_final_ty(&self) -> Ty<'tcx> {
 143         self.fcx.structurally_resolved_type(self.span, self.cur_ty)
 144     }
 145
 146     /// Returns the final type we ended up with, which may well be an
 147     /// inference variable (we will resolve it first, if possible).
 148     pub fn maybe_ambiguous_final_ty(&self) -> Ty<'tcx> {
 149         self.fcx.resolve_type_vars_if_possible(&self.cur_ty)
 150     }
 151
 152     pub fn finalize(self, pref: LvaluePreference, expr: &hir::Expr) {
 153         let fcx = self.fcx;
 154         fcx.register_infer_ok_obligations(self.finalize_as_infer_ok(pref, &[expr]));
 155     }
 156
 157     pub fn finalize_as_infer_ok<E>(self, pref: LvaluePreference, exprs: &[E])
 158                                    -> InferOk<'tcx, ()>
 159         where E: AsCoercionSite
 160     {
 161         let Autoderef { fcx, span, mut obligations, steps, .. } = self;
 162         let methods: Vec<_> = steps
 163             .iter()
 164             .map(|&(ty, kind)| {
 165                 if let AutoderefKind::Overloaded = kind {
 166                     fcx.try_overloaded_deref(span, None, ty, pref)
 167                         .map(|InferOk { value, obligations: o }| {
 168                             obligations.extend(o);
 169                             value
 170                         })
 171                 } else {
 172                     None
 173                 }
 174             })
 175             .collect();
 176
 177         debug!("finalize({:?}) - {:?},{:?}",
 178                pref,
 179                methods,
 180                obligations);
 181
 182         for expr in exprs {
 183             let expr = expr.as_coercion_site();
 184             debug!("finalize - finalizing #{} - {:?}", expr.id, expr);
 185             for (n, method) in methods.iter().enumerate() {
 186                 if let &Some(method) = method {
 187                     let method_call = MethodCall::autoderef(expr.id, n as u32);
 188                     fcx.tables.borrow_mut().method_map.insert(method_call, method);
 189                 }
 190             }
 191         }
 192
 193         InferOk {
 194             value: (),
 195             obligations
 196         }
 197     }
 198 }
 199
 200 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 201     pub fn autoderef(&'a self, span: Span, base_ty: Ty<'tcx>) -> Autoderef<'a, 'gcx, 'tcx> {
 202         Autoderef {
 203             fcx: self,
 204             steps: vec![],
 205             cur_ty: self.resolve_type_vars_if_possible(&base_ty),
 206             obligations: vec![],
 207             at_start: true,
 208             span: span,
 209         }
 210     }
 211
 212     pub fn try_overloaded_deref(&self,
 213                                 span: Span,
 214                                 base_expr: Option<&hir::Expr>,
 215                                 base_ty: Ty<'tcx>,
 216                                 pref: LvaluePreference)
 217                                 -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 218         let rcvr = base_expr.map(|base_expr| super::AdjustedRcvr {
 219             rcvr_expr: base_expr, autoderefs: 0, unsize: false
 220         });
 221
 222         self.try_overloaded_lvalue_op(span, rcvr, base_ty, &[], pref, LvalueOp::Deref)
 223     }
 224 }