src/librustc_typeck/constrained_type_params.rs

   1 // Copyright 2015 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 middle::ty::{self};
  12
  13 use std::collections::HashSet;
  14 use std::rc::Rc;
  15
  16 pub fn identify_constrained_type_params<'tcx>(_tcx: &ty::ctxt<'tcx>,
  17                                               predicates: &[ty::Predicate<'tcx>],
  18                                               impl_trait_ref: Option<Rc<ty::TraitRef<'tcx>>>,
  19                                               input_parameters: &mut HashSet<ty::ParamTy>)
  20 {
  21     loop {
  22         let num_inputs = input_parameters.len();
  23
  24         let projection_predicates =
  25             predicates.iter()
  26                       .filter_map(|predicate| {
  27                           match *predicate {
  28                               // Ignore higher-ranked binders. For the purposes
  29                               // of this check, they don't matter because they
  30                               // only affect named regions, and we're just
  31                               // concerned about type parameters here.
  32                               ty::Predicate::Projection(ref data) => Some(data.0.clone()),
  33                               _ => None,
  34                           }
  35                       });
  36
  37         for projection in projection_predicates {
  38             // Special case: watch out for some kind of sneaky attempt
  39             // to project out an associated type defined by this very trait.
  40             if Some(projection.projection_ty.trait_ref.clone()) == impl_trait_ref {
  41                 continue;
  42             }
  43
  44             let relies_only_on_inputs =
  45                 projection.projection_ty.trait_ref.input_types()
  46                                                   .iter()
  47                                                   .flat_map(|t| t.walk())
  48                                                   .filter_map(|t| t.as_opt_param_ty())
  49                                                   .all(|t| input_parameters.contains(&t));
  50
  51             if relies_only_on_inputs {
  52                 input_parameters.extend(
  53                     projection.ty.walk().filter_map(|t| t.as_opt_param_ty()));
  54             }
  55         }
  56
  57         if input_parameters.len() == num_inputs {
  58             break;
  59         }
  60     }
  61 }