src/librustc/traits/query/evaluate_obligation.rs

   1 // Copyright 2018 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 infer::InferCtxt;
  12 use rustc_data_structures::small_vec::SmallVec;
  13 use traits::{EvaluationResult, PredicateObligation, SelectionContext,
  14              TraitQueryMode, OverflowError};
  15
  16 impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
  17     /// Evaluates whether the predicate can be satisfied (by any means)
  18     /// in the given `ParamEnv`.
  19     pub fn predicate_may_hold(
  20         &self,
  21         obligation: &PredicateObligation<'tcx>,
  22     ) -> bool {
  23         self.evaluate_obligation(obligation).may_apply()
  24     }
  25
  26     /// Evaluates whether the predicate can be satisfied in the given
  27     /// `ParamEnv`, and returns `false` if not certain. However, this is
  28     /// not entirely accurate if inference variables are involved.
  29     pub fn predicate_must_hold(
  30         &self,
  31         obligation: &PredicateObligation<'tcx>,
  32     ) -> bool {
  33         self.evaluate_obligation(obligation) == EvaluationResult::EvaluatedToOk
  34     }
  35
  36     // Helper function that canonicalizes and runs the query, as well as handles
  37     // overflow.
  38     fn evaluate_obligation(
  39         &self,
  40         obligation: &PredicateObligation<'tcx>,
  41     ) -> EvaluationResult {
  42         let mut _orig_values = SmallVec::new();
  43         let c_pred = self.canonicalize_query(&obligation.param_env.and(obligation.predicate),
  44                                              &mut _orig_values);
  45         // Run canonical query. If overflow occurs, rerun from scratch but this time
  46         // in standard trait query mode so that overflow is handled appropriately
  47         // within `SelectionContext`.
  48         match self.tcx.global_tcx().evaluate_obligation(c_pred) {
  49             Ok(result) => result,
  50             Err(OverflowError) => {
  51                 let mut selcx =
  52                     SelectionContext::with_query_mode(&self, TraitQueryMode::Standard);
  53                 selcx.evaluate_obligation_recursively(obligation)
  54                      .expect("Overflow should be caught earlier in standard query mode")
  55             }
  56         }
  57     }
  58 }