src/librustc_trait_selection/traits/query/evaluate_obligation.rs

   1 use crate::infer::canonical::OriginalQueryValues;
   2 use crate::infer::InferCtxt;
   3 use crate::traits::{
   4     EvaluationResult, OverflowError, PredicateObligation, SelectionContext, TraitQueryMode,
   5 };
   6
   7 pub trait InferCtxtExt<'tcx> {
   8     fn predicate_may_hold(&self, obligation: &PredicateObligation<'tcx>) -> bool;
   9
  10     fn predicate_must_hold_considering_regions(
  11         &self,
  12         obligation: &PredicateObligation<'tcx>,
  13     ) -> bool;
  14
  15     fn predicate_must_hold_modulo_regions(&self, obligation: &PredicateObligation<'tcx>) -> bool;
  16
  17     fn evaluate_obligation(
  18         &self,
  19         obligation: &PredicateObligation<'tcx>,
  20     ) -> Result<EvaluationResult, OverflowError>;
  21
  22     // Helper function that canonicalizes and runs the query. If an
  23     // overflow results, we re-run it in the local context so we can
  24     // report a nice error.
  25     /*crate*/
  26     fn evaluate_obligation_no_overflow(
  27         &self,
  28         obligation: &PredicateObligation<'tcx>,
  29     ) -> EvaluationResult;
  30 }
  31
  32 impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
  33     /// Evaluates whether the predicate can be satisfied (by any means)
  34     /// in the given `ParamEnv`.
  35     fn predicate_may_hold(&self, obligation: &PredicateObligation<'tcx>) -> bool {
  36         self.evaluate_obligation_no_overflow(obligation).may_apply()
  37     }
  38
  39     /// Evaluates whether the predicate can be satisfied in the given
  40     /// `ParamEnv`, and returns `false` if not certain. However, this is
  41     /// not entirely accurate if inference variables are involved.
  42     ///
  43     /// This version may conservatively fail when outlives obligations
  44     /// are required.
  45     fn predicate_must_hold_considering_regions(
  46         &self,
  47         obligation: &PredicateObligation<'tcx>,
  48     ) -> bool {
  49         self.evaluate_obligation_no_overflow(obligation).must_apply_considering_regions()
  50     }
  51
  52     /// Evaluates whether the predicate can be satisfied in the given
  53     /// `ParamEnv`, and returns `false` if not certain. However, this is
  54     /// not entirely accurate if inference variables are involved.
  55     ///
  56     /// This version ignores all outlives constraints.
  57     fn predicate_must_hold_modulo_regions(&self, obligation: &PredicateObligation<'tcx>) -> bool {
  58         self.evaluate_obligation_no_overflow(obligation).must_apply_modulo_regions()
  59     }
  60
  61     /// Evaluate a given predicate, capturing overflow and propagating it back.
  62     fn evaluate_obligation(
  63         &self,
  64         obligation: &PredicateObligation<'tcx>,
  65     ) -> Result<EvaluationResult, OverflowError> {
  66         let mut _orig_values = OriginalQueryValues::default();
  67         let c_pred = self
  68             .canonicalize_query(&obligation.param_env.and(obligation.predicate), &mut _orig_values);
  69         // Run canonical query. If overflow occurs, rerun from scratch but this time
  70         // in standard trait query mode so that overflow is handled appropriately
  71         // within `SelectionContext`.
  72         self.tcx.evaluate_obligation(c_pred)
  73     }
  74
  75     // Helper function that canonicalizes and runs the query. If an
  76     // overflow results, we re-run it in the local context so we can
  77     // report a nice error.
  78     fn evaluate_obligation_no_overflow(
  79         &self,
  80         obligation: &PredicateObligation<'tcx>,
  81     ) -> EvaluationResult {
  82         match self.evaluate_obligation(obligation) {
  83             Ok(result) => result,
  84             Err(OverflowError) => {
  85                 let mut selcx = SelectionContext::with_query_mode(&self, TraitQueryMode::Standard);
  86                 selcx.evaluate_root_obligation(obligation).unwrap_or_else(|r| {
  87                     span_bug!(
  88                         obligation.cause.span,
  89                         "Overflow should be caught earlier in standard query mode: {:?}, {:?}",
  90                         obligation,
  91                         r,
  92                     )
  93                 })
  94             }
  95         }
  96     }
  97 }