src/librustc_trait_selection/traits/query/outlives_bounds.rs

   1 use crate::infer::canonical::OriginalQueryValues;
   2 use crate::infer::InferCtxt;
   3 use crate::traits::query::NoSolution;
   4 use crate::traits::{FulfillmentContext, ObligationCause, TraitEngine};
   5 use rustc_hir as hir;
   6 use rustc_infer::traits::TraitEngineExt as _;
   7 use rustc_middle::ty::{self, Ty};
   8 use rustc_span::source_map::Span;
   9
  10 pub use rustc_middle::traits::query::OutlivesBound;
  11
  12 pub trait InferCtxtExt<'tcx> {
  13     fn implied_outlives_bounds(
  14         &self,
  15         param_env: ty::ParamEnv<'tcx>,
  16         body_id: hir::HirId,
  17         ty: Ty<'tcx>,
  18         span: Span,
  19     ) -> Vec<OutlivesBound<'tcx>>;
  20 }
  21
  22 impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
  23     /// Implied bounds are region relationships that we deduce
  24     /// automatically. The idea is that (e.g.) a caller must check that a
  25     /// function's argument types are well-formed immediately before
  26     /// calling that fn, and hence the *callee* can assume that its
  27     /// argument types are well-formed. This may imply certain relationships
  28     /// between generic parameters. For example:
  29     ///
  30     ///     fn foo<'a,T>(x: &'a T)
  31     ///
  32     /// can only be called with a `'a` and `T` such that `&'a T` is WF.
  33     /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
  34     ///
  35     /// # Parameters
  36     ///
  37     /// - `param_env`, the where-clauses in scope
  38     /// - `body_id`, the body-id to use when normalizing assoc types.
  39     ///   Note that this may cause outlives obligations to be injected
  40     ///   into the inference context with this body-id.
  41     /// - `ty`, the type that we are supposed to assume is WF.
  42     /// - `span`, a span to use when normalizing, hopefully not important,
  43     ///   might be useful if a `bug!` occurs.
  44     fn implied_outlives_bounds(
  45         &self,
  46         param_env: ty::ParamEnv<'tcx>,
  47         body_id: hir::HirId,
  48         ty: Ty<'tcx>,
  49         span: Span,
  50     ) -> Vec<OutlivesBound<'tcx>> {
  51         debug!("implied_outlives_bounds(ty = {:?})", ty);
  52
  53         let mut orig_values = OriginalQueryValues::default();
  54         let key = self.canonicalize_query(&param_env.and(ty), &mut orig_values);
  55         let result = match self.tcx.implied_outlives_bounds(key) {
  56             Ok(r) => r,
  57             Err(NoSolution) => {
  58                 self.tcx.sess.delay_span_bug(
  59                     span,
  60                     "implied_outlives_bounds failed to solve all obligations",
  61                 );
  62                 return vec![];
  63             }
  64         };
  65         assert!(result.value.is_proven());
  66
  67         let result = self.instantiate_query_response_and_region_obligations(
  68             &ObligationCause::misc(span, body_id),
  69             param_env,
  70             &orig_values,
  71             &result,
  72         );
  73         debug!("implied_outlives_bounds for {:?}: {:#?}", ty, result);
  74         let result = match result {
  75             Ok(v) => v,
  76             Err(_) => {
  77                 self.tcx.sess.delay_span_bug(span, "implied_outlives_bounds failed to instantiate");
  78                 return vec![];
  79             }
  80         };
  81
  82         // Instantiation may have produced new inference variables and constraints on those
  83         // variables. Process these constraints.
  84         let mut fulfill_cx = FulfillmentContext::new();
  85         fulfill_cx.register_predicate_obligations(self, result.obligations);
  86         if fulfill_cx.select_all_or_error(self).is_err() {
  87             self.tcx.sess.delay_span_bug(
  88                 span,
  89                 "implied_outlives_bounds failed to solve obligations from instantiation",
  90             );
  91         }
  92
  93         result.value
  94     }
  95 }