compiler/rustc_trait_selection/src/traits/codegen.rs

   1 // This file contains various trait resolution methods used by codegen.
   2 // They all assume regions can be erased and monomorphic types.  It
   3 // seems likely that they should eventually be merged into more
   4 // general routines.
   5
   6 use crate::infer::{InferCtxt, TyCtxtInferExt};
   7 use crate::traits::{
   8     FulfillmentContext, ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine,
   9     Unimplemented,
  10 };
  11 use rustc_errors::ErrorReported;
  12 use rustc_middle::ty::fold::TypeFoldable;
  13 use rustc_middle::ty::{self, TyCtxt};
  14
  15 /// Attempts to resolve an obligation to an `ImplSource`. The result is
  16 /// a shallow `ImplSource` resolution, meaning that we do not
  17 /// (necessarily) resolve all nested obligations on the impl. Note
  18 /// that type check should guarantee to us that all nested
  19 /// obligations *could be* resolved if we wanted to.
  20 ///
  21 /// Assumes that this is run after the entire crate has been successfully type-checked.
  22 /// This also expects that `trait_ref` is fully normalized.
  23 pub fn codegen_fulfill_obligation<'tcx>(
  24     tcx: TyCtxt<'tcx>,
  25     (param_env, trait_ref): (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>),
  26 ) -> Result<ImplSource<'tcx, ()>, ErrorReported> {
  27     // Remove any references to regions; this helps improve caching.
  28     let trait_ref = tcx.erase_regions(trait_ref);
  29     // We expect the input to be fully normalized.
  30     debug_assert_eq!(trait_ref, tcx.normalize_erasing_regions(param_env, trait_ref));
  31     debug!(
  32         "codegen_fulfill_obligation(trait_ref={:?}, def_id={:?})",
  33         (param_env, trait_ref),
  34         trait_ref.def_id()
  35     );
  36
  37     // Do the initial selection for the obligation. This yields the
  38     // shallow result we are looking for -- that is, what specific impl.
  39     tcx.infer_ctxt().enter(|infcx| {
  40         let mut selcx = SelectionContext::new(&infcx);
  41
  42         let obligation_cause = ObligationCause::dummy();
  43         let obligation =
  44             Obligation::new(obligation_cause, param_env, trait_ref.to_poly_trait_predicate());
  45
  46         let selection = match selcx.select(&obligation) {
  47             Ok(Some(selection)) => selection,
  48             Ok(None) => {
  49                 // Ambiguity can happen when monomorphizing during trans
  50                 // expands to some humongo type that never occurred
  51                 // statically -- this humongo type can then overflow,
  52                 // leading to an ambiguous result. So report this as an
  53                 // overflow bug, since I believe this is the only case
  54                 // where ambiguity can result.
  55                 infcx.tcx.sess.delay_span_bug(
  56                     rustc_span::DUMMY_SP,
  57                     &format!(
  58                         "encountered ambiguity selecting `{:?}` during codegen, presuming due to \
  59                          overflow or prior type error",
  60                         trait_ref
  61                     ),
  62                 );
  63                 return Err(ErrorReported);
  64             }
  65             Err(Unimplemented) => {
  66                 // This can trigger when we probe for the source of a `'static` lifetime requirement
  67                 // on a trait object: `impl Foo for dyn Trait {}` has an implicit `'static` bound.
  68                 infcx.tcx.sess.delay_span_bug(
  69                     rustc_span::DUMMY_SP,
  70                     &format!(
  71                         "Encountered error `Unimplemented` selecting `{:?}` during codegen",
  72                         trait_ref
  73                     ),
  74                 );
  75                 return Err(ErrorReported);
  76             }
  77             Err(e) => {
  78                 bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
  79             }
  80         };
  81
  82         debug!("fulfill_obligation: selection={:?}", selection);
  83
  84         // Currently, we use a fulfillment context to completely resolve
  85         // all nested obligations. This is because they can inform the
  86         // inference of the impl's type parameters.
  87         let mut fulfill_cx = FulfillmentContext::new();
  88         let impl_source = selection.map(|predicate| {
  89             debug!("fulfill_obligation: register_predicate_obligation {:?}", predicate);
  90             fulfill_cx.register_predicate_obligation(&infcx, predicate);
  91         });
  92         let impl_source = drain_fulfillment_cx_or_panic(&infcx, &mut fulfill_cx, impl_source);
  93
  94         debug!("Cache miss: {:?} => {:?}", trait_ref, impl_source);
  95         Ok(impl_source)
  96     })
  97 }
  98
  99 // # Global Cache
 100
 101 /// Finishes processes any obligations that remain in the
 102 /// fulfillment context, and then returns the result with all type
 103 /// variables removed and regions erased. Because this is intended
 104 /// for use after type-check has completed, if any errors occur,
 105 /// it will panic. It is used during normalization and other cases
 106 /// where processing the obligations in `fulfill_cx` may cause
 107 /// type inference variables that appear in `result` to be
 108 /// unified, and hence we need to process those obligations to get
 109 /// the complete picture of the type.
 110 fn drain_fulfillment_cx_or_panic<T>(
 111     infcx: &InferCtxt<'_, 'tcx>,
 112     fulfill_cx: &mut FulfillmentContext<'tcx>,
 113     result: T,
 114 ) -> T
 115 where
 116     T: TypeFoldable<'tcx>,
 117 {
 118     debug!("drain_fulfillment_cx_or_panic()");
 119
 120     // In principle, we only need to do this so long as `result`
 121     // contains unbound type parameters. It could be a slight
 122     // optimization to stop iterating early.
 123     if let Err(errors) = fulfill_cx.select_all_or_error(infcx) {
 124         infcx.tcx.sess.delay_span_bug(
 125             rustc_span::DUMMY_SP,
 126             &format!("Encountered errors `{:?}` resolving bounds after type-checking", errors),
 127         );
 128     }
 129
 130     let result = infcx.resolve_vars_if_possible(result);
 131     infcx.tcx.erase_regions(result)
 132 }