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
6 use crate::infer::{DefiningAnchor, TyCtxtInferExt};
7 use crate::traits::error_reporting::TypeErrCtxtExt;
9 ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine, TraitEngineExt,
12 use rustc_infer::traits::FulfillmentErrorCode;
13 use rustc_middle::traits::CodegenObligationError;
14 use rustc_middle::ty::{self, TyCtxt};
16 /// Attempts to resolve an obligation to an `ImplSource`. The result is
17 /// a shallow `ImplSource` resolution, meaning that we do not
18 /// (necessarily) resolve all nested obligations on the impl. Note
19 /// that type check should guarantee to us that all nested
20 /// obligations *could be* resolved if we wanted to.
22 /// This also expects that `trait_ref` is fully normalized.
23 pub fn codegen_select_candidate<'tcx>(
25 (param_env, trait_ref): (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>),
26 ) -> Result<&'tcx ImplSource<'tcx, ()>, CodegenObligationError> {
27 // We expect the input to be fully normalized.
28 debug_assert_eq!(trait_ref, tcx.normalize_erasing_regions(param_env, trait_ref));
30 // Do the initial selection for the obligation. This yields the
31 // shallow result we are looking for -- that is, what specific impl.
35 .with_opaque_type_inference(DefiningAnchor::Bubble)
37 //~^ HACK `Bubble` is required for
38 // this test to pass: type-alias-impl-trait/assoc-projection-ice.rs
39 let mut selcx = SelectionContext::new(&infcx);
41 let obligation_cause = ObligationCause::dummy();
42 let obligation = Obligation::new(tcx, obligation_cause, param_env, trait_ref);
44 let selection = match selcx.select(&obligation) {
45 Ok(Some(selection)) => selection,
46 Ok(None) => return Err(CodegenObligationError::Ambiguity),
47 Err(Unimplemented) => return Err(CodegenObligationError::Unimplemented),
49 bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
55 // Currently, we use a fulfillment context to completely resolve
56 // all nested obligations. This is because they can inform the
57 // inference of the impl's type parameters.
58 let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
59 let impl_source = selection.map(|predicate| {
60 fulfill_cx.register_predicate_obligation(&infcx, predicate);
63 // In principle, we only need to do this so long as `impl_source`
64 // contains unbound type parameters. It could be a slight
65 // optimization to stop iterating early.
66 let errors = fulfill_cx.select_all_or_error(&infcx);
67 if !errors.is_empty() {
68 // `rustc_monomorphize::collector` assumes there are no type errors.
69 // Cycle errors are the only post-monomorphization errors possible; emit them now so
70 // `rustc_ty_utils::resolve_associated_item` doesn't return `None` post-monomorphization.
72 if let FulfillmentErrorCode::CodeCycle(cycle) = err.code {
73 infcx.err_ctxt().report_overflow_obligation_cycle(&cycle);
76 return Err(CodegenObligationError::FulfillmentError);
79 let impl_source = infcx.resolve_vars_if_possible(impl_source);
80 let impl_source = infcx.tcx.erase_regions(impl_source);
82 // Opaque types may have gotten their hidden types constrained, but we can ignore them safely
83 // as they will get constrained elsewhere, too.
84 // (ouz-a) This is required for `type-alias-impl-trait/assoc-projection-ice.rs` to pass
85 let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
87 Ok(&*tcx.arena.alloc(impl_source))