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::{InferCtxt, TyCtxtInferExt};
8 FulfillmentContext, ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine,
11 use rustc_errors::ErrorGuaranteed;
12 use rustc_middle::ty::fold::TypeFoldable;
13 use rustc_middle::ty::{self, TyCtxt};
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.
21 /// This also expects that `trait_ref` is fully normalized.
22 #[instrument(level = "debug", skip(tcx))]
23 pub fn codegen_fulfill_obligation<'tcx>(
25 (param_env, trait_ref): (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>),
26 ) -> Result<&'tcx ImplSource<'tcx, ()>, ErrorGuaranteed> {
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));
32 // Do the initial selection for the obligation. This yields the
33 // shallow result we are looking for -- that is, what specific impl.
34 tcx.infer_ctxt().enter(|infcx| {
35 let mut selcx = SelectionContext::new(&infcx);
37 let obligation_cause = ObligationCause::dummy();
39 Obligation::new(obligation_cause, param_env, trait_ref.to_poly_trait_predicate());
41 let selection = match selcx.select(&obligation) {
42 Ok(Some(selection)) => selection,
44 // Ambiguity can happen when monomorphizing during trans
45 // expands to some humongous type that never occurred
46 // statically -- this humongous type can then overflow,
47 // leading to an ambiguous result. So report this as an
48 // overflow bug, since I believe this is the only case
49 // where ambiguity can result.
50 let reported = infcx.tcx.sess.delay_span_bug(
53 "encountered ambiguity selecting `{:?}` during codegen, presuming due to \
54 overflow or prior type error",
60 Err(Unimplemented) => {
61 // This can trigger when we probe for the source of a `'static` lifetime requirement
62 // on a trait object: `impl Foo for dyn Trait {}` has an implicit `'static` bound.
63 // This can also trigger when we have a global bound that is not actually satisfied,
64 // but was included during typeck due to the trivial_bounds feature.
65 let guar = infcx.tcx.sess.delay_span_bug(
68 "Encountered error `Unimplemented` selecting `{:?}` during codegen",
75 bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
81 // Currently, we use a fulfillment context to completely resolve
82 // all nested obligations. This is because they can inform the
83 // inference of the impl's type parameters.
84 let mut fulfill_cx = FulfillmentContext::new();
85 let impl_source = selection.map(|predicate| {
86 fulfill_cx.register_predicate_obligation(&infcx, predicate);
88 let impl_source = drain_fulfillment_cx_or_panic(&infcx, &mut fulfill_cx, impl_source);
90 // Opaque types may have gotten their hidden types constrained, but we can ignore them safely
91 // as they will get constrained elsewhere, too.
92 let _opaque_types = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
94 debug!("Cache miss: {trait_ref:?} => {impl_source:?}");
95 Ok(&*tcx.arena.alloc(impl_source))
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 outside of type inference, 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<'tcx, T>(
111 infcx: &InferCtxt<'_, 'tcx>,
112 fulfill_cx: &mut FulfillmentContext<'tcx>,
116 T: TypeFoldable<'tcx>,
118 debug!("drain_fulfillment_cx_or_panic()");
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 let errors = fulfill_cx.select_all_or_error(infcx);
124 if !errors.is_empty() {
125 infcx.tcx.sess.delay_span_bug(
126 rustc_span::DUMMY_SP,
128 "Encountered errors `{:?}` resolving bounds outside of type inference",
134 let result = infcx.resolve_vars_if_possible(result);
135 infcx.tcx.erase_regions(result)