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// This file contains various trait resolution methods used by codegen.
// They all assume regions can be erased and monomorphic types. It
// seems likely that they should eventually be merged into more
// general routines.
use crate::infer::{DefiningAnchor, TyCtxtInferExt};
use crate::traits::{
ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine, TraitEngineExt,
Unimplemented,
};
use rustc_middle::traits::CodegenObligationError;
use rustc_middle::ty::{self, TyCtxt};
/// Attempts to resolve an obligation to an `ImplSource`. The result is
/// a shallow `ImplSource` resolution, meaning that we do not
/// (necessarily) resolve all nested obligations on the impl. Note
/// that type check should guarantee to us that all nested
/// obligations *could be* resolved if we wanted to.
///
/// This also expects that `trait_ref` is fully normalized.
pub fn codegen_select_candidate<'tcx>(
tcx: TyCtxt<'tcx>,
(param_env, trait_ref): (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>),
) -> Result<&'tcx ImplSource<'tcx, ()>, CodegenObligationError> {
// We expect the input to be fully normalized.
debug_assert_eq!(trait_ref, tcx.normalize_erasing_regions(param_env, trait_ref));
// Do the initial selection for the obligation. This yields the
// shallow result we are looking for -- that is, what specific impl.
let mut infcx_builder =
tcx.infer_ctxt().ignoring_regions().with_opaque_type_inference(DefiningAnchor::Bubble);
infcx_builder.enter(|infcx| {
//~^ HACK `Bubble` is required for
// this test to pass: type-alias-impl-trait/assoc-projection-ice.rs
let mut selcx = SelectionContext::new(&infcx);
let obligation_cause = ObligationCause::dummy();
let obligation =
Obligation::new(obligation_cause, param_env, trait_ref.to_poly_trait_predicate());
let selection = match selcx.select(&obligation) {
Ok(Some(selection)) => selection,
Ok(None) => return Err(CodegenObligationError::Ambiguity),
Err(Unimplemented) => return Err(CodegenObligationError::Unimplemented),
Err(e) => {
bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
}
};
debug!(?selection);
// Currently, we use a fulfillment context to completely resolve
// all nested obligations. This is because they can inform the
// inference of the impl's type parameters.
let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
let impl_source = selection.map(|predicate| {
fulfill_cx.register_predicate_obligation(&infcx, predicate);
});
// In principle, we only need to do this so long as `impl_source`
// contains unbound type parameters. It could be a slight
// optimization to stop iterating early.
let errors = fulfill_cx.select_all_or_error(&infcx);
if !errors.is_empty() {
return Err(CodegenObligationError::FulfillmentError);
}
let impl_source = infcx.resolve_vars_if_possible(impl_source);
let impl_source = infcx.tcx.erase_regions(impl_source);
// Opaque types may have gotten their hidden types constrained, but we can ignore them safely
// as they will get constrained elsewhere, too.
// (ouz-a) This is required for `type-alias-impl-trait/assoc-projection-ice.rs` to pass
let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
Ok(&*tcx.arena.alloc(impl_source))
})
}
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