Move codegen_select_candidate to a rustc_traits

1 files changed, 88 insertions, 0 deletions

diff --git a/compiler/rustc_traits/src/codegen.rs b/compiler/rustc_traits/src/codegen.rs
new file mode 100644
index 00000000000..f8f74b732ef
--- /dev/null
+++ b/compiler/rustc_traits/src/codegen.rs
@@ -0,0 +1,88 @@
+// 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 rustc_infer::infer::{DefiningAnchor, TyCtxtInferExt};
+use rustc_infer::traits::FulfillmentErrorCode;
+use rustc_middle::traits::CodegenObligationError;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
+use rustc_trait_selection::traits::{
+    ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine, TraitEngineExt,
+    Unimplemented,
+};
+
+/// 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 infcx = tcx
+        .infer_ctxt()
+        .ignoring_regions()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .build();
+    //~^ 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(tcx, obligation_cause, param_env, trait_ref);
+
+    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() {
+        // `rustc_monomorphize::collector` assumes there are no type errors.
+        // Cycle errors are the only post-monomorphization errors possible; emit them now so
+        // `rustc_ty_utils::resolve_associated_item` doesn't return `None` post-monomorphization.
+        for err in errors {
+            if let FulfillmentErrorCode::CodeCycle(cycle) = err.code {
+                infcx.err_ctxt().report_overflow_obligation_cycle(&cycle);
+            }
+        }
+        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))
+}