Auto merge of #92007 - oli-obk:lazy_tait2, r=nikomatsakis

Lazy type-alias-impl-trait

Previously opaque types were processed by

1. replacing all mentions of them with inference variables
2. memorizing these inference variables in a side-table
3. at the end of typeck, resolve the inference variables in the side table and use the resolved type as the hidden type of the opaque type

This worked okayish for `impl Trait` in return position, but required lots of roundabout type inference hacks and processing.

This PR instead stops this process of replacing opaque types with inference variables, and just keeps the opaque types around.
Whenever an opaque type `O` is compared with another type `T`, we make the comparison succeed and record `T` as the hidden type. If `O` is compared to `U` while there is a recorded hidden type for it, we grab the recorded type (`T`) and compare that against `U`. This makes implementing

* rust-lang/rfcs#2515

much simpler (previous attempts on the inference based scheme were very prone to ICEs and general misbehaviour that was not explainable except by random implementation defined oddities).

r? `@nikomatsakis`

fixes #93411
fixes #88236

compiler/rustc_borrowck/src/lib.rs

@@ -124,8 +124,9 @@ fn mir_borrowck<'tcx>(
 ) -> &'tcx BorrowCheckResult<'tcx> {
     let (input_body, promoted) = tcx.mir_promoted(def);
     debug!("run query mir_borrowck: {}", tcx.def_path_str(def.did.to_def_id()));
+    let hir_owner = tcx.hir().local_def_id_to_hir_id(def.did).owner;
 
-    let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(def.did).enter(|infcx| {
+    let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(hir_owner).enter(|infcx| {
         let input_body: &Body<'_> = &input_body.borrow();
         let promoted: &IndexVec<_, _> = &promoted.borrow();
         do_mir_borrowck(&infcx, input_body, promoted, false).0
@@ -140,7 +141,7 @@ fn mir_borrowck<'tcx>(
 /// If `return_body_with_facts` is true, then return the body with non-erased
 /// region ids on which the borrow checking was performed together with Polonius
 /// facts.
-#[instrument(skip(infcx, input_body, input_promoted), level = "debug")]
+#[instrument(skip(infcx, input_body, input_promoted), fields(id=?input_body.source.with_opt_param().as_local().unwrap()), level = "debug")]
 fn do_mir_borrowck<'a, 'tcx>(
     infcx: &InferCtxt<'a, 'tcx>,
     input_body: &Body<'tcx>,

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -1,7 +1,6 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::vec_map::VecMap;
 use rustc_hir::OpaqueTyOrigin;
-use rustc_infer::infer::opaque_types::OpaqueTypeDecl;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable};
@@ -54,27 +53,44 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     pub(crate) fn infer_opaque_types(
         &self,
         infcx: &InferCtxt<'_, 'tcx>,
-        opaque_ty_decls: VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>,
+        opaque_ty_decls: VecMap<OpaqueTypeKey<'tcx>, (Ty<'tcx>, Span, OpaqueTyOrigin)>,
         span: Span,
     ) -> VecMap<OpaqueTypeKey<'tcx>, Ty<'tcx>> {
         opaque_ty_decls
             .into_iter()
-            .filter_map(|(opaque_type_key, decl)| {
+            .map(|(opaque_type_key, (concrete_type, decl_span, origin))| {
                 let substs = opaque_type_key.substs;
-                let concrete_type = decl.concrete_ty;
+                // FIXME: why are the spans in decl_span often DUMMY_SP?
+                let span = decl_span.substitute_dummy(span);
                 debug!(?concrete_type, ?substs);
 
                 let mut subst_regions = vec![self.universal_regions.fr_static];
                 let universal_substs = infcx.tcx.fold_regions(substs, &mut false, |region, _| {
-                    let vid = self.universal_regions.to_region_vid(region);
-                    subst_regions.push(vid);
-                    self.definitions[vid].external_name.unwrap_or_else(|| {
-                        infcx
-                            .tcx
-                            .sess
-                            .delay_span_bug(span, "opaque type with non-universal region substs");
-                        infcx.tcx.lifetimes.re_static
-                    })
+                    if let ty::RePlaceholder(..) = region {
+                        // Higher kinded regions don't need remapping, they don't refer to anything outside of this the substs.
+                        return region;
+                    }
+                    let vid = self.to_region_vid(region);
+                    trace!(?vid);
+                    let scc = self.constraint_sccs.scc(vid);
+                    trace!(?scc);
+                    match self.scc_values.universal_regions_outlived_by(scc).find_map(|lb| {
+                        self.eval_equal(vid, lb).then_some(self.definitions[lb].external_name?)
+                    }) {
+                        Some(region) => {
+                            let vid = self.universal_regions.to_region_vid(region);
+                            subst_regions.push(vid);
+                            region
+                        }
+                        None => {
+                            subst_regions.push(vid);
+                            infcx.tcx.sess.delay_span_bug(
+                                span,
+                                "opaque type with non-universal region substs",
+                            );
+                            infcx.tcx.lifetimes.re_static
+                        }
+                    }
                 });
 
                 subst_regions.sort();
@@ -100,12 +116,14 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     span,
                 );
 
-                check_opaque_type_parameter_valid(
-                    infcx.tcx,
+                (
                     opaque_type_key,
-                    OpaqueTypeDecl { concrete_ty: remapped_type, ..decl },
+                    if check_opaque_type_parameter_valid(infcx.tcx, opaque_type_key, origin, span) {
+                        remapped_type
+                    } else {
+                        infcx.tcx.ty_error()
+                    },
                 )
-                .then_some((opaque_type_key, remapped_type))
             })
             .collect()
     }
@@ -149,9 +167,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 fn check_opaque_type_parameter_valid(
     tcx: TyCtxt<'_>,
     opaque_type_key: OpaqueTypeKey<'_>,
-    decl: OpaqueTypeDecl<'_>,
+    origin: OpaqueTyOrigin,
+    span: Span,
 ) -> bool {
-    match decl.origin {
+    match origin {
         // No need to check return position impl trait (RPIT)
         // because for type and const parameters they are correct
         // by construction: we convert
@@ -177,7 +196,6 @@ fn check_opaque_type_parameter_valid(
         // Check these
         OpaqueTyOrigin::TyAlias => {}
     }
-    let span = decl.definition_span;
     let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
     let mut seen_params: FxHashMap<_, Vec<_>> = FxHashMap::default();
     for (i, arg) in opaque_type_key.substs.iter().enumerate() {

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -147,9 +147,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         // Return types are a bit more complex. They may contain opaque `impl Trait` types.
         let mir_output_ty = body.local_decls[RETURN_PLACE].ty;
         let output_span = body.local_decls[RETURN_PLACE].source_info.span;
-        if let Err(terr) = self.eq_opaque_type_and_type(
-            mir_output_ty,
+        if let Err(terr) = self.eq_types(
             normalized_output_ty,
+            mir_output_ty,
             Locations::All(output_span),
             ConstraintCategory::BoringNoLocation,
         ) {
@@ -169,9 +169,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             let user_provided_output_ty = user_provided_sig.output();
             let user_provided_output_ty =
                 self.normalize(user_provided_output_ty, Locations::All(output_span));
-            if let Err(err) = self.eq_opaque_type_and_type(
-                mir_output_ty,
+            if let Err(err) = self.eq_types(
                 user_provided_output_ty,
+                mir_output_ty,
                 Locations::All(output_span),
                 ConstraintCategory::BoringNoLocation,
             ) {