Rollup merge of rust-lang#97346 - JohnTitor:remove-back-compat-hacks, r=oli-obk

Remove a back-compat hack on lazy TAIT

This PR's motivation is here: rust-lang#72614 (comment)
~~But removing a hack doesn't seem to reject the code on the issue, there're some more hacks?~~
r? ``@oli-obk``

Author: Dylan-DPC

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -39,21 +39,19 @@ pub struct OpaqueTypeDecl<'tcx> {
 }
 
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
-    /// This is a backwards compatibility hack to prevent breaking changes from
-    /// lazy TAIT around RPIT handling.
-    pub fn replace_opaque_types_with_inference_vars<T: TypeFoldable<'tcx>>(
+    pub fn replace_opaque_types_with_inference_vars(
         &self,
-        value: T,
+        ty: Ty<'tcx>,
         body_id: HirId,
         span: Span,
         code: ObligationCauseCode<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
-    ) -> InferOk<'tcx, T> {
-        if !value.has_opaque_types() {
-            return InferOk { value, obligations: vec![] };
+    ) -> InferOk<'tcx, Ty<'tcx>> {
+        if !ty.has_opaque_types() {
+            return InferOk { value: ty, obligations: vec![] };
         }
         let mut obligations = vec![];
-        let value = value.fold_with(&mut ty::fold::BottomUpFolder {
+        let value = ty.fold_with(&mut ty::fold::BottomUpFolder {
             tcx: self.tcx,
             lt_op: |lt| lt,
             ct_op: |ct| ct,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -28,7 +28,6 @@ use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
-use rustc_infer::traits::ObligationCauseCode;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::fold::{MaxUniverse, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::Subst;
@@ -252,22 +251,10 @@ fn project_and_unify_type<'cx, 'tcx>(
         Err(InProgress) => return ProjectAndUnifyResult::Recursive,
     };
     debug!(?normalized, ?obligations, "project_and_unify_type result");
-    let actual = obligation.predicate.term;
-    // HACK: lazy TAIT would regress src/test/ui/impl-trait/nested-return-type2.rs, so we add
-    // a back-compat hack hat converts the RPITs into inference vars, just like they were before
-    // lazy TAIT.
-    // This does not affect TAITs in general, as tested in the nested-return-type-tait* tests.
-    let InferOk { value: actual, obligations: new } =
-        selcx.infcx().replace_opaque_types_with_inference_vars(
-            actual,
-            obligation.cause.body_id,
-            obligation.cause.span,
-            ObligationCauseCode::MiscObligation,
-            obligation.param_env,
-        );
-    obligations.extend(new);
-
-    match infcx.at(&obligation.cause, obligation.param_env).eq(normalized, actual) {
+    match infcx
+        .at(&obligation.cause, obligation.param_env)
+        .eq(normalized, obligation.predicate.term)
+    {
         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
             obligations.extend(inferred_obligations);
             ProjectAndUnifyResult::Holds(obligations)

src/test/ui/impl-trait/nested-return-type2.rs

@@ -1,5 +1,3 @@
-// check-pass
-
 trait Duh {}
 
 impl Duh for i32 {}
@@ -20,11 +18,9 @@ impl<R: Duh, F: FnMut() -> R> Trait for F {
 // the hidden type. We already have obligations registered on the inference
 // var to make it uphold the `: Duh` bound on `Trait::Assoc`. The opaque
 // type does not implement `Duh`, even if its hidden type does.
-// Lazy TAIT would error out, but we inserted a hack to make it work again,
-// keeping backwards compatibility.
 fn foo() -> impl Trait<Assoc = impl Send> {
+    //~^ ERROR `impl Send: Duh` is not satisfied
     || 42
 }
 
-fn main() {
-}
+fn main() {}

src/test/ui/impl-trait/nested-return-type2.stderr

@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `impl Send: Duh` is not satisfied
+  --> $DIR/nested-return-type2.rs:21:13
+   |
+LL | fn foo() -> impl Trait<Assoc = impl Send> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Duh` is not implemented for `impl Send`
+   |
+   = help: the trait `Duh` is implemented for `i32`
+note: required because of the requirements on the impl of `Trait` for `[closure@$DIR/nested-return-type2.rs:23:5: 23:10]`
+  --> $DIR/nested-return-type2.rs:12:31
+   |
+LL | impl<R: Duh, F: FnMut() -> R> Trait for F {
+   |                               ^^^^^     ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.