Rollup merge of rust-lang#133768 - compiler-errors:gate, r=lcnr,jackh726

Remove `generic_associated_types_extended` feature gate

This PR retires nightly support for the `generic_associated_types_extended` feature. This feature hasn't received much attention in the last two years or so, and I believe the feature still remains both unsound and ICEy to use. I think that if we were to redesign and reimplement it, we'd want to first figure out how to implement it soundly, but in the mean time I'd prefer to clean this up.

r? ``@lcnr`` cc ``@jackh726`` who added this feature gate I think

Author: matthiaskrgr

compiler/rustc_feature/src/removed.rs

@@ -119,6 +119,13 @@ declare_features! (
     (removed, generator_clone, "1.65.0", Some(95360), Some("renamed to `coroutine_clone`")),
     /// Allows defining generators.
     (removed, generators, "1.21.0", Some(43122), Some("renamed to `coroutines`")),
+    /// An extension to the `generic_associated_types` feature, allowing incomplete features.
+    (removed, generic_associated_types_extended, "CURRENT_RUSTC_VERSION", Some(95451),
+        Some(
+            "feature needs overhaul and reimplementation pending \
+            better implied higher-ranked implied bounds support"
+        )
+    ),
     /// Allows `impl Trait` in bindings (`let`, `const`, `static`).
     (removed, impl_trait_in_bindings, "1.55.0", Some(63065),
      Some("the implementation was not maintainable, the feature may get reintroduced once the current refactorings are done")),

compiler/rustc_feature/src/unstable.rs

@@ -497,8 +497,6 @@ declare_features! (
     (unstable, gen_blocks, "1.75.0", Some(117078)),
     /// Infer generic args for both consts and types.
     (unstable, generic_arg_infer, "1.55.0", Some(85077)),
-    /// An extension to the `generic_associated_types` feature, allowing incomplete features.
-    (incomplete, generic_associated_types_extended, "1.61.0", Some(95451)),
     /// Allows non-trivial generic constants which have to have wfness manually propagated to callers
     (incomplete, generic_const_exprs, "1.56.0", Some(76560)),
     /// Allows generic parameters and where-clauses on free & associated const items.

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -329,10 +329,7 @@ pub fn dyn_compatibility_violations_for_assoc_item(
             .collect(),
         // Associated types can only be dyn-compatible if they have `Self: Sized` bounds.
         ty::AssocKind::Type => {
-            if !tcx.features().generic_associated_types_extended()
-                && !tcx.generics_of(item.def_id).is_own_empty()
-                && !item.is_impl_trait_in_trait()
-            {
+            if !tcx.generics_of(item.def_id).is_own_empty() && !item.is_impl_trait_in_trait() {
                 vec![DynCompatibilityViolation::GAT(item.name, item.ident(tcx).span)]
             } else {
                 // We will permit associated types if they are explicitly mentioned in the trait object.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -14,7 +14,7 @@ use rustc_middle::traits::select::OverflowError;
 use rustc_middle::traits::{BuiltinImplSource, ImplSource, ImplSourceUserDefinedData};
 use rustc_middle::ty::fast_reject::DeepRejectCtxt;
 use rustc_middle::ty::fold::TypeFoldable;
-use rustc_middle::ty::visit::{MaxUniverse, TypeVisitable, TypeVisitableExt};
+use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, Term, Ty, TyCtxt, TypingMode, Upcast};
 use rustc_middle::{bug, span_bug};
 use rustc_span::symbol::sym;
@@ -179,35 +179,11 @@ pub(super) fn poly_project_and_unify_term<'cx, 'tcx>(
 ) -> ProjectAndUnifyResult<'tcx> {
     let infcx = selcx.infcx;
     let r = infcx.commit_if_ok(|_snapshot| {
-        let old_universe = infcx.universe();
         let placeholder_predicate = infcx.enter_forall_and_leak_universe(obligation.predicate);
-        let new_universe = infcx.universe();
 
         let placeholder_obligation = obligation.with(infcx.tcx, placeholder_predicate);
         match project_and_unify_term(selcx, &placeholder_obligation) {
             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => Err(e),
-            ProjectAndUnifyResult::Holds(obligations)
-                if old_universe != new_universe
-                    && selcx.tcx().features().generic_associated_types_extended() =>
-            {
-                // If the `generic_associated_types_extended` feature is active, then we ignore any
-                // obligations references lifetimes from any universe greater than or equal to the
-                // universe just created. Otherwise, we can end up with something like `for<'a> I: 'a`,
-                // which isn't quite what we want. Ideally, we want either an implied
-                // `for<'a where I: 'a> I: 'a` or we want to "lazily" check these hold when we
-                // instantiate concrete regions. There is design work to be done here; until then,
-                // however, this allows experimenting potential GAT features without running into
-                // well-formedness issues.
-                let new_obligations = obligations
-                    .into_iter()
-                    .filter(|obligation| {
-                        let mut visitor = MaxUniverse::new();
-                        obligation.predicate.visit_with(&mut visitor);
-                        visitor.max_universe() < new_universe
-                    })
-                    .collect();
-                Ok(ProjectAndUnifyResult::Holds(new_obligations))
-            }
             other => Ok(other),
         }
     });

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -626,7 +626,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         for assoc_type in assoc_types {
             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 
-            if !defs.own_params.is_empty() && !tcx.features().generic_associated_types_extended() {
+            if !defs.own_params.is_empty() {
                 tcx.dcx().span_delayed_bug(
                     obligation.cause.span,
                     "GATs in trait object shouldn't have been considered",

tests/crashes/131538.rs

@@ -1,17 +1,11 @@
-//@ revisions: base extended
-//@[base] check-fail
-//@[extended] check-pass
-
-#![cfg_attr(extended, feature(generic_associated_types_extended))]
-#![cfg_attr(extended, allow(incomplete_features))]
+//@ known-bug: #133805
 
 pub trait FromLendingIterator<A>: Sized {
     fn from_iter<T: for<'x> LendingIterator<Item<'x> = A>>(iter: T) -> Self;
 }
 
 impl<A> FromLendingIterator<A> for Vec<A> {
     fn from_iter<I: for<'x> LendingIterator<Item<'x> = A>>(mut iter: I) -> Self {
-        //[base]~^ impl has stricter
         let mut v = vec![];
         while let Some(item) = iter.next() {
             v.push(item);
@@ -32,7 +26,6 @@ pub trait LendingIterator {
         Self: for<'q> LendingIterator<Item<'q> = A>,
     {
         <B as FromLendingIterator<A>>::from_iter(self)
-        //[base]~^ ERROR: does not live long enough
     }
 }
 

tests/ui/feature-gates/feature-gate-generic_associated_types_extended.rs

@@ -1,5 +1,5 @@
 error[E0276]: impl has stricter requirements than trait
-  --> $DIR/lending_iterator.rs:13:45
+  --> $DIR/lending_iterator.rs:8:45
    |
 LL |     fn from_iter<T: for<'x> LendingIterator<Item<'x> = A>>(iter: T) -> Self;
    |     ------------------------------------------------------------------------ definition of `from_iter` from trait
@@ -8,7 +8,7 @@ LL |     fn from_iter<I: for<'x> LendingIterator<Item<'x> = A>>(mut iter: I) ->
    |                                             ^^^^^^^^^^^^ impl has extra requirement `I: 'x`
 
 error: `Self` does not live long enough
-  --> $DIR/lending_iterator.rs:34:9
+  --> $DIR/lending_iterator.rs:28:9
    |
 LL |         <B as FromLendingIterator<A>>::from_iter(self)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^