Auto merge of rust-lang#124029 - compiler-errors:rpitit-variances, r=<try>

Give RPITITs variances, so they can (not) capture lifetimes

This is needed for edition 2021 and the `precise_captures` feature, so we can specify that we *don't* want to capture a lifetime implicitly for an RPITIT. I'll elaborate this further and try to motivate its soundness once perf comes back.

r? `@ghost`

Author: bors

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -5,7 +5,8 @@ use rustc_infer::infer::{outlives::env::OutlivesEnvironment, TyCtxtInferExt};
 use rustc_lint_defs::builtin::{REFINING_IMPL_TRAIT_INTERNAL, REFINING_IMPL_TRAIT_REACHABLE};
 use rustc_middle::traits::{ObligationCause, Reveal};
 use rustc_middle::ty::{
-    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable, TypeVisitor,
+    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable,
+    TypeVisitableExt, TypeVisitor,
 };
 use rustc_span::Span;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
@@ -62,6 +63,10 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
         return;
     };
 
+    if hidden_tys.items().any(|(_, &ty)| ty.skip_binder().references_error()) {
+        return;
+    }
+
     let mut collector = ImplTraitInTraitCollector { tcx, types: FxIndexSet::default() };
     trait_m_sig.visit_with(&mut collector);
 

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -61,6 +61,13 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
             let crate_map = tcx.crate_variances(());
             return crate_map.variances.get(&item_def_id.to_def_id()).copied().unwrap_or(&[]);
         }
+        DefKind::AssocTy => match tcx.opt_rpitit_info(item_def_id.to_def_id()) {
+            Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) => {
+                return variance_of_opaque(tcx, opaque_def_id.expect_local());
+            }
+            None => {}
+            Some(ty::ImplTraitInTraitData::Impl { .. }) => {}
+        },
         DefKind::OpaqueTy => {
             return variance_of_opaque(tcx, item_def_id);
         }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1086,9 +1086,12 @@ fn should_encode_variances<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, def_kind: Def
         | DefKind::Fn
         | DefKind::Ctor(..)
         | DefKind::AssocFn => true,
+        DefKind::AssocTy => {
+            // Only encode variances for RPITITs (for traits)
+            matches!(tcx.opt_rpitit_info(def_id), Some(ty::ImplTraitInTraitData::Trait { .. }))
+        }
         DefKind::Mod
         | DefKind::Field
-        | DefKind::AssocTy
         | DefKind::AssocConst
         | DefKind::TyParam
         | DefKind::ConstParam

compiler/rustc_middle/src/ty/relate.rs

@@ -233,6 +233,16 @@ impl<'tcx> Relate<'tcx> for ty::AliasTy<'tcx> {
                     b.args,
                     false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
                 )?,
+                DefKind::AssocTy if relation.tcx().is_impl_trait_in_trait(a.def_id) => {
+                    relate_args_with_variances(
+                        relation,
+                        a.def_id,
+                        relation.tcx().variances_of(a.def_id),
+                        a.args,
+                        b.args,
+                        false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
+                    )?
+                }
                 DefKind::AssocTy | DefKind::AssocConst | DefKind::TyAlias => {
                     relate_args_invariantly(relation, a.args, b.args)?
                 }

tests/ui/impl-trait/precise-capturing/rpitit-impl-captures-too-much.rs

@@ -0,0 +1,15 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+struct Invariant<'a>(&'a mut &'a mut ());
+
+trait Trait {
+    fn hello(self_: Invariant<'_>) -> impl use<Self> Sized;
+}
+
+impl Trait for () {
+    fn hello(self_: Invariant<'_>) -> impl use<'_> Sized {}
+    //~^ return type captures more lifetimes than trait definition
+}
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/rpitit-impl-captures-too-much.stderr

@@ -0,0 +1,26 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/rpitit-impl-captures-too-much.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: return type captures more lifetimes than trait definition
+  --> $DIR/rpitit-impl-captures-too-much.rs:11:39
+   |
+LL |     fn hello(self_: Invariant<'_>) -> impl use<'_> Sized {}
+   |                               --      ^^^^^^^^^^^^^^^^^^
+   |                               |
+   |                               this lifetime was captured
+   |
+note: hidden type must only reference lifetimes captured by this impl trait
+  --> $DIR/rpitit-impl-captures-too-much.rs:7:39
+   |
+LL |     fn hello(self_: Invariant<'_>) -> impl use<Self> Sized;
+   |                                       ^^^^^^^^^^^^^^^^^^^^
+   = note: hidden type inferred to be `impl Sized`
+
+error: aborting due to 1 previous error; 1 warning emitted
+

tests/ui/impl-trait/precise-capturing/rpitit.rs

@@ -0,0 +1,17 @@
+//@ check-pass
+
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+struct Invariant<'a>(&'a mut &'a mut ());
+
+trait Trait {
+    fn hello(self_: Invariant<'_>) -> impl use<Self> Sized;
+}
+
+fn eq<'a, 'b, T: Trait>(x: Invariant<'a>, y: Invariant<'b>) {
+    let mut out = T::hello(x);
+    out = T::hello(y);
+}
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/rpitit.stderr

@@ -0,0 +1,11 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/rpitit.rs:3:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+