Auto merge of #112682 - spastorino:new-rpitit-21, r=compiler-errors

Add bidirectional where clauses on RPITIT synthesized GATs

Given the following:

```rust
struct MyStruct<'a, T>(&'a T);

trait MyTrait<'a, T> {
    fn my_fn<'b, 'c, 'd, V>(item: &'c String) -> impl Sized + 'a + 'b + 'c where V: 'b, V: 'd;
}

impl<'a, T> MyTrait<'a, T> for MyStruct<'a, T> {
    fn my_fn<'b, 'c, 'd, V>(_: &'c String) -> impl Sized + 'a + 'b + 'c
    where
        V: 'b,
        V: 'd,
    {
        unimplemented!();
    }
}
```

We need the desugaring to be:

```rust
trait MyTrait<'a, T> {
    type MyFn<'bf, 'df, Vf, 'a2, 'b2, 'c2>: Sized + 'a2 + 'b2 + 'c2 where Vf: 'b2, 'a2: 'a, 'a: 'a2, 'b2: 'bf, 'bf: 'b2;

    fn my_fn<'b, 'c, 'd, V>(item: &'c String) -> MyStruct<'a>::MyFn<'b, 'd, V, 'a, 'b, 'c> where V: 'b, V: 'd {
        type opaque<'a3, 'b3, 'c3>;
    };
}

impl<'a, T> MyIter<'a, T> for MyStruct<'a, T> {
    type MyFn<'bf, 'df, Vf, 'a2, 'b2, 'c2> = impl Sized + 'a2 + 'b2 + 'c2 where Vf: b2, 'a2: 'a, 'a: 'a2, 'b2: 'bf, 'bf: 'b2;

    fn my_fn<'b, 'c, 'd, V>(_: &'c String) -> MyStruct<'a>::MyFn<'a, 'b, 'c, V> where V: 'b, V: 'd {
        type opaque<'a3, 'b3, 'c3>;
        unimplemented!();
    }
}
```

This PR adds the where clauses for the `MyFn` generated GATs.

This is a draft with a very ugly solution so we can make comments over concrete code.

r? `@compiler-errors`

compiler/rustc_ast_lowering/src/lib.rs

@@ -1539,9 +1539,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         );
         debug!(?opaque_ty_def_id);
 
-        // Contains the new lifetime definitions created for the TAIT (if any).
-        let mut collected_lifetimes = Vec::new();
-
         // If this came from a TAIT (as opposed to a function that returns an RPIT), we only want
         // to capture the lifetimes that appear in the bounds. So visit the bounds to find out
         // exactly which ones those are.
@@ -1558,20 +1555,31 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
         debug!(?lifetimes_to_remap);
 
-        self.with_hir_id_owner(opaque_ty_node_id, |lctx| {
-            let mut new_remapping = FxHashMap::default();
+        let mut new_remapping = FxHashMap::default();
 
-            // If this opaque type is only capturing a subset of the lifetimes (those that appear
-            // in bounds), then create the new lifetime parameters required and create a mapping
-            // from the old `'a` (on the function) to the new `'a` (on the opaque type).
-            collected_lifetimes = lctx.create_lifetime_defs(
-                opaque_ty_def_id,
-                &lifetimes_to_remap,
-                &mut new_remapping,
-            );
-            debug!(?collected_lifetimes);
-            debug!(?new_remapping);
+        // Contains the new lifetime definitions created for the TAIT (if any).
+        // If this opaque type is only capturing a subset of the lifetimes (those that appear in
+        // bounds), then create the new lifetime parameters required and create a mapping from the
+        // old `'a` (on the function) to the new `'a` (on the opaque type).
+        let collected_lifetimes =
+            self.create_lifetime_defs(opaque_ty_def_id, &lifetimes_to_remap, &mut new_remapping);
+        debug!(?collected_lifetimes);
+        debug!(?new_remapping);
+
+        // This creates HIR lifetime arguments as `hir::GenericArg`, in the given example `type
+        // TestReturn<'a, T, 'x> = impl Debug + 'x`, it creates a collection containing `&['x]`.
+        let collected_lifetime_mapping: Vec<_> = collected_lifetimes
+            .iter()
+            .map(|(node_id, lifetime)| {
+                let id = self.next_node_id();
+                let lifetime = self.new_named_lifetime(lifetime.id, id, lifetime.ident);
+                let def_id = self.local_def_id(*node_id);
+                (lifetime, def_id)
+            })
+            .collect();
+        debug!(?collected_lifetime_mapping);
 
+        self.with_hir_id_owner(opaque_ty_node_id, |lctx| {
             // Install the remapping from old to new (if any):
             lctx.with_remapping(new_remapping, |lctx| {
                 // This creates HIR lifetime definitions as `hir::GenericParam`, in the given
@@ -1610,6 +1618,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let hir_bounds = lctx.lower_param_bounds(bounds, itctx);
                 debug!(?hir_bounds);
 
+                let lifetime_mapping = if in_trait {
+                    self.arena.alloc_from_iter(
+                        collected_lifetime_mapping
+                            .iter()
+                            .map(|(lifetime, def_id)| (**lifetime, *def_id)),
+                    )
+                } else {
+                    &mut []
+                };
+
                 let opaque_ty_item = hir::OpaqueTy {
                     generics: self.arena.alloc(hir::Generics {
                         params: lifetime_defs,
@@ -1620,6 +1638,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }),
                     bounds: hir_bounds,
                     origin,
+                    lifetime_mapping,
                     in_trait,
                 };
                 debug!(?opaque_ty_item);
@@ -1628,20 +1647,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             })
         });
 
-        // This creates HIR lifetime arguments as `hir::GenericArg`, in the given example `type
-        // TestReturn<'a, T, 'x> = impl Debug + 'x`, it creates a collection containing `&['x]`.
-        let lifetimes =
-            self.arena.alloc_from_iter(collected_lifetimes.into_iter().map(|(_, lifetime)| {
-                let id = self.next_node_id();
-                let l = self.new_named_lifetime(lifetime.id, id, lifetime.ident);
-                hir::GenericArg::Lifetime(l)
-            }));
-        debug!(?lifetimes);
-
         // `impl Trait` now just becomes `Foo<'a, 'b, ..>`.
         hir::TyKind::OpaqueDef(
             hir::ItemId { owner_id: hir::OwnerId { def_id: opaque_ty_def_id } },
-            lifetimes,
+            self.arena.alloc_from_iter(
+                collected_lifetime_mapping
+                    .iter()
+                    .map(|(lifetime, _)| hir::GenericArg::Lifetime(*lifetime)),
+            ),
             in_trait,
         )
     }
@@ -1655,7 +1668,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         span: Span,
         opaque_ty_span: Span,
     ) -> hir::OwnerNode<'hir> {
-        let opaque_ty_item_kind = hir::ItemKind::OpaqueTy(opaque_ty_item);
+        let opaque_ty_item_kind = hir::ItemKind::OpaqueTy(self.arena.alloc(opaque_ty_item));
         // Generate an `type Foo = impl Trait;` declaration.
         trace!("registering opaque type with id {:#?}", opaque_ty_id);
         let opaque_ty_item = hir::Item {
@@ -1983,7 +1996,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let lifetime = Lifetime { id: outer_node_id, ident };
             collected_lifetimes.push((inner_node_id, lifetime, Some(inner_res)));
         }
-
         debug!(?collected_lifetimes);
 
         // We only want to capture the lifetimes that appear in the bounds. So visit the bounds to
@@ -1993,22 +2005,36 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let lifetimes_to_remap = lifetime_collector::lifetimes_in_ret_ty(&self.resolver, output);
         debug!(?lifetimes_to_remap);
 
-        self.with_hir_id_owner(opaque_ty_node_id, |this| {
-            // If this opaque type is only capturing a subset of the lifetimes (those that appear
-            // in bounds), then create the new lifetime parameters required and create a mapping
-            // from the old `'a` (on the function) to the new `'a` (on the opaque type).
-            collected_lifetimes.extend(
-                this.create_lifetime_defs(
-                    opaque_ty_def_id,
-                    &lifetimes_to_remap,
-                    &mut new_remapping,
-                )
+        // If this opaque type is only capturing a subset of the lifetimes (those that appear in
+        // bounds), then create the new lifetime parameters required and create a mapping from the
+        // old `'a` (on the function) to the new `'a` (on the opaque type).
+        collected_lifetimes.extend(
+            self.create_lifetime_defs(opaque_ty_def_id, &lifetimes_to_remap, &mut new_remapping)
                 .into_iter()
                 .map(|(new_node_id, lifetime)| (new_node_id, lifetime, None)),
-            );
-            debug!(?collected_lifetimes);
-            debug!(?new_remapping);
+        );
+        debug!(?collected_lifetimes);
+        debug!(?new_remapping);
+
+        // This creates pairs of HIR lifetimes and def_ids. In the given example `type
+        // TestReturn<'a, T, 'x> = impl Debug + 'x`, it creates a collection containing the
+        // new lifetime of the RPIT 'x and the def_id of the lifetime 'x corresponding to
+        // `TestReturn`.
+        let collected_lifetime_mapping: Vec<_> = collected_lifetimes
+            .iter()
+            .map(|(node_id, lifetime, res)| {
+                let id = self.next_node_id();
+                let res = res.unwrap_or(
+                    self.resolver.get_lifetime_res(lifetime.id).unwrap_or(LifetimeRes::Error),
+                );
+                let lifetime = self.new_named_lifetime_with_res(id, lifetime.ident, res);
+                let def_id = self.local_def_id(*node_id);
+                (lifetime, def_id)
+            })
+            .collect();
+        debug!(?collected_lifetime_mapping);
 
+        self.with_hir_id_owner(opaque_ty_node_id, |this| {
             // Install the remapping from old to new (if any):
             this.with_remapping(new_remapping, |this| {
                 // We have to be careful to get elision right here. The
@@ -2063,6 +2089,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 ));
                 debug!("lower_async_fn_ret_ty: generic_params={:#?}", generic_params);
 
+                let lifetime_mapping = if in_trait {
+                    self.arena.alloc_from_iter(
+                        collected_lifetime_mapping
+                            .iter()
+                            .map(|(lifetime, def_id)| (**lifetime, *def_id)),
+                    )
+                } else {
+                    &mut []
+                };
+
                 let opaque_ty_item = hir::OpaqueTy {
                     generics: this.arena.alloc(hir::Generics {
                         params: generic_params,
@@ -2073,6 +2109,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }),
                     bounds: arena_vec![this; future_bound],
                     origin: hir::OpaqueTyOrigin::AsyncFn(fn_def_id),
+                    lifetime_mapping,
                     in_trait,
                 };
 
@@ -2096,15 +2133,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         //
         // For the "output" lifetime parameters, we just want to
         // generate `'_`.
-        let generic_args = self.arena.alloc_from_iter(collected_lifetimes.into_iter().map(
-            |(_, lifetime, res)| {
-                let id = self.next_node_id();
-                let res = res.unwrap_or(
-                    self.resolver.get_lifetime_res(lifetime.id).unwrap_or(LifetimeRes::Error),
-                );
-                hir::GenericArg::Lifetime(self.new_named_lifetime_with_res(id, lifetime.ident, res))
-            },
-        ));
+        let generic_args = self.arena.alloc_from_iter(
+            collected_lifetime_mapping
+                .iter()
+                .map(|(lifetime, _)| hir::GenericArg::Lifetime(*lifetime)),
+        );
 
         // Create the `Foo<...>` reference itself. Note that the `type
         // Foo = impl Trait` is, internally, created as a child of the

compiler/rustc_hir/src/hir.rs

@@ -2664,6 +2664,10 @@ pub struct OpaqueTy<'hir> {
     pub generics: &'hir Generics<'hir>,
     pub bounds: GenericBounds<'hir>,
     pub origin: OpaqueTyOrigin,
+    // Opaques have duplicated lifetimes, this mapping connects the original lifetime with the copy
+    // so we can later generate bidirectional outlives predicates to enforce that these lifetimes
+    // stay in sync.
+    pub lifetime_mapping: &'hir [(Lifetime, LocalDefId)],
     pub in_trait: bool,
 }
 
@@ -3315,7 +3319,7 @@ pub enum ItemKind<'hir> {
     /// A type alias, e.g., `type Foo = Bar<u8>`.
     TyAlias(&'hir Ty<'hir>, &'hir Generics<'hir>),
     /// An opaque `impl Trait` type alias, e.g., `type Foo = impl Bar;`.
-    OpaqueTy(OpaqueTy<'hir>),
+    OpaqueTy(&'hir OpaqueTy<'hir>),
     /// An enum definition, e.g., `enum Foo<A, B> {C<A>, D<B>}`.
     Enum(EnumDef<'hir>, &'hir Generics<'hir>),
     /// A struct definition, e.g., `struct Foo<A> {x: A}`.

compiler/rustc_hir/src/intravisit.rs

@@ -502,7 +502,7 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item<'v>) {
             visitor.visit_ty(ty);
             visitor.visit_generics(generics)
         }
-        ItemKind::OpaqueTy(OpaqueTy { ref generics, bounds, .. }) => {
+        ItemKind::OpaqueTy(&OpaqueTy { generics, bounds, .. }) => {
             visitor.visit_id(item.hir_id());
             walk_generics(visitor, generics);
             walk_list!(visitor, visit_param_bound, bounds);

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2809,7 +2809,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 let opaque_ty = tcx.hir().item(item_id);
 
                 match opaque_ty.kind {
-                    hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => {
+                    hir::ItemKind::OpaqueTy(&hir::OpaqueTy { origin, .. }) => {
                         let local_def_id = item_id.owner_id.def_id;
                         // If this is an RPITIT and we are using the new RPITIT lowering scheme, we
                         // generate the def_id of an associated type for the trait and return as

compiler/rustc_hir_analysis/src/check/check.rs

@@ -299,7 +299,7 @@ pub(super) fn check_opaque_for_inheriting_lifetimes(
         }
     }
 
-    if let ItemKind::OpaqueTy(hir::OpaqueTy {
+    if let ItemKind::OpaqueTy(&hir::OpaqueTy {
         origin: hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::FnReturn(..),
         in_trait,
         ..

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -144,7 +144,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             Some(tcx.typeck_root_def_id(def_id.to_def_id()))
         }
         Node::Item(item) => match item.kind {
-            ItemKind::OpaqueTy(hir::OpaqueTy {
+            ItemKind::OpaqueTy(&hir::OpaqueTy {
                 origin:
                     hir::OpaqueTyOrigin::FnReturn(fn_def_id) | hir::OpaqueTyOrigin::AsyncFn(fn_def_id),
                 in_trait,