Inherit lifetimes for async fn instead of duplicating them.

Author: cjgillot

compiler/rustc_ast_lowering/src/lib.rs

@@ -1649,11 +1649,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         // When we create the opaque type for this async fn, it is going to have
         // to capture all the lifetimes involved in the signature (including in the
-        // return type). This is done by introducing lifetime parameters for:
+        // return type). This is done by:
         //
-        // - all the explicitly declared lifetimes from the impl and function itself;
-        // - all the elided lifetimes in the fn arguments;
-        // - all the elided lifetimes in the return type.
+        // - making the opaque type inherit all lifetime parameters from its parent;
+        // - make all the elided lifetimes in the fn arguments into parameters;
+        // - manually introducing parameters on the opaque type for elided
+        //   lifetimes in the return type.
         //
         // So for example in this snippet:
         //
@@ -1669,44 +1670,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // we would create an opaque type like:
         //
         // ```
-        // type Bar<'a, 'b, '0, '1, '2> = impl Future<Output = &'2 u32>;
+        // type Foo<'a>::bar<'b, '0, '1>::Bar<'2> = impl Future<Output = &'2 u32>;
         // ```
         //
         // and we would then desugar `bar` to the equivalent of:
         //
         // ```rust
         // impl<'a> Foo<'a> {
-        //   fn bar<'b, '0, '1>(&'0 self, x: &'b Vec<f64>, y: &'1 str) -> Bar<'a, 'b, '0, '1, '_>
+        //   fn bar<'b, '0, '1>(&'0 self, x: &'b Vec<f64>, y: &'1 str) -> Bar<'_>
         // }
         // ```
         //
         // Note that the final parameter to `Bar` is `'_`, not `'2` --
         // this is because the elided lifetimes from the return type
         // should be figured out using the ordinary elision rules, and
         // this desugaring achieves that.
-
-        debug!("lower_async_fn_ret_ty: in_scope_lifetimes={:#?}", self.in_scope_lifetimes);
-        debug!("lower_async_fn_ret_ty: lifetimes_to_define={:#?}", self.lifetimes_to_define);
-
-        // Calculate all the lifetimes that should be captured
-        // by the opaque type. This should include all in-scope
-        // lifetime parameters, including those defined in-band.
-        //
-        // `lifetime_params` is a vector of tuple (span, parameter name, lifetime name).
-
-        // Input lifetime like `'a` or `'1`:
-        let mut lifetime_params: Vec<_> = self
-            .in_scope_lifetimes
-            .iter()
-            .cloned()
-            .map(|name| (name.ident().span, name, hir::LifetimeName::Param(name)))
-            .chain(
-                self.lifetimes_to_define
-                    .iter()
-                    .map(|&(span, name)| (span, name, hir::LifetimeName::Param(name))),
-            )
-            .collect();
-
+        let mut lifetime_params = Vec::new();
         self.with_hir_id_owner(opaque_ty_node_id, |this| {
             // We have to be careful to get elision right here. The
             // idea is that we create a lifetime parameter for each
@@ -1725,16 +1704,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             debug!("lower_async_fn_ret_ty: future_bound={:#?}", future_bound);
             debug!("lower_async_fn_ret_ty: lifetimes_to_define={:#?}", lifetimes_to_define);
 
-            lifetime_params.extend(
-                // Output lifetime like `'_`:
-                lifetimes_to_define
-                    .into_iter()
-                    .map(|(span, name)| (span, name, hir::LifetimeName::Implicit)),
-            );
+            // Output lifetime like `'_`:
+            lifetime_params = lifetimes_to_define;
             debug!("lower_async_fn_ret_ty: lifetime_params={:#?}", lifetime_params);
 
             let generic_params =
-                this.arena.alloc_from_iter(lifetime_params.iter().map(|&(span, hir_name, _)| {
+                this.arena.alloc_from_iter(lifetime_params.iter().map(|&(span, hir_name)| {
                     this.lifetime_to_generic_param(span, hir_name, opaque_ty_def_id)
                 }));
 
@@ -1752,28 +1727,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             this.generate_opaque_type(opaque_ty_def_id, opaque_ty_item, span, opaque_ty_span)
         });
 
-        // As documented above on the variable
-        // `input_lifetimes_count`, we need to create the lifetime
-        // arguments to our opaque type. Continuing with our example,
-        // we're creating the type arguments for the return type:
+        // We need to create the lifetime arguments to our opaque type.
+        // Continuing with our example, we're creating the type arguments
+        // for the return type:
         //
         // ```
-        // Bar<'a, 'b, '0, '1, '_>
+        // For<'a>::bar<'b, '0, '1>::Bar<'_>
         // ```
         //
-        // For the "input" lifetime parameters, we wish to create
-        // references to the parameters themselves, including the
-        // "implicit" ones created from parameter types (`'a`, `'b`,
-        // '`0`, `'1`).
-        //
-        // For the "output" lifetime parameters, we just want to
-        // generate `'_`.
+        // For the "input" lifetime parameters are inherited automatically.
+        // For the "output" lifetime parameters, we just want to generate `'_`.
         let generic_args =
-            self.arena.alloc_from_iter(lifetime_params.into_iter().map(|(span, _, name)| {
+            self.arena.alloc_from_iter(lifetime_params.into_iter().map(|(span, _)| {
                 GenericArg::Lifetime(hir::Lifetime {
                     hir_id: self.next_id(),
                     span: self.lower_span(span),
-                    name,
+                    name: hir::LifetimeName::Implicit,
                 })
             }));
 

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -276,7 +276,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         debug!(?concrete_ty);
 
         let first_own_region = match opaque_defn.origin {
-            hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
+            hir::OpaqueTyOrigin::FnReturn(..) => {
                 // We lower
                 //
                 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
@@ -291,7 +291,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
             // These opaque type inherit all lifetime parameters from their
             // parent, so we have to check them all.
-            hir::OpaqueTyOrigin::TyAlias => 0,
+            hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::TyAlias => 0,
         };
 
         // For a case like `impl Foo<'a, 'b>`, we would generate a constraint

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -731,9 +731,16 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
         match item.kind {
             hir::ItemKind::Fn(ref sig, ref generics, _) => {
                 self.missing_named_lifetime_spots.push(generics.into());
-                self.visit_early_late(None, item.hir_id(), &sig.decl, generics, |this| {
-                    intravisit::walk_item(this, item);
-                });
+                self.visit_early_late(
+                    None,
+                    item.hir_id(),
+                    &sig.decl,
+                    generics,
+                    sig.header.asyncness,
+                    |this| {
+                        intravisit::walk_item(this, item);
+                    },
+                );
                 self.missing_named_lifetime_spots.pop();
             }
 
@@ -851,11 +858,16 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 
     fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem<'tcx>) {
         match item.kind {
-            hir::ForeignItemKind::Fn(ref decl, _, ref generics) => {
-                self.visit_early_late(None, item.hir_id(), decl, generics, |this| {
+            hir::ForeignItemKind::Fn(ref decl, _, ref generics) => self.visit_early_late(
+                None,
+                item.hir_id(),
+                decl,
+                generics,
+                hir::IsAsync::NotAsync,
+                |this| {
                     intravisit::walk_foreign_item(this, item);
-                })
-            }
+                },
+            ),
             hir::ForeignItemKind::Static(..) => {
                 intravisit::walk_foreign_item(self, item);
             }
@@ -1141,6 +1153,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     trait_item.hir_id(),
                     &sig.decl,
                     &trait_item.generics,
+                    sig.header.asyncness,
                     |this| intravisit::walk_trait_item(this, trait_item),
                 );
                 self.missing_named_lifetime_spots.pop();
@@ -1210,6 +1223,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     impl_item.hir_id(),
                     &sig.decl,
                     &impl_item.generics,
+                    sig.header.asyncness,
                     |this| intravisit::walk_impl_item(this, impl_item),
                 );
                 self.missing_named_lifetime_spots.pop();
@@ -2187,11 +2201,15 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         hir_id: hir::HirId,
         decl: &'tcx hir::FnDecl<'tcx>,
         generics: &'tcx hir::Generics<'tcx>,
+        asyncness: hir::IsAsync,
         walk: F,
     ) where
         F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
     {
-        insert_late_bound_lifetimes(self.map, decl, generics);
+        // Async fns need all their lifetime parameters to be early bound.
+        if asyncness != hir::IsAsync::Async {
+            insert_late_bound_lifetimes(self.map, decl, generics);
+        }
 
         // Find the start of nested early scopes, e.g., in methods.
         let mut next_early_index = 0;

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2337,16 +2337,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 let def_id = item_id.def_id.to_def_id();
 
                 match opaque_ty.kind {
-                    hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => self
-                        .impl_trait_ty_to_ty(
-                            def_id,
-                            lifetimes,
-                            matches!(
-                                origin,
-                                hir::OpaqueTyOrigin::FnReturn(..)
-                                    | hir::OpaqueTyOrigin::AsyncFn(..)
-                            ),
-                        ),
+                    hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => {
+                        let replace_parent_lifetimes =
+                            matches!(origin, hir::OpaqueTyOrigin::FnReturn(..));
+                        self.impl_trait_ty_to_ty(def_id, lifetimes, replace_parent_lifetimes)
+                    }
                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
                 }
             }

compiler/rustc_typeck/src/check/check.rs

@@ -540,10 +540,8 @@ pub(super) fn check_opaque_for_inheriting_lifetimes(
         }
     }
 
-    if let ItemKind::OpaqueTy(hir::OpaqueTy {
-        origin: hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::FnReturn(..),
-        ..
-    }) = item.kind
+    if let ItemKind::OpaqueTy(hir::OpaqueTy { origin: hir::OpaqueTyOrigin::FnReturn(..), .. }) =
+        item.kind
     {
         let mut visitor = ProhibitOpaqueVisitor {
             opaque_identity_ty: tcx.mk_opaque(
@@ -565,20 +563,13 @@ pub(super) fn check_opaque_for_inheriting_lifetimes(
 
         if let Some(ty) = prohibit_opaque.break_value() {
             visitor.visit_item(&item);
-            let is_async = match item.kind {
-                ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => {
-                    matches!(origin, hir::OpaqueTyOrigin::AsyncFn(..))
-                }
-                _ => unreachable!(),
-            };
 
             let mut err = struct_span_err!(
                 tcx.sess,
                 span,
                 E0760,
-                "`{}` return type cannot contain a projection or `Self` that references lifetimes from \
+                "`impl Trait` return type cannot contain a projection or `Self` that references lifetimes from \
                  a parent scope",
-                if is_async { "async fn" } else { "impl Trait" },
             );
 
             for (span, name) in visitor.selftys {

compiler/rustc_typeck/src/collect.rs

@@ -2063,8 +2063,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
                     generics
                 }
                 ItemKind::OpaqueTy(OpaqueTy {
-                    origin: hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::FnReturn(..),
-                    ..
+                    origin: hir::OpaqueTyOrigin::FnReturn(..), ..
                 }) => {
                     // return-position impl trait
                     //
@@ -2084,7 +2083,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
                 }
                 ItemKind::OpaqueTy(OpaqueTy {
                     ref generics,
-                    origin: hir::OpaqueTyOrigin::TyAlias,
+                    origin: hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::TyAlias,
                     ..
                 }) => {
                     // type-alias impl trait

src/test/ui/async-await/issue-61949-self-return-type.rs

@@ -8,7 +8,6 @@ pub struct Foo<'a> {
 
 impl<'a> Foo<'a> {
     pub async fn new(_bar: &'a i32) -> Self {
-    //~^ ERROR `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
         Foo {
             bar: &22
         }
@@ -19,6 +18,7 @@ async fn foo() {
     let x = {
         let bar = 22;
         Foo::new(&bar).await
+        //~^ ERROR `bar` does not live long enough [E0597]
     };
     drop(x);
 }

src/test/ui/async-await/issue-61949-self-return-type.stderr

@@ -1,9 +1,15 @@
-error[E0760]: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
-  --> $DIR/issue-61949-self-return-type.rs:10:40
+error[E0597]: `bar` does not live long enough
+  --> $DIR/issue-61949-self-return-type.rs:20:18
    |
-LL |     pub async fn new(_bar: &'a i32) -> Self {
-   |                                        ^^^^ help: consider spelling out the type instead: `Foo<'a>`
+LL |     let x = {
+   |         - borrow later stored here
+LL |         let bar = 22;
+LL |         Foo::new(&bar).await
+   |                  ^^^^ borrowed value does not live long enough
+LL |
+LL |     };
+   |     - `bar` dropped here while still borrowed
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0760`.
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/async-await/issue-74072-lifetime-name-annotations.stderr

@@ -1,14 +1,10 @@
 error[E0506]: cannot assign to `*x` because it is borrowed
   --> $DIR/issue-74072-lifetime-name-annotations.rs:9:5
    |
-LL | pub async fn async_fn(x: &mut i32) -> &i32 {
-   |                          - let's call the lifetime of this reference `'1`
 LL |     let y = &*x;
    |             --- borrow of `*x` occurs here
 LL |     *x += 1;
    |     ^^^^^^^ assignment to borrowed `*x` occurs here
-LL |     y
-   |     - returning this value requires that `*x` is borrowed for `'1`
 
 error[E0506]: cannot assign to `*x` because it is borrowed
   --> $DIR/issue-74072-lifetime-name-annotations.rs:16:9

src/test/ui/async-await/issue-75785-confusing-named-region.stderr

@@ -1,14 +1,10 @@
 error[E0506]: cannot assign to `*x` because it is borrowed
   --> $DIR/issue-75785-confusing-named-region.rs:9:5
    |
-LL | pub async fn async_fn(x: &mut i32) -> (&i32, &i32) {
-   |                          - let's call the lifetime of this reference `'1`
 LL |     let y = &*x;
    |             --- borrow of `*x` occurs here
 LL |     *x += 1;
    |     ^^^^^^^ assignment to borrowed `*x` occurs here
-LL |     (&32, y)
-   |     -------- returning this value requires that `*x` is borrowed for `'1`
 
 error: aborting due to previous error