Refactor async fn return type lowering

async fn now lowers directly to an existential type declaration
rather than reusing the `impl Trait` return type lowering.

As part of this, it lowers all argument-position elided lifetimes
using the in-band-lifetimes machinery, creating fresh parameter
names for each of them, using each lifetime parameter as a generic
argument to the generated existential type.

This doesn't currently successfully allow multiple
argument-position elided lifetimes since `existential type`
doesn't yet support multiple lifetimes where neither outlive
the other. This requires a separate fix.

Author: cramertj

src/librustc/hir/intravisit.rs

@@ -490,7 +490,11 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item) {
             visitor.visit_ty(ty);
             visitor.visit_generics(generics)
         }
-        ItemKind::Existential(ExistTy { ref generics, ref bounds, impl_trait_fn: _ }) => {
+        ItemKind::Existential(ExistTy {
+            ref generics,
+            ref bounds,
+            ..
+        }) => {
             visitor.visit_id(item.hir_id);
             walk_generics(visitor, generics);
             walk_list!(visitor, visit_param_bound, bounds);

src/librustc/hir/lowering.rs

@@ -1799,6 +1799,18 @@ pub struct ExistTy {
     pub generics: Generics,
     pub bounds: GenericBounds,
     pub impl_trait_fn: Option<DefId>,
+    pub origin: ExistTyOrigin,
+}
+
+/// Where the existential type came from
+#[derive(Copy, Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
+pub enum ExistTyOrigin {
+    /// `existential type Foo: Trait;`
+    ExistentialType,
+    /// `-> impl Trait`
+    ReturnImplTrait,
+    /// `async fn`
+    AsyncFn,
 }
 
 /// The various kinds of types recognized by the compiler.

src/librustc/hir/mod.rs

@@ -67,6 +67,9 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// the fn body). (Ultimately, writeback is responsible for this
     /// check.)
     pub has_required_region_bounds: bool,
+
+    /// The origin of the existential type
+    pub origin: hir::ExistTyOrigin,
 }
 
 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
@@ -326,14 +329,39 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                         // There are two regions (`lr` and
                         // `subst_arg`) which are not relatable. We can't
                         // find a best choice.
-                        self.tcx
+                        let context_name = match opaque_defn.origin {
+                            hir::ExistTyOrigin::ExistentialType => "existential type",
+                            hir::ExistTyOrigin::ReturnImplTrait => "impl Trait",
+                            hir::ExistTyOrigin::AsyncFn => "async fn",
+                        };
+                        let msg = format!("ambiguous lifetime bound in `{}`", context_name);
+                        let mut err = self.tcx
                             .sess
-                            .struct_span_err(span, "ambiguous lifetime bound in `impl Trait`")
-                            .span_label(
-                                span,
-                                format!("neither `{}` nor `{}` outlives the other", lr, subst_arg),
-                            )
-                            .emit();
+                            .struct_span_err(span, &msg);
+
+                        let lr_name = lr.to_string();
+                        let subst_arg_name = subst_arg.to_string();
+                        let label_owned;
+                        let label = match (&*lr_name, &*subst_arg_name) {
+                            ("'_", "'_") => "the elided lifetimes here do not outlive one another",
+                            _ => {
+                                label_owned = format!(
+                                    "neither `{}` nor `{}` outlives the other",
+                                    lr_name,
+                                    subst_arg_name,
+                                );
+                                &label_owned
+                            }
+                        };
+                        err.span_label(span, label);
+
+                        if let hir::ExistTyOrigin::AsyncFn = opaque_defn.origin {
+                            err.note("multiple unrelated lifetimes are not allowed in \
+                                     `async fn`.");
+                            err.note("if you're using argument-position elided lifetimes, consider \
+                                switching to a single named lifetime.");
+                        }
+                        err.emit();
 
                         least_region = Some(self.tcx.mk_region(ty::ReEmpty));
                         break;
@@ -692,39 +720,49 @@ impl<'a, 'gcx, 'tcx> Instantiator<'a, 'gcx, 'tcx> {
                             parent_def_id == tcx.hir()
                                                 .local_def_id_from_hir_id(opaque_parent_hir_id)
                         };
-                        let in_definition_scope = match tcx.hir().find_by_hir_id(opaque_hir_id) {
+                        let (in_definition_scope, origin) =
+                            match tcx.hir().find_by_hir_id(opaque_hir_id)
+                        {
                             Some(Node::Item(item)) => match item.node {
                                 // impl trait
                                 hir::ItemKind::Existential(hir::ExistTy {
                                     impl_trait_fn: Some(parent),
+                                    origin,
                                     ..
-                                }) => parent == self.parent_def_id,
+                                }) => (parent == self.parent_def_id, origin),
                                 // named existential types
                                 hir::ItemKind::Existential(hir::ExistTy {
                                     impl_trait_fn: None,
+                                    origin,
                                     ..
-                                }) => may_define_existential_type(
-                                    tcx,
-                                    self.parent_def_id,
-                                    opaque_hir_id,
+                                }) => (
+                                    may_define_existential_type(
+                                        tcx,
+                                        self.parent_def_id,
+                                        opaque_hir_id,
+                                    ),
+                                    origin,
                                 ),
-                                _ => def_scope_default(),
+                                _ => (def_scope_default(), hir::ExistTyOrigin::ExistentialType),
                             },
                             Some(Node::ImplItem(item)) => match item.node {
-                                hir::ImplItemKind::Existential(_) => may_define_existential_type(
-                                    tcx,
-                                    self.parent_def_id,
-                                    opaque_hir_id,
+                                hir::ImplItemKind::Existential(_) => (
+                                    may_define_existential_type(
+                                        tcx,
+                                        self.parent_def_id,
+                                        opaque_hir_id,
+                                    ),
+                                    hir::ExistTyOrigin::ExistentialType,
                                 ),
-                                _ => def_scope_default(),
+                                _ => (def_scope_default(), hir::ExistTyOrigin::ExistentialType),
                             },
                             _ => bug!(
                                 "expected (impl) item, found {}",
                                 tcx.hir().hir_to_string(opaque_hir_id),
                             ),
                         };
                         if in_definition_scope {
-                            return self.fold_opaque_ty(ty, def_id, substs);
+                            return self.fold_opaque_ty(ty, def_id, substs, origin);
                         }
 
                         debug!(
@@ -746,6 +784,7 @@ impl<'a, 'gcx, 'tcx> Instantiator<'a, 'gcx, 'tcx> {
         ty: Ty<'tcx>,
         def_id: DefId,
         substs: SubstsRef<'tcx>,
+        origin: hir::ExistTyOrigin,
     ) -> Ty<'tcx> {
         let infcx = self.infcx;
         let tcx = infcx.tcx;
@@ -795,6 +834,7 @@ impl<'a, 'gcx, 'tcx> Instantiator<'a, 'gcx, 'tcx> {
                 substs,
                 concrete_ty: ty_var,
                 has_required_region_bounds: !required_region_bounds.is_empty(),
+                origin,
             },
         );
         debug!("instantiate_opaque_types: ty_var={:?}", ty_var);

src/librustc/infer/opaque_types/mod.rs

@@ -2891,7 +2891,7 @@ fn insert_late_bound_lifetimes(
     }
 }
 
-fn report_missing_lifetime_specifiers(
+pub fn report_missing_lifetime_specifiers(
     sess: &Session,
     span: Span,
     count: usize,

src/librustc/middle/resolve_lifetime.rs

@@ -1979,6 +1979,7 @@ fn explicit_predicates_of<'a, 'tcx>(
                     ref bounds,
                     impl_trait_fn,
                     ref generics,
+                    origin: _,
                 }) => {
                     let substs = InternalSubsts::identity_for_item(tcx, def_id);
                     let opaque_ty = tcx.mk_opaque(def_id, substs);

src/librustc_typeck/collect.rs

@@ -79,6 +79,11 @@ async fn async_fn(x: u8) -> u8 {
     x
 }
 
+async fn generic_async_fn<T>(x: T) -> T {
+    await!(wake_and_yield_once());
+    x
+}
+
 async fn async_fn_with_borrow(x: &u8) -> u8 {
     await!(wake_and_yield_once());
     *x
@@ -96,14 +101,21 @@ fn async_fn_with_impl_future_named_lifetime<'a>(x: &'a u8) -> impl Future<Output
     }
 }
 
-async fn async_fn_with_named_lifetime_multiple_args<'a>(x: &'a u8, _y: &'a u8) -> u8 {
+/* FIXME(cramertj) support when `existential type T<'a, 'b>:;` works
+async fn async_fn_multiple_args(x: &u8, _y: &u8) -> u8 {
+    await!(wake_and_yield_once());
+    *x
+}
+*/
+
+async fn async_fn_multiple_args_named_lifetime<'a>(x: &'a u8, _y: &'a u8) -> u8 {
     await!(wake_and_yield_once());
     *x
 }
 
 fn async_fn_with_internal_borrow(y: u8) -> impl Future<Output = u8> {
     async move {
-        await!(async_fn_with_borrow(&y))
+        await!(async_fn_with_borrow_named_lifetime(&y))
     }
 }
 
@@ -162,6 +174,7 @@ fn main() {
         async_nonmove_block,
         async_closure,
         async_fn,
+        generic_async_fn,
         async_fn_with_internal_borrow,
         Foo::async_method,
         |x| {
@@ -170,15 +183,14 @@ fn main() {
             }
         },
     }
-
     test_with_borrow! {
         async_block_with_borrow_named_lifetime,
         async_fn_with_borrow,
         async_fn_with_borrow_named_lifetime,
         async_fn_with_impl_future_named_lifetime,
         |x| {
             async move {
-                await!(async_fn_with_named_lifetime_multiple_args(x, x))
+                await!(async_fn_multiple_args_named_lifetime(x, x))
             }
         },
     }

src/test/run-pass/async-await.rs

@@ -5,7 +5,7 @@
 use std::ops::Add;
 
 async fn multiple_named_lifetimes<'a, 'b>(_: &'a u8, _: &'b u8) {}
-//~^ ERROR multiple different lifetimes used in arguments of `async fn`
+//~^ ERROR ambiguous lifetime bound in `async fn`
 
 async fn multiple_hrtb_and_single_named_lifetime_ok<'c>(
     _: impl for<'a> Add<&'a u8>,
@@ -14,7 +14,6 @@ async fn multiple_hrtb_and_single_named_lifetime_ok<'c>(
 ) {}
 
 async fn multiple_elided_lifetimes(_: &u8, _: &u8) {}
-//~^ ERROR multiple elided lifetimes used
-//~^^ ERROR missing lifetime specifier
+//~^ ambiguous lifetime bound in `async fn`
 
 fn main() {}

src/test/ui/async-fn-multiple-lifetimes.rs

@@ -1,32 +1,20 @@
-error[E0709]: multiple different lifetimes used in arguments of `async fn`
-  --> $DIR/async-fn-multiple-lifetimes.rs:7:47
+error: ambiguous lifetime bound in `async fn`
+  --> $DIR/async-fn-multiple-lifetimes.rs:7:65
    |
 LL | async fn multiple_named_lifetimes<'a, 'b>(_: &'a u8, _: &'b u8) {}
-   |                                               ^^         ^^ different lifetime here
-   |                                               |
-   |                                               first lifetime here
+   |                                                                 ^ neither `'a` nor `'b` outlives the other
    |
-   = help: `async fn` can only accept borrowed values with identical lifetimes
+   = note: multiple unrelated lifetimes are not allowed in `async fn`.
+   = note: if you're using argument-position elided lifetimes, consider switching to a single named lifetime.
 
-error[E0707]: multiple elided lifetimes used in arguments of `async fn`
-  --> $DIR/async-fn-multiple-lifetimes.rs:16:39
+error: ambiguous lifetime bound in `async fn`
+  --> $DIR/async-fn-multiple-lifetimes.rs:16:52
    |
 LL | async fn multiple_elided_lifetimes(_: &u8, _: &u8) {}
-   |                                       ^       ^ different lifetime here
-   |                                       |
-   |                                       first lifetime here
+   |                                                    ^ the elided lifetimes here do not outlive one another
    |
-   = help: consider giving these arguments named lifetimes
+   = note: multiple unrelated lifetimes are not allowed in `async fn`.
+   = note: if you're using argument-position elided lifetimes, consider switching to a single named lifetime.
 
-error[E0106]: missing lifetime specifier
-  --> $DIR/async-fn-multiple-lifetimes.rs:16:39
-   |
-LL | async fn multiple_elided_lifetimes(_: &u8, _: &u8) {}
-   |                                       ^ expected lifetime parameter
-   |
-   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `_` or `_`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors occurred: E0106, E0707, E0709.
-For more information about an error, try `rustc --explain E0106`.

src/test/ui/async-fn-multiple-lifetimes.stderr

@@ -0,0 +1,16 @@
+// compile-pass
+// edition:2018
+
+#![feature(async_await, await_macro, futures_api)]
+
+use std::sync::Arc;
+
+trait SomeTrait: Send + Sync + 'static {
+    fn do_something(&self);
+}
+
+async fn my_task(obj: Arc<SomeTrait>) {
+    unimplemented!()
+}
+
+fn main() {}