revamp how we handle elision in async fn

We now always make fresh lifetimne parameters for all elided
lifetimes, whether they are in the inputs or outputs. But then
we generate `'_` in the case of elided lifetimes from the outputs.

Example:

```rust
async fn foo<'a>(x: &'a u32) -> &u32 { .. }
```

becomes

```rust
type Foo<'a, 'b> = impl Future<Output = &'b u32>;
fn foo<'a>(x: &'a u32) -> Foo<'a, '_>
```

src/librustc/hir/lowering.rs

@@ -341,49 +341,6 @@ enum AnonymousLifetimeMode {
 
     /// Pass responsibility to `resolve_lifetime` code for all cases.
     PassThrough,
-
-    /// Used in the return types of `async fn` where there exists
-    /// exactly one argument-position elided lifetime.
-    ///
-    /// In `async fn`, we lower the arguments types using the `CreateParameter`
-    /// mode, meaning that non-`dyn` elided lifetimes are assigned a fresh name.
-    /// If any corresponding elided lifetimes appear in the output, we need to
-    /// replace them with references to the fresh name assigned to the corresponding
-    /// elided lifetime in the arguments.
-    ///
-    /// For **Modern cases**, replace the anonymous parameter with a
-    /// reference to a specific freshly-named lifetime that was
-    /// introduced in argument
-    ///
-    /// For **Dyn Bound** cases, pass responsibility to
-    /// `resole_lifetime` code.
-    Replace(LtReplacement),
-}
-
-/// The type of elided lifetime replacement to perform on `async fn` return types.
-#[derive(Copy, Clone)]
-enum LtReplacement {
-    /// Fresh name introduced by the single non-dyn elided lifetime
-    /// in the arguments of the async fn.
-    Some(ParamName),
-
-    /// There is no single non-dyn elided lifetime because no lifetimes
-    /// appeared in the arguments.
-    NoLifetimes,
-
-    /// There is no single non-dyn elided lifetime because multiple
-    /// lifetimes appeared in the arguments.
-    MultipleLifetimes,
-}
-
-/// Calculates the `LtReplacement` to use for elided lifetimes in the return
-/// type based on the fresh elided lifetimes introduced in argument position.
-fn get_elided_lt_replacement(arg_position_lifetimes: &[(Span, ParamName)]) -> LtReplacement {
-    match arg_position_lifetimes {
-        [] => LtReplacement::NoLifetimes,
-        [(_span, param)] => LtReplacement::Some(*param),
-        _ => LtReplacement::MultipleLifetimes,
-    }
 }
 
 struct ImplTraitTypeIdVisitor<'a> { ids: &'a mut SmallVec<[NodeId; 1]> }
@@ -2318,8 +2275,7 @@ impl<'a> LoweringContext<'a> {
                         err.emit();
                     }
                     AnonymousLifetimeMode::PassThrough |
-                    AnonymousLifetimeMode::ReportError |
-                    AnonymousLifetimeMode::Replace(_) => {
+                    AnonymousLifetimeMode::ReportError => {
                         self.sess.buffer_lint_with_diagnostic(
                             ELIDED_LIFETIMES_IN_PATHS,
                             CRATE_NODE_ID,
@@ -2515,7 +2471,6 @@ impl<'a> LoweringContext<'a> {
 
         // Remember how many lifetimes were already around so that we can
         // only look at the lifetime parameters introduced by the arguments.
-        let lifetime_count_before_args = self.lifetimes_to_define.len();
         let inputs = self.with_anonymous_lifetime_mode(lt_mode, |this| {
             decl.inputs
                 .iter()
@@ -2530,16 +2485,10 @@ impl<'a> LoweringContext<'a> {
         });
 
         let output = if let Some(ret_id) = make_ret_async {
-            // Calculate the `LtReplacement` to use for any return-position elided
-            // lifetimes based on the elided lifetime parameters introduced in the args.
-            let lt_replacement = get_elided_lt_replacement(
-                &self.lifetimes_to_define[lifetime_count_before_args..]
-            );
             self.lower_async_fn_ret_ty(
                 &decl.output,
                 in_band_ty_params.expect("`make_ret_async` but no `fn_def_id`").0,
                 ret_id,
-                lt_replacement,
             )
         } else {
             match decl.output {
@@ -2604,7 +2553,6 @@ impl<'a> LoweringContext<'a> {
         output: &FunctionRetTy,
         fn_def_id: DefId,
         opaque_ty_node_id: NodeId,
-        elided_lt_replacement: LtReplacement,
     ) -> hir::FunctionRetTy {
         let span = output.span();
 
@@ -2622,9 +2570,18 @@ impl<'a> LoweringContext<'a> {
 
         self.allocate_hir_id_counter(opaque_ty_node_id);
 
+        let input_lifetimes_count = self.in_scope_lifetimes.len() + self.lifetimes_to_define.len();
         let (opaque_ty_id, lifetime_params) = 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
+            // lifetime in the return type.  So, given a return type
+            // like `async fn foo(..) -> &[&u32]`, we lower to `impl
+            // Future<Output = &'1 [ &'2 u32 ]>`.
+            //
+            // Then, we will create `fn foo(..) -> Foo<'_, '_>`, and
+            // hence the elision takes place at the fn site.
             let future_bound = this.with_anonymous_lifetime_mode(
-                AnonymousLifetimeMode::Replace(elided_lt_replacement),
+                AnonymousLifetimeMode::CreateParameter,
                 |this| this.lower_async_fn_output_type_to_future_bound(
                     output,
                     fn_def_id,
@@ -2678,19 +2635,53 @@ impl<'a> LoweringContext<'a> {
             (opaque_ty_id, lifetime_params)
         });
 
-        let generic_args =
-            lifetime_params
-                .iter().cloned()
-                .map(|(span, hir_name)| {
-                    GenericArg::Lifetime(hir::Lifetime {
-                        hir_id: self.next_id(),
-                        span,
-                        name: hir::LifetimeName::Param(hir_name),
-                    })
+        // Create the generic lifetime arguments that we will supply
+        // to the opaque return type. Consider:
+        //
+        // ```rust
+        // async fn foo(x: &u32, ) -> &[&u32] { .. }
+        // ```
+        //
+        // Here, we would create something like:
+        //
+        // ```rust
+        // type Foo<'a, 'b, 'c> = impl Future<Output = &'a [&'b u32]>;
+        // fn foo<'a>(x: &'a u32) -> Foo<'a, '_, '_>
+        // ```
+        //
+        // Note that for the lifetimes which came from the input
+        // (`'a`, here), we supply them as arguments to the return
+        // type `Foo`. But for those lifetime parameters (`'b`, `'c`)
+        // that we created from the return type, we want to use `'_`
+        // in the return type, so as to trigger elision.
+        let mut generic_args: Vec<_> =
+            lifetime_params[..input_lifetimes_count]
+            .iter()
+            .map(|&(span, hir_name)| {
+                GenericArg::Lifetime(hir::Lifetime {
+                    hir_id: self.next_id(),
+                    span,
+                    name: hir::LifetimeName::Param(hir_name),
                 })
-                .collect();
+            })
+            .collect();
+        generic_args.extend(
+            lifetime_params[input_lifetimes_count..]
+            .iter()
+            .map(|&(span, _)| {
+                GenericArg::Lifetime(hir::Lifetime {
+                    hir_id: self.next_id(),
+                    span,
+                    name: hir::LifetimeName::Implicit,
+                })
+            })
+        );
 
-        let opaque_ty_ref = hir::TyKind::Def(hir::ItemId { id: opaque_ty_id }, generic_args);
+        // Create the `Foo<...>` refernece itself. Note that the `type
+        // Foo = impl Trait` is, internally, created as a child of the
+        // async fn, so the *type parameters* are inherited.  It's
+        // only the lifetime parameters that we must supply.
+        let opaque_ty_ref = hir::TyKind::Def(hir::ItemId { id: opaque_ty_id }, generic_args.into());
 
         hir::FunctionRetTy::Return(P(hir::Ty {
             node: opaque_ty_ref,
@@ -2786,11 +2777,6 @@ impl<'a> LoweringContext<'a> {
                     }
 
                     AnonymousLifetimeMode::ReportError => self.new_error_lifetime(Some(l.id), span),
-
-                    AnonymousLifetimeMode::Replace(replacement) => {
-                        let hir_id = self.lower_node_id(l.id);
-                        self.replace_elided_lifetime(hir_id, span, replacement)
-                    }
                 },
             ident => {
                 self.maybe_collect_in_band_lifetime(ident);
@@ -2813,39 +2799,6 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
-    /// Replace a return-position elided lifetime with the elided lifetime
-    /// from the arguments.
-    fn replace_elided_lifetime(
-        &mut self,
-        hir_id: hir::HirId,
-        span: Span,
-        replacement: LtReplacement,
-    ) -> hir::Lifetime {
-        let multiple_or_none = match replacement {
-            LtReplacement::Some(name) => {
-                return hir::Lifetime {
-                    hir_id,
-                    span,
-                    name: hir::LifetimeName::Param(name),
-                };
-            }
-            LtReplacement::MultipleLifetimes => "multiple",
-            LtReplacement::NoLifetimes => "none",
-        };
-
-        let mut err = crate::middle::resolve_lifetime::report_missing_lifetime_specifiers(
-            self.sess,
-            span,
-            1,
-        );
-        err.note(&format!(
-            "return-position elided lifetimes require exactly one \
-             input-position elided lifetime, found {}.", multiple_or_none));
-        err.emit();
-
-        hir::Lifetime { hir_id, span, name: hir::LifetimeName::Error }
-    }
-
     fn lower_generic_params(
         &mut self,
         params: &[GenericParam],
@@ -5791,10 +5744,6 @@ impl<'a> LoweringContext<'a> {
             AnonymousLifetimeMode::ReportError => self.new_error_lifetime(None, span),
 
             AnonymousLifetimeMode::PassThrough => self.new_implicit_lifetime(span),
-
-            AnonymousLifetimeMode::Replace(replacement) => {
-                self.new_replacement_lifetime(replacement, span)
-            }
         }
     }
 
@@ -5848,10 +5797,6 @@ impl<'a> LoweringContext<'a> {
             // This is the normal case.
             AnonymousLifetimeMode::PassThrough => self.new_implicit_lifetime(span),
 
-            AnonymousLifetimeMode::Replace(replacement) => {
-                self.new_replacement_lifetime(replacement, span)
-            }
-
             AnonymousLifetimeMode::ReportError => self.new_error_lifetime(None, span),
         }
     }
@@ -5883,25 +5828,11 @@ impl<'a> LoweringContext<'a> {
 
             // This is the normal case.
             AnonymousLifetimeMode::PassThrough => {}
-
-            // We don't need to do any replacement here as this lifetime
-            // doesn't refer to an elided lifetime elsewhere in the function
-            // signature.
-            AnonymousLifetimeMode::Replace(_) => {}
         }
 
         self.new_implicit_lifetime(span)
     }
 
-    fn new_replacement_lifetime(
-        &mut self,
-        replacement: LtReplacement,
-        span: Span,
-    ) -> hir::Lifetime {
-        let hir_id = self.next_id();
-        self.replace_elided_lifetime(hir_id, span, replacement)
-    }
-
     fn new_implicit_lifetime(&mut self, span: Span) -> hir::Lifetime {
         hir::Lifetime {
             hir_id: self.next_id(),

src/test/ui/async-await/issues/issue-63388-1.rs

@@ -0,0 +1,20 @@
+// edition:2018
+
+#![feature(async_await)]
+
+struct Xyz {
+    a: u64,
+}
+
+trait Foo {}
+
+impl Xyz {
+    async fn do_sth<'a>(
+        &'a self, foo: &dyn Foo
+    ) -> &dyn Foo //~ ERROR lifetime mismatch
+    {
+        foo
+    }
+}
+
+fn main() {}

src/test/ui/async-await/issues/issue-63388-1.stderr

@@ -0,0 +1,12 @@
+error[E0623]: lifetime mismatch
+  --> $DIR/issue-63388-1.rs:14:10
+   |
+LL |         &'a self, foo: &dyn Foo
+   |         -------- this parameter and the return type are declared with different lifetimes...
+LL |     ) -> &dyn Foo
+   |          ^^^^^^^^
+   |          |
+   |          ...but data from `foo` is returned here
+
+error: aborting due to previous error
+

src/test/ui/async-await/issues/issue-63388-2.rs

@@ -0,0 +1,20 @@
+// edition:2018
+
+#![feature(async_await)]
+
+struct Xyz {
+    a: u64,
+}
+
+trait Foo {}
+
+impl Xyz {
+    async fn do_sth<'a>(
+        foo: &dyn Foo, bar: &'a dyn Foo //~ ERROR cannot infer
+    ) -> &dyn Foo //~ ERROR missing lifetime specifier
+    {
+        foo
+    }
+}
+
+fn main() {}

src/test/ui/async-await/issues/issue-63388-2.stderr

@@ -0,0 +1,29 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/issue-63388-2.rs:14:10
+   |
+LL |     ) -> &dyn Foo
+   |          ^ help: consider using the named lifetime: `&'a`
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `foo` or `bar`
+
+error: cannot infer an appropriate lifetime
+  --> $DIR/issue-63388-2.rs:13:9
+   |
+LL |         foo: &dyn Foo, bar: &'a dyn Foo
+   |         ^^^ ...but this borrow...
+LL |     ) -> &dyn Foo
+   |          -------- this return type evaluates to the `'static` lifetime...
+   |
+note: ...can't outlive the lifetime '_ as defined on the method body at 13:14
+  --> $DIR/issue-63388-2.rs:13:14
+   |
+LL |         foo: &dyn Foo, bar: &'a dyn Foo
+   |              ^
+help: you can add a constraint to the return type to make it last less than `'static` and match the lifetime '_ as defined on the method body at 13:14
+   |
+LL |     ) -> &dyn Foo + '_
+   |          ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/async-await/issues/issue-63388-3.rs

@@ -0,0 +1,19 @@
+// edition:2018
+// check-pass
+
+#![feature(async_await)]
+
+struct Xyz {
+    a: u64,
+}
+
+trait Foo {}
+
+impl Xyz {
+    async fn do_sth(
+        &self, foo: &dyn Foo
+    ) {
+    }
+}
+
+fn main() {}

src/test/ui/async-await/issues/issue-63388-4.rs

@@ -0,0 +1,12 @@
+// check-pass
+// edition:2018
+
+#![feature(async_await)]
+
+struct A;
+
+impl A {
+    async fn foo(&self, f: &u32) -> &A { self }
+}
+
+fn main() { }