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, '_>
```

6 files changed, 112 insertions, 0 deletions

diff --git a/src/test/ui/async-await/issues/issue-63388-1.rs b/src/test/ui/async-await/issues/issue-63388-1.rs
new file mode 100644
index 00000000000..80003b0d701
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/async-await/issues/issue-63388-1.stderr b/src/test/ui/async-await/issues/issue-63388-1.stderr
new file mode 100644
index 00000000000..5302adce5a0
--- /dev/null
+++ b/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
+
diff --git a/src/test/ui/async-await/issues/issue-63388-2.rs b/src/test/ui/async-await/issues/issue-63388-2.rs
new file mode 100644
index 00000000000..ca9bbef0d50
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/async-await/issues/issue-63388-2.stderr b/src/test/ui/async-await/issues/issue-63388-2.stderr
new file mode 100644
index 00000000000..1810138dc80
--- /dev/null
+++ b/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`.
diff --git a/src/test/ui/async-await/issues/issue-63388-3.rs b/src/test/ui/async-await/issues/issue-63388-3.rs
new file mode 100644
index 00000000000..05f23f95965
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/async-await/issues/issue-63388-4.rs b/src/test/ui/async-await/issues/issue-63388-4.rs
new file mode 100644
index 00000000000..0939242d7fc
--- /dev/null
+++ b/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() { }