Avoid unused lifetime warning for lifetimes introduced when desugering async.

src/librustc/middle/resolve_lifetime.rs

@@ -709,15 +709,22 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     match param.kind {
                         GenericParamKind::Lifetime { .. } => {
                             let (name, reg) = Region::early(&self.tcx.hir(), &mut index, &param);
+                            let def_id = if let Region::EarlyBound(_ ,def_id , _) = reg {
+                                def_id
+                            } else {
+                                bug!();
+                            };
                             if let hir::ParamName::Plain(param_name) = name {
                                 if param_name.name == kw::UnderscoreLifetime {
                                     // Pick the elided lifetime "definition" if one exists
                                     // and use it to make an elision scope.
+                                    self.lifetime_uses.insert(def_id.clone(), LifetimeUseSet::Many);
                                     elision = Some(reg);
                                 } else {
                                     lifetimes.insert(name, reg);
                                 }
                             } else {
+                                self.lifetime_uses.insert(def_id.clone(), LifetimeUseSet::Many);
                                 lifetimes.insert(name, reg);
                             }
                         }
@@ -1623,7 +1630,6 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                         _ => None,
                     } {
                         debug!("id = {:?} span = {:?} name = {:?}", id, span, name);
-
                         if name.name == kw::UnderscoreLifetime {
                             continue;
                         }

src/test/ui/async-await/unused-lifetime.rs

@@ -0,0 +1,42 @@
+// edition:2018
+
+// Avoid spurious warnings of unused lifetime. The below async functions
+// are desugered to have an unused lifetime
+// but we don't want to warn about that as there's nothing they can do about it.
+
+#![deny(unused_lifetimes)]
+#![allow(dead_code)]
+
+pub async fn october(s: &str) {
+    println!("{}", s);
+}
+
+pub async fn async_fn(&mut ref s: &mut[i32]) {
+    println!("{:?}", s);
+}
+
+macro_rules! foo_macro {
+    () => {
+        pub async fn async_fn_in_macro(&mut ref _s: &mut[i32]) {}
+    };
+}
+
+foo_macro!();
+
+pub async fn func_with_unused_lifetime<'a>(s: &'a str) {
+    //~^ ERROR lifetime parameter `'a` never used
+    println!("{}", s);
+}
+
+pub async fn func_with_two_unused_lifetime<'a, 'b>(s: &'a str, t: &'b str) {
+    //~^ ERROR lifetime parameter `'a` never used
+    //~^^ ERROR lifetime parameter `'b` never used
+    println!("{}", s);
+}
+
+pub async fn func_with_unused_lifetime_in_two_params<'c>(s: &'c str, t: &'c str) {
+    //~^ ERROR lifetime parameter `'c` never used
+    println!("{}", s);
+}
+
+fn main() {}

src/test/ui/async-await/unused-lifetime.stderr

@@ -0,0 +1,32 @@
+error: lifetime parameter `'a` never used
+  --> $DIR/unused-lifetime.rs:26:40
+   |
+LL | pub async fn func_with_unused_lifetime<'a>(s: &'a str) {
+   |                                        ^^
+   |
+note: lint level defined here
+  --> $DIR/unused-lifetime.rs:7:9
+   |
+LL | #![deny(unused_lifetimes)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: lifetime parameter `'a` never used
+  --> $DIR/unused-lifetime.rs:31:44
+   |
+LL | pub async fn func_with_two_unused_lifetime<'a, 'b>(s: &'a str, t: &'b str) {
+   |                                            ^^
+
+error: lifetime parameter `'b` never used
+  --> $DIR/unused-lifetime.rs:31:48
+   |
+LL | pub async fn func_with_two_unused_lifetime<'a, 'b>(s: &'a str, t: &'b str) {
+   |                                                ^^
+
+error: lifetime parameter `'c` never used
+  --> $DIR/unused-lifetime.rs:37:54
+   |
+LL | pub async fn func_with_unused_lifetime_in_two_params<'c>(s: &'c str, t: &'c str) {
+   |                                                      ^^
+
+error: aborting due to 4 previous errors
+