From d95a7768a1c5497c6ccb0c2ded30f5968552513c Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Wed, 18 Sep 2019 14:13:36 -0400
Subject: [PATCH] don't record all intermediate adjustment types

That's way more than is needed, and winds up recording types
that will never appear in MIR.
---
 .../check/generator_interior.rs               | 33 +++++++++++++++----
 src/test/ui/async-await/issues/issue-64477.rs | 20 +++++++++++
 2 files changed, 47 insertions(+), 6 deletions(-)
 create mode 100644 src/test/ui/async-await/issues/issue-64477.rs

diff --git a/src/librustc_typeck/check/generator_interior.rs b/src/librustc_typeck/check/generator_interior.rs
index 5cee3d63ffc..a7c307fdf89 100644
--- a/src/librustc_typeck/check/generator_interior.rs
+++ b/src/librustc_typeck/check/generator_interior.rs
@@ -181,13 +181,34 @@ impl<'a, 'tcx> Visitor<'tcx> for InteriorVisitor<'a, 'tcx> {
 
         let scope = self.region_scope_tree.temporary_scope(expr.hir_id.local_id);
 
-        // Record the unadjusted type
+        // If there are adjustments, then record the final type --
+        // this is the actual value that is being produced.
+        if let Some(adjusted_ty) = self.fcx.tables.borrow().expr_ty_adjusted_opt(expr) {
+            self.record(adjusted_ty, scope, Some(expr), expr.span);
+        }
+
+        // Also record the unadjusted type (which is the only type if
+        // there are no adjustments). The reason for this is that the
+        // unadjusted value is sometimes a "temporary" that would wind
+        // up in a MIR temporary.
+        //
+        // As an example, consider an expression like `vec![].push()`.
+        // Here, the `vec![]` would wind up MIR stored into a
+        // temporary variable `t` which we can borrow to invoke
+        // `<Vec<_>>::push(&mut t)`.
+        //
+        // Note that an expression can have many adjustments, and we
+        // are just ignoring those intermediate types. This is because
+        // those intermediate values are always linearly "consumed" by
+        // the other adjustments, and hence would never be directly
+        // captured in the MIR.
+        //
+        // (Note that this partly relies on the fact that the `Deref`
+        // traits always return references, which means their content
+        // can be reborrowed without needing to spill to a temporary.
+        // If this were not the case, then we could conceivably have
+        // to create intermediate temporaries.)
         let ty = self.fcx.tables.borrow().expr_ty(expr);
         self.record(ty, scope, Some(expr), expr.span);
-
-        // Also include the adjusted types, since these can result in MIR locals
-        for adjustment in self.fcx.tables.borrow().expr_adjustments(expr) {
-            self.record(adjustment.target, scope, Some(expr), expr.span);
-        }
     }
 }
diff --git a/src/test/ui/async-await/issues/issue-64477.rs b/src/test/ui/async-await/issues/issue-64477.rs
new file mode 100644
index 00000000000..5bd52d44a58
--- /dev/null
+++ b/src/test/ui/async-await/issues/issue-64477.rs
@@ -0,0 +1,20 @@
+// Regression test for #64477.
+//
+// We were incorrectly claiming that the `f(x).await` future captured
+// a value of type `T`, and hence that `T: Send` would have to hold.
+//
+// check-pass
+// edition:2018
+
+use std::future::Future;
+use std::pin::Pin;
+
+fn f<T>(_: &T) -> Pin<Box<dyn Future<Output = ()> + Send>> {
+    unimplemented!()
+}
+
+pub fn g<T: Sync>(x: &'static T) -> impl Future<Output = ()> + Send {
+    async move { f(x).await }
+}
+
+fn main() { }