Don't mark generators as !Send if a Copy + !Send value is dropped across a yield

Copy types can neither read nor write their values when dropped (the language guarantees Drop is a no-op).
So it doesn't make sense for them to make the generator non-Send.

Author: jyn514

compiler/rustc_typeck/src/check/generator_interior.rs

@@ -17,6 +17,7 @@ use rustc_middle::middle::region::{self, Scope, ScopeData, YieldData};
 use rustc_middle::ty::{self, RvalueScopes, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
+use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::debug;
 
 mod drop_ranges;
@@ -51,6 +52,19 @@ impl<'a, 'tcx> InteriorVisitor<'a, 'tcx> {
             ty, hir_id, scope, expr, source_span, self.expr_count,
         );
 
+        if self.fcx.type_is_copy_modulo_regions(self.fcx.param_env, ty, source_span) {
+            // This is only used because it's dropped after the yield.
+            // But it's a Copy type, so it's not possible for the drop to read or write the value.
+            // Ignore it instead of giving an error.
+            if matches!(
+                scope,
+                Some(region::Scope { data: region::ScopeData::Remainder { .. }, .. })
+            ) {
+                debug!("ignoring Copy type only used in drop");
+                return;
+            }
+        }
+
         let live_across_yield = scope
             .map(|s| {
                 self.region_scope_tree.yield_in_scope(s).and_then(|yield_data| {

src/test/ui/async-await/trivial-drop.rs

@@ -0,0 +1,12 @@
+// edition:2018
+// build-pass
+async fn trivial_drop() {
+    let x: *const usize = &0;
+    async {}.await;
+}
+
+fn assert_send<T: Send>(_: T) {}
+
+fn main() {
+    assert_send(trivial_drop());
+}

src/test/ui/generator/trivial-drop-non-copy.rs

@@ -0,0 +1,15 @@
+#![feature(generators, negative_impls)]
+
+fn assert_send<T: Send>(_: T) {}
+
+struct S;
+impl !Send for S {}
+
+fn main() {
+    println!("{}", std::mem::needs_drop::<S>());
+    let g = || {
+        let x = S; //~ type `S`
+        yield; //~ `x` maybe used later
+    };
+    assert_send(g); //~ ERROR generator cannot be sent between threads
+}

src/test/ui/generator/trivial-drop-non-copy.stderr

@@ -0,0 +1,24 @@
+error: generator cannot be sent between threads safely
+  --> $DIR/trivial-drop-non-copy.rs:14:5
+   |
+LL |     assert_send(g);
+   |     ^^^^^^^^^^^ generator is not `Send`
+   |
+   = help: within `[generator@$DIR/trivial-drop-non-copy.rs:10:13: 10:15]`, the trait `Send` is not implemented for `S`
+note: generator is not `Send` as this value is used across a yield
+  --> $DIR/trivial-drop-non-copy.rs:12:9
+   |
+LL |         let x = S;
+   |             - has type `S` which is not `Send`
+LL |         yield;
+   |         ^^^^^ yield occurs here, with `x` maybe used later
+LL |     };
+   |     - `x` is later dropped here
+note: required by a bound in `assert_send`
+  --> $DIR/trivial-drop-non-copy.rs:3:19
+   |
+LL | fn assert_send<T: Send>(_: T) {}
+   |                   ^^^^ required by this bound in `assert_send`
+
+error: aborting due to previous error
+

src/test/ui/generator/trivial-drop.rs

@@ -0,0 +1,12 @@
+// build-pass
+#![feature(generators)]
+
+fn assert_send<T: Send>(_: T) {}
+
+fn main() {
+    let g = || {
+        let x: *const usize = &0;
+        yield;
+    };
+    assert_send(g);
+}