8 files changed, 181 insertions, 0 deletions
diff --git a/tests/ui/sync/mutexguard-sync.rs b/tests/ui/sync/mutexguard-sync.rs
new file mode 100644
index 000000000..b56418383
--- /dev/null
+++ b/tests/ui/sync/mutexguard-sync.rs
@@ -0,0 +1,13 @@
+// MutexGuard<Cell<i32>> must not be Sync, that would be unsound.
+use std::sync::Mutex;
+use std::cell::Cell;
+
+fn test_sync<T: Sync>(_t: T) {}
+
+fn main()
+{
+    let m = Mutex::new(Cell::new(0i32));
+    let guard = m.lock().unwrap();
+    test_sync(guard);
+    //~^ ERROR `Cell<i32>` cannot be shared between threads safely [E0277]
+}
diff --git a/tests/ui/sync/mutexguard-sync.stderr b/tests/ui/sync/mutexguard-sync.stderr
new file mode 100644
index 000000000..4dc557119
--- /dev/null
+++ b/tests/ui/sync/mutexguard-sync.stderr
@@ -0,0 +1,20 @@
+error[E0277]: `Cell<i32>` cannot be shared between threads safely
+  --> $DIR/mutexguard-sync.rs:11:15
+   |
+LL |     test_sync(guard);
+   |     --------- ^^^^^ `Cell<i32>` cannot be shared between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
+   = note: required for `MutexGuard<'_, Cell<i32>>` to implement `Sync`
+note: required by a bound in `test_sync`
+  --> $DIR/mutexguard-sync.rs:5:17
+   |
+LL | fn test_sync<T: Sync>(_t: T) {}
+   |                 ^^^^ required by this bound in `test_sync`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/sync/suggest-cell.rs b/tests/ui/sync/suggest-cell.rs
new file mode 100644
index 000000000..3284eae7b
--- /dev/null
+++ b/tests/ui/sync/suggest-cell.rs
@@ -0,0 +1,31 @@
+fn require_sync<T: Sync>() {}
+//~^ NOTE required by this bound in `require_sync`
+//~| NOTE required by this bound in `require_sync`
+//~| NOTE required by this bound in `require_sync`
+//~| NOTE required by this bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+
+fn main() {
+    require_sync::<std::cell::Cell<()>>();
+    //~^ ERROR `Cell<()>` cannot be shared between threads safely
+    //~| NOTE `Cell<()>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+
+    require_sync::<std::cell::Cell<u8>>();
+    //~^ ERROR `Cell<u8>` cannot be shared between threads safely
+    //~| NOTE `Cell<u8>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead
+
+    require_sync::<std::cell::Cell<i32>>();
+    //~^ ERROR `Cell<i32>` cannot be shared between threads safely
+    //~| NOTE `Cell<i32>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
+
+    require_sync::<std::cell::Cell<bool>>();
+    //~^ ERROR `Cell<bool>` cannot be shared between threads safely
+    //~| NOTE `Cell<bool>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead
+}
diff --git a/tests/ui/sync/suggest-cell.stderr b/tests/ui/sync/suggest-cell.stderr
new file mode 100644
index 000000000..c232c1ccd
--- /dev/null
+++ b/tests/ui/sync/suggest-cell.stderr
@@ -0,0 +1,59 @@
+error[E0277]: `Cell<()>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:12:20
+   |
+LL |     require_sync::<std::cell::Cell<()>>();
+   |                    ^^^^^^^^^^^^^^^^^^^ `Cell<()>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<()>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error[E0277]: `Cell<u8>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:17:20
+   |
+LL |     require_sync::<std::cell::Cell<u8>>();
+   |                    ^^^^^^^^^^^^^^^^^^^ `Cell<u8>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<u8>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error[E0277]: `Cell<i32>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:22:20
+   |
+LL |     require_sync::<std::cell::Cell<i32>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^ `Cell<i32>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error[E0277]: `Cell<bool>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:27:20
+   |
+LL |     require_sync::<std::cell::Cell<bool>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^ `Cell<bool>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<bool>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/sync/suggest-once-cell.rs b/tests/ui/sync/suggest-once-cell.rs
new file mode 100644
index 000000000..82fca45b1
--- /dev/null
+++ b/tests/ui/sync/suggest-once-cell.rs
@@ -0,0 +1,12 @@
+#![feature(once_cell)]
+
+fn require_sync<T: Sync>() {}
+//~^ NOTE required by this bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+
+fn main() {
+    require_sync::<std::cell::OnceCell<()>>();
+    //~^ ERROR `OnceCell<()>` cannot be shared between threads safely
+    //~| NOTE `OnceCell<()>` cannot be shared between threads safely
+    //~| NOTE use `std::sync::OnceLock` instead
+}
diff --git a/tests/ui/sync/suggest-once-cell.stderr b/tests/ui/sync/suggest-once-cell.stderr
new file mode 100644
index 000000000..fadf05374
--- /dev/null
+++ b/tests/ui/sync/suggest-once-cell.stderr
@@ -0,0 +1,17 @@
+error[E0277]: `OnceCell<()>` cannot be shared between threads safely
+  --> $DIR/suggest-once-cell.rs:8:20
+   |
+LL |     require_sync::<std::cell::OnceCell<()>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^ `OnceCell<()>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `OnceCell<()>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::OnceLock` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-once-cell.rs:3:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/sync/suggest-ref-cell.rs b/tests/ui/sync/suggest-ref-cell.rs
new file mode 100644
index 000000000..6b972ae09
--- /dev/null
+++ b/tests/ui/sync/suggest-ref-cell.rs
@@ -0,0 +1,12 @@
+#![feature(once_cell)]
+
+fn require_sync<T: Sync>() {}
+//~^ NOTE required by this bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+
+fn main() {
+    require_sync::<std::cell::RefCell<()>>();
+    //~^ ERROR `RefCell<()>` cannot be shared between threads safely
+    //~| NOTE `RefCell<()>` cannot be shared between threads safely
+    //~| NOTE use `std::sync::RwLock` instead
+}
diff --git a/tests/ui/sync/suggest-ref-cell.stderr b/tests/ui/sync/suggest-ref-cell.stderr
new file mode 100644
index 000000000..9e8b8fcb4
--- /dev/null
+++ b/tests/ui/sync/suggest-ref-cell.stderr
@@ -0,0 +1,17 @@
+error[E0277]: `RefCell<()>` cannot be shared between threads safely
+  --> $DIR/suggest-ref-cell.rs:8:20
+   |
+LL |     require_sync::<std::cell::RefCell<()>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ `RefCell<()>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `RefCell<()>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-ref-cell.rs:3:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.