Introduce #[diagnostic::on_move] on Arc

This annotates the `Arc` type with the diagnostic attribute
`#[diagnostic::on_move]`. Now when a moved `Arc` is borrowed,
a suggestion to clone it is made, with a label explaining why.

library/alloc/src/lib.rs

@@ -111,6 +111,7 @@
 #![feature(core_intrinsics)]
 #![feature(deprecated_suggestion)]
 #![feature(deref_pure_trait)]
+#![feature(diagnostic_on_move)]
 #![feature(dispatch_from_dyn)]
 #![feature(ergonomic_clones)]
 #![feature(error_generic_member_access)]

library/alloc/src/sync.rs

@@ -261,6 +261,11 @@ macro_rules! acquire {
 #[rustc_diagnostic_item = "Arc"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_insignificant_dtor]
+#[diagnostic::on_move(
+    message = "the type `{Self}` does not implement `Copy`",
+    label = "this move could be avoided by cloning the original `{Self}`, which is inexpensive",
+    note = "consider using `Arc::clone`"
+)]
 pub struct Arc<
     T: ?Sized,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,

tests/ui/moves/arc-consumed-in-looped-closure.rs

@@ -19,7 +19,7 @@ fn execute<F>(&self, f: F)
 }
 
 fn main() {
-    let results = Arc::new(Mutex::new(Vec::new())); //~ NOTE move occurs because
+    let results = Arc::new(Mutex::new(Vec::new())); //~ NOTE this move could be avoided by cloning the original `Arc`, which is inexpensive
     let pool = ThreadPool {
         workers: vec![],
         queue: Arc::new(()),
@@ -29,6 +29,7 @@ fn main() {
         // let results = Arc::clone(&results); // Forgot this.
         pool.execute(move || { //~ ERROR E0382
             //~^ NOTE value moved into closure here, in previous iteration of loop
+            //~| NOTE consider using `Arc::clone`
             //~| HELP consider cloning the value before moving it into the closure
             let mut r = results.lock().unwrap(); //~ NOTE use occurs due to use in closure
             r.push(i);

tests/ui/moves/arc-consumed-in-looped-closure.stderr

@@ -1,8 +1,8 @@
-error[E0382]: use of moved value: `results`
+error[E0382]: the type `Arc` does not implement `Copy`
   --> $DIR/arc-consumed-in-looped-closure.rs:30:22
    |
 LL |     let results = Arc::new(Mutex::new(Vec::new()));
-   |         ------- move occurs because `results` has type `Arc<std::sync::Mutex<Vec<i32>>>`, which does not implement the `Copy` trait
+   |         ------- this move could be avoided by cloning the original `Arc`, which is inexpensive
 ...
 LL |     for i in 0..20 {
    |     -------------- inside of this loop
@@ -13,12 +13,14 @@ LL |         pool.execute(move || {
 LL |             let mut r = results.lock().unwrap();
    |                         ------- use occurs due to use in closure
    |
+   = note: consider using `Arc::clone`
 help: consider cloning the value before moving it into the closure
    |
 LL ~         let value = results.clone();
 LL ~         pool.execute(move || {
 LL |
 LL |
+LL |
 LL ~             let mut r = value.lock().unwrap();
    |
 

tests/ui/moves/no-capture-arc.rs

@@ -9,7 +9,7 @@ fn main() {
         assert_eq!((*arc_v)[3], 4);
     });
 
-    assert_eq!((*arc_v)[2], 3); //~ ERROR borrow of moved value: `arc_v`
+    assert_eq!((*arc_v)[2], 3); //~ ERROR the type `Arc` does not implement `Copy`
 
     println!("{:?}", *arc_v);
 }

tests/ui/moves/no-capture-arc.stderr

@@ -1,8 +1,8 @@
-error[E0382]: borrow of moved value: `arc_v`
+error[E0382]: the type `Arc` does not implement `Copy`
   --> $DIR/no-capture-arc.rs:12:18
    |
 LL |     let arc_v = Arc::new(v);
-   |         ----- move occurs because `arc_v` has type `Arc<Vec<i32>>`, which does not implement the `Copy` trait
+   |         ----- this move could be avoided by cloning the original `Arc`, which is inexpensive
 LL |
 LL |     thread::spawn(move|| {
    |                   ------ value moved into closure here
@@ -12,6 +12,7 @@ LL |         assert_eq!((*arc_v)[3], 4);
 LL |     assert_eq!((*arc_v)[2], 3);
    |                  ^^^^^ value borrowed here after move
    |
+   = note: consider using `Arc::clone`
    = note: borrow occurs due to deref coercion to `Vec<i32>`
 help: consider cloning the value before moving it into the closure
    |

tests/ui/moves/no-reuse-move-arc.fixed

@@ -11,7 +11,7 @@ fn main() {
         assert_eq!((*value)[3], 4);
     });
 
-    assert_eq!((*arc_v)[2], 3); //~ ERROR borrow of moved value: `arc_v`
+    assert_eq!((*arc_v)[2], 3); //~ ERROR the type `Arc` does not implement `Copy`
 
     println!("{:?}", *arc_v);
 }

tests/ui/moves/no-reuse-move-arc.rs

@@ -10,7 +10,7 @@ fn main() {
         assert_eq!((*arc_v)[3], 4);
     });
 
-    assert_eq!((*arc_v)[2], 3); //~ ERROR borrow of moved value: `arc_v`
+    assert_eq!((*arc_v)[2], 3); //~ ERROR the type `Arc` does not implement `Copy`
 
     println!("{:?}", *arc_v);
 }

tests/ui/moves/no-reuse-move-arc.stderr

@@ -1,8 +1,8 @@
-error[E0382]: borrow of moved value: `arc_v`
+error[E0382]: the type `Arc` does not implement `Copy`
   --> $DIR/no-reuse-move-arc.rs:13:18
    |
 LL |     let arc_v = Arc::new(v);
-   |         ----- move occurs because `arc_v` has type `Arc<Vec<i32>>`, which does not implement the `Copy` trait
+   |         ----- this move could be avoided by cloning the original `Arc`, which is inexpensive
 LL |
 LL |     thread::spawn(move|| {
    |                   ------ value moved into closure here
@@ -12,6 +12,7 @@ LL |         assert_eq!((*arc_v)[3], 4);
 LL |     assert_eq!((*arc_v)[2], 3);
    |                  ^^^^^ value borrowed here after move
    |
+   = note: consider using `Arc::clone`
    = note: borrow occurs due to deref coercion to `Vec<i32>`
 help: consider cloning the value before moving it into the closure
    |