Auto merge of rust-lang#12609 - Alexendoo:arc-with-non-send-sync-message, r=Jarcho

Reword `arc_with_non_send_sync` note and help messages

Addresses rust-lang/rust-clippy#12608 (comment)

Makes the note more concise and reframes the `Rc` suggestion around whether it crosses threads currently due to a manual `Send`/`Sync` impl or may do in the future

changelog: none

Author: bors

clippy_lints/src/arc_with_non_send_sync.rs

@@ -56,7 +56,12 @@ impl<'tcx> LateLintPass<'tcx> for ArcWithNonSendSync {
             && let Some(send) = cx.tcx.get_diagnostic_item(sym::Send)
             && let Some(sync) = cx.tcx.lang_items().sync_trait()
             && let [is_send, is_sync] = [send, sync].map(|id| implements_trait(cx, arg_ty, id, &[]))
-            && !(is_send && is_sync)
+            && let reason = match (is_send, is_sync) {
+                (false, false) => "neither `Send` nor `Sync`",
+                (false, true) => "not `Send`",
+                (true, false) => "not `Sync`",
+                _ => return,
+            }
             && !is_from_proc_macro(cx, expr)
         {
             span_lint_and_then(
@@ -66,21 +71,12 @@ impl<'tcx> LateLintPass<'tcx> for ArcWithNonSendSync {
                 "usage of an `Arc` that is not `Send` and `Sync`",
                 |diag| {
                     with_forced_trimmed_paths!({
-                        diag.note(format!("`Arc<{arg_ty}>` is not `Send` and `Sync` as:"));
-
-                        if !is_send {
-                            diag.note(format!("- the trait `Send` is not implemented for `{arg_ty}`"));
-                        }
-                        if !is_sync {
-                            diag.note(format!("- the trait `Sync` is not implemented for `{arg_ty}`"));
-                        }
-
-                        diag.help("consider using an `Rc` instead. `Arc` does not provide benefits for non `Send` and `Sync` types");
-
-                        diag.note("if you intend to use `Arc` with `Send` and `Sync` traits");
-
                         diag.note(format!(
-                            "wrap the inner type with a `Mutex` or implement `Send` and `Sync` for `{arg_ty}`"
+                            "`Arc<{arg_ty}>` is not `Send` and `Sync` as `{arg_ty}` is {reason}"
+                        ));
+                        diag.help("if the `Arc` will not used be across threads replace it with an `Rc`");
+                        diag.help(format!(
+                            "otherwise make `{arg_ty}` `Send` and `Sync` or consider a wrapper type such as `Mutex`"
                         ));
                     });
                 },

tests/ui/arc_with_non_send_sync.rs

@@ -33,16 +33,9 @@ fn main() {
     let _ = Arc::new(42);
 
     let _ = Arc::new(RefCell::new(42));
-    //~^ ERROR: usage of an `Arc` that is not `Send` and `Sync`
-    //~| NOTE: the trait `Sync` is not implemented for `RefCell<i32>`
 
     let mutex = Mutex::new(1);
     let _ = Arc::new(mutex.lock().unwrap());
-    //~^ ERROR: usage of an `Arc` that is not `Send` and `Sync`
-    //~| NOTE: the trait `Send` is not implemented for `MutexGuard<'_, i32>`
 
     let _ = Arc::new(&42 as *const i32);
-    //~^ ERROR: usage of an `Arc` that is not `Send` and `Sync`
-    //~| NOTE: the trait `Send` is not implemented for `*const i32`
-    //~| NOTE: the trait `Sync` is not implemented for `*const i32`
 }

tests/ui/arc_with_non_send_sync.stderr

@@ -4,38 +4,31 @@ error: usage of an `Arc` that is not `Send` and `Sync`
 LL |     let _ = Arc::new(RefCell::new(42));
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `Arc<RefCell<i32>>` is not `Send` and `Sync` as:
-   = note: - the trait `Sync` is not implemented for `RefCell<i32>`
-   = help: consider using an `Rc` instead. `Arc` does not provide benefits for non `Send` and `Sync` types
-   = note: if you intend to use `Arc` with `Send` and `Sync` traits
-   = note: wrap the inner type with a `Mutex` or implement `Send` and `Sync` for `RefCell<i32>`
+   = note: `Arc<RefCell<i32>>` is not `Send` and `Sync` as `RefCell<i32>` is not `Sync`
+   = help: if the `Arc` will not used be across threads replace it with an `Rc`
+   = help: otherwise make `RefCell<i32>` `Send` and `Sync` or consider a wrapper type such as `Mutex`
    = note: `-D clippy::arc-with-non-send-sync` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::arc_with_non_send_sync)]`
 
 error: usage of an `Arc` that is not `Send` and `Sync`
-  --> tests/ui/arc_with_non_send_sync.rs:40:13
+  --> tests/ui/arc_with_non_send_sync.rs:38:13
    |
 LL |     let _ = Arc::new(mutex.lock().unwrap());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `Arc<MutexGuard<'_, i32>>` is not `Send` and `Sync` as:
-   = note: - the trait `Send` is not implemented for `MutexGuard<'_, i32>`
-   = help: consider using an `Rc` instead. `Arc` does not provide benefits for non `Send` and `Sync` types
-   = note: if you intend to use `Arc` with `Send` and `Sync` traits
-   = note: wrap the inner type with a `Mutex` or implement `Send` and `Sync` for `MutexGuard<'_, i32>`
+   = note: `Arc<MutexGuard<'_, i32>>` is not `Send` and `Sync` as `MutexGuard<'_, i32>` is not `Send`
+   = help: if the `Arc` will not used be across threads replace it with an `Rc`
+   = help: otherwise make `MutexGuard<'_, i32>` `Send` and `Sync` or consider a wrapper type such as `Mutex`
 
 error: usage of an `Arc` that is not `Send` and `Sync`
-  --> tests/ui/arc_with_non_send_sync.rs:44:13
+  --> tests/ui/arc_with_non_send_sync.rs:40:13
    |
 LL |     let _ = Arc::new(&42 as *const i32);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `Arc<*const i32>` is not `Send` and `Sync` as:
-   = note: - the trait `Send` is not implemented for `*const i32`
-   = note: - the trait `Sync` is not implemented for `*const i32`
-   = help: consider using an `Rc` instead. `Arc` does not provide benefits for non `Send` and `Sync` types
-   = note: if you intend to use `Arc` with `Send` and `Sync` traits
-   = note: wrap the inner type with a `Mutex` or implement `Send` and `Sync` for `*const i32`
+   = note: `Arc<*const i32>` is not `Send` and `Sync` as `*const i32` is neither `Send` nor `Sync`
+   = help: if the `Arc` will not used be across threads replace it with an `Rc`
+   = help: otherwise make `*const i32` `Send` and `Sync` or consider a wrapper type such as `Mutex`
 
 error: aborting due to 3 previous errors