Rollup merge of #105839 - LegionMammal978:mut-upvar-not-send, r=lcnr

Suggest a `T: Send` bound for `&mut T` upvars in `Send` generators

Right now, we suggest a `T: Sync` bound for both `&T` and `&mut T` upvars. A user on URLO [found this confusing](https://users.rust-lang.org/t/error-complains-about-missing-sync-but-send-is-whats-missing/86021), so I wrote this quick fix to look at the mutability before making the suggestion.

Author: Dylan-DPC

Diff for: compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2344,28 +2344,33 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
-                // `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
-                let refers_to_non_sync = match target_ty.kind() {
-                    ty::Ref(_, ref_ty, _) => match self.evaluate_obligation(&obligation) {
-                        Ok(eval) if !eval.may_apply() => Some(ref_ty),
+                // `Some((ref_ty, is_mut))` if `target_ty` is `&T` or `&mut T` and fails to impl `Send`
+                let non_send = match target_ty.kind() {
+                    ty::Ref(_, ref_ty, mutability) => match self.evaluate_obligation(&obligation) {
+                        Ok(eval) if !eval.may_apply() => Some((ref_ty, mutability.is_mut())),
                         _ => None,
                     },
                     _ => None,
                 };
 
-                let (span_label, span_note) = match refers_to_non_sync {
-                    // if `target_ty` is `&T` and `T` fails to impl `Sync`,
-                    // include suggestions to make `T: Sync` so that `&T: Send`
-                    Some(ref_ty) => (
-                        format!(
-                            "has type `{}` which {}, because `{}` is not `Sync`",
-                            target_ty, trait_explanation, ref_ty
-                        ),
-                        format!(
-                            "captured value {} because `&` references cannot be sent unless their referent is `Sync`",
-                            trait_explanation
-                        ),
-                    ),
+                let (span_label, span_note) = match non_send {
+                    // if `target_ty` is `&T` or `&mut T` and fails to impl `Send`,
+                    // include suggestions to make `T: Sync` so that `&T: Send`,
+                    // or to make `T: Send` so that `&mut T: Send`
+                    Some((ref_ty, is_mut)) => {
+                        let ref_ty_trait = if is_mut { "Send" } else { "Sync" };
+                        let ref_kind = if is_mut { "&mut" } else { "&" };
+                        (
+                            format!(
+                                "has type `{}` which {}, because `{}` is not `{}`",
+                                target_ty, trait_explanation, ref_ty, ref_ty_trait
+                            ),
+                            format!(
+                                "captured value {} because `{}` references cannot be sent unless their referent is `{}`",
+                                trait_explanation, ref_kind, ref_ty_trait
+                            ),
+                        )
+                    }
                     None => (
                         format!("has type `{}` which {}", target_ty, trait_explanation),
                         format!("captured value {}", trait_explanation),

Diff for: src/test/ui/generator/ref-upvar-not-send.rs

@@ -0,0 +1,31 @@
+// For `Send` generators, suggest a `T: Sync` requirement for `&T` upvars,
+// and suggest a `T: Send` requirement for `&mut T` upvars.
+
+#![feature(generators)]
+
+fn assert_send<T: Send>(_: T) {}
+//~^ NOTE required by a bound in `assert_send`
+//~| NOTE required by this bound in `assert_send`
+//~| NOTE required by a bound in `assert_send`
+//~| NOTE required by this bound in `assert_send`
+
+fn main() {
+    let x: &*mut () = &std::ptr::null_mut();
+    let y: &mut *mut () = &mut std::ptr::null_mut();
+    assert_send(move || {
+        //~^ ERROR generator cannot be sent between threads safely
+        //~| NOTE generator is not `Send`
+        yield;
+        let _x = x;
+    });
+    //~^^ NOTE captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync`
+    //~| NOTE has type `&*mut ()` which is not `Send`, because `*mut ()` is not `Sync`
+    assert_send(move || {
+        //~^ ERROR generator cannot be sent between threads safely
+        //~| NOTE generator is not `Send`
+        yield;
+        let _y = y;
+    });
+    //~^^ NOTE captured value is not `Send` because `&mut` references cannot be sent unless their referent is `Send`
+    //~| NOTE has type `&mut *mut ()` which is not `Send`, because `*mut ()` is not `Send`
+}

Diff for: src/test/ui/generator/ref-upvar-not-send.stderr

@@ -0,0 +1,50 @@
+error: generator cannot be sent between threads safely
+  --> $DIR/ref-upvar-not-send.rs:15:17
+   |
+LL |       assert_send(move || {
+   |  _________________^
+LL | |
+LL | |
+LL | |         yield;
+LL | |         let _x = x;
+LL | |     });
+   | |_____^ generator is not `Send`
+   |
+   = help: the trait `Sync` is not implemented for `*mut ()`
+note: captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync`
+  --> $DIR/ref-upvar-not-send.rs:19:18
+   |
+LL |         let _x = x;
+   |                  ^ has type `&*mut ()` which is not `Send`, because `*mut ()` is not `Sync`
+note: required by a bound in `assert_send`
+  --> $DIR/ref-upvar-not-send.rs:6:19
+   |
+LL | fn assert_send<T: Send>(_: T) {}
+   |                   ^^^^ required by this bound in `assert_send`
+
+error: generator cannot be sent between threads safely
+  --> $DIR/ref-upvar-not-send.rs:23:17
+   |
+LL |       assert_send(move || {
+   |  _________________^
+LL | |
+LL | |
+LL | |         yield;
+LL | |         let _y = y;
+LL | |     });
+   | |_____^ generator is not `Send`
+   |
+   = help: within `[generator@$DIR/ref-upvar-not-send.rs:23:17: 23:24]`, the trait `Send` is not implemented for `*mut ()`
+note: captured value is not `Send` because `&mut` references cannot be sent unless their referent is `Send`
+  --> $DIR/ref-upvar-not-send.rs:27:18
+   |
+LL |         let _y = y;
+   |                  ^ has type `&mut *mut ()` which is not `Send`, because `*mut ()` is not `Send`
+note: required by a bound in `assert_send`
+  --> $DIR/ref-upvar-not-send.rs:6:19
+   |
+LL | fn assert_send<T: Send>(_: T) {}
+   |                   ^^^^ required by this bound in `assert_send`
+
+error: aborting due to 2 previous errors
+