Create unnecessary_send_constraint lint for &(dyn ... + Send)

compiler/rustc_lint/messages.ftl

@@ -192,6 +192,16 @@ lint_redundant_semicolons =
         *[false] this semicolon
     }
 
+lint_useless_send_constraint =
+    constraining a reference to `Send` without `Sync` is useless, consider {$only_trait ->
+        [true] replacing it with `Any`
+        *[false] removing it
+    }
+    .suggestion = {$only_trait ->
+        [true] replace this with `Any`
+        *[false] remove this
+    }
+
 lint_drop_trait_constraints =
     bounds on `{$predicate}` are most likely incorrect, consider instead using `{$needs_drop}` to detect whether a type can be trivially dropped
 

compiler/rustc_lint/src/lib.rs

@@ -77,6 +77,7 @@ mod redundant_semicolon;
 mod traits;
 mod types;
 mod unused;
+mod useless_send_constraint;
 
 pub use array_into_iter::ARRAY_INTO_ITER;
 
@@ -125,6 +126,7 @@ pub use passes::{EarlyLintPass, LateLintPass};
 pub use rustc_session::lint::Level::{self, *};
 pub use rustc_session::lint::{BufferedEarlyLint, FutureIncompatibleInfo, Lint, LintId};
 pub use rustc_session::lint::{LintArray, LintPass};
+use useless_send_constraint::UselessSendConstraint;
 
 fluent_messages! { "../messages.ftl" }
 
@@ -242,6 +244,7 @@ late_lint_methods!(
             OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
             MultipleSupertraitUpcastable: MultipleSupertraitUpcastable,
             MapUnitFn: MapUnitFn,
+            UselessSendConstraint: UselessSendConstraint,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -1168,6 +1168,15 @@ pub struct RedundantSemicolonsDiag {
     pub suggestion: Span,
 }
 
+// lint_useless_send_constraint.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_useless_send_constraint)]
+pub struct UselessSendConstraintDiag {
+    pub only_trait: bool,
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub suggestion: Span,
+}
+
 // traits.rs
 pub struct DropTraitConstraintsDiag<'a> {
     pub predicate: Predicate<'a>,

compiler/rustc_lint/src/useless_send_constraint.rs

@@ -0,0 +1,58 @@
+use rustc_span::sym;
+
+use crate::hir;
+
+use crate::{lints::UselessSendConstraintDiag, LateContext, LateLintPass};
+
+declare_lint! {
+    /// The `lint_useless_send_constraint` lints useless constraint of references to `Send`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// fn foo(_: &(dyn Any + Send>) {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// References cannot be sent across threads unless thay have a `Sync` bound, so constraining them to `Send` without `Sync` is useless.
+    pub USELESS_SEND_CONSTRAINT,
+    Warn,
+    "constraining a reference to `Send` without `Sync` is useless, consider removing it"
+}
+
+declare_lint_pass!(UselessSendConstraint => [USELESS_SEND_CONSTRAINT]);
+
+impl<'tcx> LateLintPass<'tcx> for UselessSendConstraint {
+    fn check_ty(&mut self, cx: &LateContext<'tcx>, ty: &'tcx hir::Ty<'tcx>) {
+        let hir::TyKind::Ref(
+            hir::MutTy {
+                ty: hir::Ty {
+                    kind: hir::TyKind::TraitObject(bounds, ..),
+                    ..
+                },
+                mutbl: hir::Mutability::Not, // only immutable references
+            },
+            ..
+        ) = ty.kind else { return; };
+
+        let send = cx.tcx.get_diagnostic_item(sym::Send);
+        let sync = cx.tcx.get_diagnostic_item(sym::Sync);
+
+        let send_bound = bounds.iter().find(|b| b.trait_ref.trait_def_id() == send);
+        let sync_bound = bounds.iter().find(|b| b.trait_ref.trait_def_id() == sync);
+
+        if let Some(send_bound) = send_bound && sync_bound.is_none() {
+            let only_trait = bounds.len() == 1;
+
+            cx.tcx.emit_spanned_lint(
+                USELESS_SEND_CONSTRAINT,
+                send_bound.trait_ref.hir_ref_id, // is this correct?
+                send_bound.span,
+                UselessSendConstraintDiag { only_trait, suggestion: send_bound.span },
+            )
+        }
+    }
+}

library/core/src/any.rs

@@ -398,7 +398,7 @@ impl dyn Any + Send {
     /// ```
     /// use std::any::Any;
     ///
-    /// fn is_string(s: &(dyn Any + Send)) {
+    /// fn is_string(s: &dyn Any) {
     ///     if s.is::<String>() {
     ///         println!("It's a string!");
     ///     } else {
@@ -422,7 +422,7 @@ impl dyn Any + Send {
     /// ```
     /// use std::any::Any;
     ///
-    /// fn print_if_string(s: &(dyn Any + Send)) {
+    /// fn print_if_string(s: &dyn Any) {
     ///     if let Some(string) = s.downcast_ref::<String>() {
     ///         println!("It's a string({}): '{}'", string.len(), string);
     ///     } else {

library/core/src/cell.rs

@@ -2246,6 +2246,7 @@ impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
 
 #[allow(unused)]
+#[cfg_attr(not(bootstrap), allow(useless_send_constraint))]
 fn assert_coerce_unsized(
     a: UnsafeCell<&i32>,
     b: SyncUnsafeCell<&i32>,

library/core/src/panic.rs

@@ -107,5 +107,5 @@ pub unsafe trait BoxMeUp {
     fn take_box(&mut self) -> *mut (dyn Any + Send);
 
     /// Just borrow the contents.
-    fn get(&mut self) -> &(dyn Any + Send);
+    fn get(&mut self) -> &dyn Any;
 }

library/core/src/panic/panic_info.rs

@@ -24,7 +24,7 @@ use crate::panic::Location;
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 #[derive(Debug)]
 pub struct PanicInfo<'a> {
-    payload: &'a (dyn Any + Send),
+    payload: &'a dyn Any,
     message: Option<&'a fmt::Arguments<'a>>,
     location: &'a Location<'a>,
     can_unwind: bool,
@@ -54,7 +54,7 @@ impl<'a> PanicInfo<'a> {
     )]
     #[doc(hidden)]
     #[inline]
-    pub fn set_payload(&mut self, info: &'a (dyn Any + Send)) {
+    pub fn set_payload(&mut self, info: &'a dyn Any) {
         self.payload = info;
     }
 
@@ -81,7 +81,7 @@ impl<'a> PanicInfo<'a> {
     /// ```
     #[must_use]
     #[stable(feature = "panic_hooks", since = "1.10.0")]
-    pub fn payload(&self) -> &(dyn Any + Send) {
+    pub fn payload(&self) -> &dyn Any {
         self.payload
     }
 

library/core/src/panicking.rs

@@ -11,7 +11,7 @@
 //! ```
 //!
 //! This definition allows for panicking with any general message, but it does not
-//! allow for failing with a `Box<Any>` value. (`PanicInfo` just contains a `&(dyn Any + Send)`,
+//! allow for failing with a `Box<Any>` value. (`PanicInfo` just contains a `&dyn Any`,
 //! for which we fill in a dummy value in `PanicInfo::internal_constructor`.)
 //! The reason for this is that core is not allowed to allocate.
 //!

library/std/src/io/error.rs

@@ -695,7 +695,7 @@ impl Error {
     #[stable(feature = "io_error_inner", since = "1.3.0")]
     #[must_use]
     #[inline]
-    pub fn get_ref(&self) -> Option<&(dyn error::Error + Send + Sync + 'static)> {
+    pub fn get_ref(&self) -> Option<&(dyn error::Error + Sync + 'static)> {
         match self.repr.data() {
             ErrorData::Os(..) => None,
             ErrorData::Simple(..) => None,

library/std/src/panicking.rs

@@ -220,10 +220,7 @@ pub fn take_hook() -> Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send> {
 #[unstable(feature = "panic_update_hook", issue = "92649")]
 pub fn update_hook<F>(hook_fn: F)
 where
-    F: Fn(&(dyn Fn(&PanicInfo<'_>) + Send + Sync + 'static), &PanicInfo<'_>)
-        + Sync
-        + Send
-        + 'static,
+    F: Fn(&(dyn Fn(&PanicInfo<'_>) + Sync + 'static), &PanicInfo<'_>) + Sync + Send + 'static,
 {
     if thread::panicking() {
         panic!("cannot modify the panic hook from a panicking thread");
@@ -556,7 +553,7 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
             Box::into_raw(Box::new(contents))
         }
 
-        fn get(&mut self) -> &(dyn Any + Send) {
+        fn get(&mut self) -> &dyn Any {
             self.fill()
         }
     }
@@ -568,7 +565,7 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
             Box::into_raw(Box::new(self.0))
         }
 
-        fn get(&mut self) -> &(dyn Any + Send) {
+        fn get(&mut self) -> &dyn Any {
             &self.0
         }
     }
@@ -635,7 +632,7 @@ pub const fn begin_panic<M: Any + Send>(msg: M) -> ! {
             Box::into_raw(data)
         }
 
-        fn get(&mut self) -> &(dyn Any + Send) {
+        fn get(&mut self) -> &dyn Any {
             match self.inner {
                 Some(ref a) => a,
                 None => process::abort(),
@@ -725,7 +722,7 @@ pub fn rust_panic_without_hook(payload: Box<dyn Any + Send>) -> ! {
             Box::into_raw(mem::replace(&mut self.0, Box::new(())))
         }
 
-        fn get(&mut self) -> &(dyn Any + Send) {
+        fn get(&mut self) -> &dyn Any {
             &*self.0
         }
     }

library/test/src/test_result.rs

@@ -27,7 +27,7 @@ pub enum TestResult {
 /// and associated data.
 pub fn calc_result<'a>(
     desc: &TestDesc,
-    task_result: Result<(), &'a (dyn Any + 'static + Send)>,
+    task_result: Result<(), &'a (dyn Any + 'static)>,
     time_opts: &Option<time::TestTimeOptions>,
     exec_time: &Option<time::TestExecTime>,
 ) -> TestResult {

tests/ui/lint/useless-send-constraint/useless-send-constraint.rs

@@ -0,0 +1,13 @@
+#![warn(useless_send_constraint)]
+
+use std::any::Any;
+
+fn main() {}
+
+fn fine(_a: &dyn Any) {}
+
+fn should_replace_with_any(_a: &(dyn Send)) {}
+
+fn should_remove_send(_a: &(dyn Any + Send)) {}
+
+fn should_remove_send_duplicate(_a: &(dyn Any + Send)) {}