check for auto impls on field representing types

Author: BennoLossin

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -259,6 +259,17 @@ pub(super) fn check_item<'tcx>(
                         .with_span_label(sp, "auto trait")
                         .emit());
                 }
+                if is_auto && let rustc_hir::TyKind::FieldOf(..) = impl_.self_ty.kind {
+                    res = res.and(Err(tcx
+                        .dcx()
+                        .struct_span_err(
+                            item.span,
+                            "impls of auto traits for field representing types not supported",
+                        )
+                        .with_span_label(impl_.self_ty.span, "field representing type")
+                        .with_span_label(of_trait.trait_ref.path.span, "auto trait")
+                        .emit()));
+                }
                 match header.polarity {
                     ty::ImplPolarity::Positive => {
                         res = res.and(check_impl(tcx, item, impl_));

tests/ui/field_projections/send.next.stderr

@@ -0,0 +1,26 @@
+error: impls of auto traits for field representing types not supported
+  --> $DIR/send.rs:13:1
+   |
+LL | unsafe impl Send for field_of!(Bar, bar_field) {}
+   | ^^^^^^^^^^^^----^^^^^-------------------------^^^
+   |             |        |
+   |             |        field representing type
+   |             auto trait
+
+error[E0277]: `field_of!(Foo, field)` cannot be sent between threads safely
+  --> $DIR/send.rs:20:15
+   |
+LL |     is_send::<field_of!(Foo, field)>();
+   |               ^^^^^^^^^^^^^^^^^^^^^ `field_of!(Foo, field)` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `field_of!(Foo, field)`
+   = help: the trait `Send` is implemented for `field_of!(Bar, bar_field)`
+note: required by a bound in `is_send`
+  --> $DIR/send.rs:16:15
+   |
+LL | fn is_send<T: Send>() {}
+   |               ^^^^ required by this bound in `is_send`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/field_projections/send.old.stderr

@@ -0,0 +1,26 @@
+error: impls of auto traits for field representing types not supported
+  --> $DIR/send.rs:13:1
+   |
+LL | unsafe impl Send for field_of!(Bar, bar_field) {}
+   | ^^^^^^^^^^^^----^^^^^-------------------------^^^
+   |             |        |
+   |             |        field representing type
+   |             auto trait
+
+error[E0277]: `field_of!(Foo, field)` cannot be sent between threads safely
+  --> $DIR/send.rs:20:15
+   |
+LL |     is_send::<field_of!(Foo, field)>();
+   |               ^^^^^^^^^^^^^^^^^^^^^ `field_of!(Foo, field)` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `field_of!(Foo, field)`
+   = help: the trait `Send` is implemented for `field_of!(Bar, bar_field)`
+note: required by a bound in `is_send`
+  --> $DIR/send.rs:16:15
+   |
+LL | fn is_send<T: Send>() {}
+   |               ^^^^ required by this bound in `is_send`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/field_projections/send.rs

@@ -1,6 +1,5 @@
 //@ revisions: old next
 //@ [next] compile-flags: -Znext-solver
-//@ run-pass
 #![feature(field_projections)]
 #![allow(incomplete_features)]
 use std::field::field_of;
@@ -12,10 +11,12 @@ struct Bar {
     bar_field: u32,
 }
 unsafe impl Send for field_of!(Bar, bar_field) {}
+//~^ ERROR: impls of auto traits for field representing types not supported
 
 fn is_send<T: Send>() {}
 
 fn main() {
     is_send::<field_of!(Bar, bar_field)>();
     is_send::<field_of!(Foo, field)>();
+    //~^ ERROR: `field_of!(Foo, field)` cannot be sent between threads safely [E0277]
 }