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Deduplicate more E0038 object safety errors by pointing at type more often
On casts to `dyn Trait` caused by explicit `as` or return type, we point at the type instead of the expression. Silence note with context about the cast when we'd point at the type, which allows automatic deduplication to happen.
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‎compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs‎

Lines changed: 40 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -986,7 +986,46 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
986986

987987
TraitNotObjectSafe(did) => {
988988
let violations = self.tcx.object_safety_violations(did);
989-
report_object_safety_error(self.tcx, span, None, did, violations)
989+
let (hir_id, early) = match root_obligation.cause.code() {
990+
ObligationCauseCode::Coercion { parent_code, .. } => {
991+
match &**parent_code {
992+
ObligationCauseCode::ExprAssignable(hir_id) => (*hir_id, true),
993+
ObligationCauseCode::ReturnValue(_) => {
994+
let node = self.tcx.hir_node(self.tcx.local_def_id_to_hir_id(root_obligation.cause.body_id));
995+
if let Some(decl) = node.fn_decl()
996+
&& let hir::FnRetTy::Return(ty) = decl.output
997+
{
998+
// We'll point at the return type on type safety errors.
999+
(Some(ty.hir_id), true)
1000+
} else {
1001+
(None, false)
1002+
}
1003+
}
1004+
ObligationCauseCode::BlockTailExpression(hir_id, _) => {
1005+
if let Some((_, decl, _)) = self.tcx.get_fn_decl(*hir_id)
1006+
&& let hir::FnRetTy::Return(ty) = decl.output
1007+
{
1008+
// We'll point at the return type on type safety errors.
1009+
(Some(ty.hir_id), true)
1010+
} else {
1011+
(None, false)
1012+
}
1013+
}
1014+
_ => (None, false),
1015+
}
1016+
}
1017+
_ => (None, false),
1018+
};
1019+
let mut err = report_object_safety_error(self.tcx, span, hir_id, did, violations);
1020+
if early {
1021+
// We want to skip `note_obligation_cause` because we don't want to note that
1022+
// the requirement came from a cast, because that note causes the error
1023+
// deduplication to not trigger and we'll have two errors instead of one.
1024+
self.point_at_returns_when_relevant(&mut err, &obligation);
1025+
return err.emit();
1026+
} else {
1027+
err
1028+
}
9901029
}
9911030

9921031
SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {

‎tests/ui/dyn-keyword/trait-dyn-in-qualified-path-for-object-unsafe-trait.stderr‎

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -42,7 +42,6 @@ LL | trait Trait: Sized {}
4242
| |
4343
| this trait cannot be made into an object...
4444
= help: only type `()` implements the trait, consider using it directly instead
45-
= note: required for the cast from `&()` to `&(dyn Trait + 'static)`
4645

4746
error[E0038]: the trait `Trait` cannot be made into an object
4847
--> $DIR/trait-dyn-in-qualified-path-for-object-unsafe-trait.rs:10:6

‎tests/ui/feature-gates/feature-gate-dispatch-from-dyn-missing-impl.stderr‎

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -17,13 +17,13 @@ LL | fn ptr(self: Ptr<Self>);
1717
= help: only type `i32` implements the trait, consider using it directly instead
1818

1919
error[E0038]: the trait `Trait` cannot be made into an object
20-
--> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:5
20+
--> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:29
2121
|
2222
LL | fn ptr(self: Ptr<Self>);
2323
| --------- help: consider changing method `ptr`'s `self` parameter to be `&self`: `&Self`
2424
...
2525
LL | Ptr(Box::new(4)) as Ptr<dyn Trait>;
26-
| ^^^^^^^^^^^^^^^^ `Trait` cannot be made into an object
26+
| ^^^^^^^^^ `Trait` cannot be made into an object
2727
|
2828
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
2929
--> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:25:18
@@ -33,7 +33,7 @@ LL | trait Trait {
3333
LL | fn ptr(self: Ptr<Self>);
3434
| ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
3535
= help: only type `i32` implements the trait, consider using it directly instead
36-
= note: required for the cast from `Ptr<{integer}>` to `Ptr<dyn Trait>`
36+
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
3737

3838
error: aborting due to 2 previous errors
3939

‎tests/ui/generic-associated-types/gat-in-trait-path.base.stderr‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -51,7 +51,7 @@ LL | type A<'a> where Self: 'a;
5151
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `Foo` for this new enum and using it instead:
5252
Fooer<T>
5353
Fooy
54-
= note: required for the cast from `Box<Fooer<{integer}>>` to `Box<(dyn Foo<A = &'a ()> + 'static)>`
54+
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
5555

5656
error: aborting due to 3 previous errors
5757

‎tests/ui/generic-associated-types/issue-76535.base.stderr‎

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -32,10 +32,10 @@ LL | type SubType<'a>: SubTrait where Self: 'a;
3232
= note: `SuperTrait` can be implemented in other crates; if you want to support your users passing their own types here, you can't refer to a specific type
3333

3434
error[E0038]: the trait `SuperTrait` cannot be made into an object
35-
--> $DIR/issue-76535.rs:39:57
35+
--> $DIR/issue-76535.rs:39:18
3636
|
3737
LL | let sub: Box<dyn SuperTrait<SubType = SubStruct>> = Box::new(SuperStruct::new(0));
38-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SuperTrait` cannot be made into an object
38+
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SuperTrait` cannot be made into an object
3939
|
4040
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
4141
--> $DIR/issue-76535.rs:9:10
@@ -47,7 +47,7 @@ LL | type SubType<'a>: SubTrait where Self: 'a;
4747
= help: consider moving `SubType` to another trait
4848
= help: only type `SuperStruct` is seen to implement the trait in this crate, consider using it directly instead
4949
= note: `SuperTrait` can be implemented in other crates; if you want to support your users passing their own types here, you can't refer to a specific type
50-
= note: required for the cast from `Box<SuperStruct>` to `Box<dyn SuperTrait<SubType = SubStruct<'_>>>`
50+
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
5151

5252
error: aborting due to 3 previous errors
5353

‎tests/ui/generic-associated-types/issue-79422.base.stderr‎

Lines changed: 6 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
error[E0107]: missing generics for associated type `MapLike::VRefCont`
2-
--> $DIR/issue-79422.rs:47:36
2+
--> $DIR/issue-79422.rs:45:36
33
|
44
LL | as Box<dyn MapLike<u8, u8, VRefCont = dyn RefCont<'_, u8>>>;
55
| ^^^^^^^^ expected 1 lifetime argument
@@ -15,7 +15,7 @@ LL | as Box<dyn MapLike<u8, u8, VRefCont<'a> = dyn RefCont<'_, u8>>>;
1515
| ++++
1616

1717
error[E0038]: the trait `MapLike` cannot be made into an object
18-
--> $DIR/issue-79422.rs:47:16
18+
--> $DIR/issue-79422.rs:45:16
1919
|
2020
LL | as Box<dyn MapLike<u8, u8, VRefCont = dyn RefCont<'_, u8>>>;
2121
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `MapLike` cannot be made into an object
@@ -33,10 +33,10 @@ LL | type VRefCont<'a>: RefCont<'a, V> where Self: 'a;
3333
std::collections::BTreeMap<K, V>
3434

3535
error[E0038]: the trait `MapLike` cannot be made into an object
36-
--> $DIR/issue-79422.rs:44:13
36+
--> $DIR/issue-79422.rs:45:16
3737
|
38-
LL | let m = Box::new(std::collections::BTreeMap::<u8, u8>::new())
39-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `MapLike` cannot be made into an object
38+
LL | as Box<dyn MapLike<u8, u8, VRefCont = dyn RefCont<'_, u8>>>;
39+
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `MapLike` cannot be made into an object
4040
|
4141
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
4242
--> $DIR/issue-79422.rs:23:10
@@ -49,7 +49,7 @@ LL | type VRefCont<'a>: RefCont<'a, V> where Self: 'a;
4949
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `MapLike` for this new enum and using it instead:
5050
Source
5151
std::collections::BTreeMap<K, V>
52-
= note: required for the cast from `Box<BTreeMap<u8, u8>>` to `Box<dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>>`
52+
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
5353

5454
error: aborting due to 3 previous errors
5555

‎tests/ui/generic-associated-types/issue-79422.extended.stderr‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
error[E0107]: missing generics for associated type `MapLike::VRefCont`
2-
--> $DIR/issue-79422.rs:47:36
2+
--> $DIR/issue-79422.rs:45:36
33
|
44
LL | as Box<dyn MapLike<u8, u8, VRefCont = dyn RefCont<'_, u8>>>;
55
| ^^^^^^^^ expected 1 lifetime argument

‎tests/ui/generic-associated-types/issue-79422.rs‎

Lines changed: 2 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -41,10 +41,9 @@ impl<K, V: Default> MapLike<K, V> for Source {
4141
}
4242

4343
fn main() {
44-
let m = Box::new(std::collections::BTreeMap::<u8, u8>::new())
45-
//[base]~^ ERROR the trait
46-
//[extended]~^^ type mismatch
44+
let m = Box::new(std::collections::BTreeMap::<u8, u8>::new()) //[extended]~ type mismatch
4745
as Box<dyn MapLike<u8, u8, VRefCont = dyn RefCont<'_, u8>>>;
4846
//~^ ERROR missing generics for associated type
4947
//[base]~^^ ERROR the trait
48+
//[base]~| ERROR the trait
5049
}

‎tests/ui/higher-ranked/trait-bounds/span-bug-issue-121597.stderr‎

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1,8 +1,8 @@
11
error[E0038]: the trait `Foo` cannot be made into an object
2-
--> $DIR/span-bug-issue-121597.rs:14:23
2+
--> $DIR/span-bug-issue-121597.rs:14:13
33
|
44
LL | let x: &dyn Foo = &();
5-
| ^^^ `Foo` cannot be made into an object
5+
| ^^^^^^^ `Foo` cannot be made into an object
66
|
77
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
88
--> $DIR/span-bug-issue-121597.rs:4:12
@@ -11,7 +11,6 @@ LL | trait Foo: for<T> Bar<T> {}
1111
| --- ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
1212
| |
1313
| this trait cannot be made into an object...
14-
= note: required for the cast from `&()` to `&dyn Foo`
1514

1615
error[E0038]: the trait `Foo` cannot be made into an object
1716
--> $DIR/span-bug-issue-121597.rs:14:13
@@ -26,6 +25,7 @@ LL | trait Foo: for<T> Bar<T> {}
2625
| --- ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
2726
| |
2827
| this trait cannot be made into an object...
28+
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
2929

3030
error[E0308]: mismatched types
3131
--> $DIR/span-bug-issue-121597.rs:18:15

‎tests/ui/impl-trait/in-trait/object-safety.stderr‎

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -47,10 +47,10 @@ LL | fn baz(&self) -> impl Debug;
4747
= help: only type `u32` implements the trait, consider using it directly instead
4848

4949
error[E0038]: the trait `Foo` cannot be made into an object
50-
--> $DIR/object-safety.rs:14:13
50+
--> $DIR/object-safety.rs:14:37
5151
|
5252
LL | let i = Box::new(42_u32) as Box<dyn Foo>;
53-
| ^^^^^^^^^^^^^^^^ `Foo` cannot be made into an object
53+
| ^^^^^^^ `Foo` cannot be made into an object
5454
|
5555
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
5656
--> $DIR/object-safety.rs:4:22
@@ -61,7 +61,7 @@ LL | fn baz(&self) -> impl Debug;
6161
| ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
6262
= help: consider moving `baz` to another trait
6363
= help: only type `u32` implements the trait, consider using it directly instead
64-
= note: required for the cast from `Box<u32>` to `Box<dyn Foo>`
64+
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
6565

6666
error: aborting due to 4 previous errors
6767

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