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On object safety error, mention new enum as alternative
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When we encounter a `dyn Trait` that isn't object safe, look for its
implementors. If there's one, mention using it directly If there are
less than 9, mention the possibility of creating a new enum and using
that instead.

Account for object unsafe `impl Trait on dyn Trait {}`.  Make a
distinction between public and sealed traits.

Fix #80194.
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estebank committed Oct 29, 2023
1 parent 608e968 commit 8c04999
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64 changes: 63 additions & 1 deletion compiler/rustc_infer/src/traits/error_reporting/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,8 @@ use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_middle::ty::TyCtxt;
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, TyCtxt};
use rustc_span::Span;
use std::fmt;
use std::iter;
Expand Down Expand Up @@ -108,5 +109,66 @@ pub fn report_object_safety_error<'tcx>(
violation.solution(&mut err);
}
}

let impls_of = tcx.trait_impls_of(trait_def_id);
let impls = if impls_of.blanket_impls().is_empty() {
impls_of
.non_blanket_impls()
.values()
.flatten()
.filter(|def_id| {
!matches!(tcx.type_of(*def_id).instantiate_identity().kind(), ty::Dynamic(..))
})
.collect::<Vec<_>>()
} else {
vec![]
};
let externally_visible = if !impls.is_empty()
&& let Some(def_id) = trait_def_id.as_local()
&& tcx.effective_visibilities(()).is_exported(def_id)
{
true
} else {
false
};
match &impls[..] {
[] => {}
_ if impls.len() > 9 => {}
[only] if externally_visible => {
err.help(with_no_trimmed_paths!(format!(
"only type `{}` is seen to implement the trait in this crate, consider using it \
directly instead",
tcx.type_of(*only).instantiate_identity(),
)));
}
[only] => {
err.help(with_no_trimmed_paths!(format!(
"only type `{}` implements the trait, consider using it directly instead",
tcx.type_of(*only).instantiate_identity(),
)));
}
impls => {
let types = impls
.iter()
.map(|t| {
with_no_trimmed_paths!(format!(" {}", tcx.type_of(*t).instantiate_identity(),))
})
.collect::<Vec<_>>();
err.help(format!(
"the following types implement the trait, consider defining an enum where each \
variant holds one of these types, implementing `{}` for this new enum and using \
it instead:\n{}",
trait_str,
types.join("\n"),
));
}
}
if externally_visible {
err.note(format!(
"`{trait_str}` 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",
));
}

err
}
Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@ LL | fn test(&self) -> [u8; bar::<Self>()];
| ...because method `test` references the `Self` type in its `where` clause
= help: consider moving `test` to another trait
= help: consider moving `test` to another trait
= help: only type `()` implements the trait, consider using it directly instead

error: aborting due to previous error

Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ LL | trait Trait {
| ----- this trait cannot be made into an object...
LL | fn ptr(self: Ptr<Self>);
| ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
= help: only type `i32` implements the trait, consider using it directly instead

error[E0038]: the trait `Trait` cannot be made into an object
--> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:5
Expand All @@ -31,6 +32,7 @@ LL | trait Trait {
| ----- this trait cannot be made into an object...
LL | fn ptr(self: Ptr<Self>);
| ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
= help: only type `i32` implements the trait, consider using it directly instead
= note: required for the cast from `Ptr<{integer}>` to `Ptr<dyn Trait>`

error: aborting due to 2 previous errors
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,9 @@ LL | trait Foo {
LL | type A<'a> where Self: 'a;
| ^ ...because it contains the generic associated type `A`
= help: consider moving `A` to another trait
= 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:
Fooy
Fooer<T>

error: aborting due to previous error

Expand Down
4 changes: 4 additions & 0 deletions tests/ui/generic-associated-types/issue-76535.base.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,8 @@ LL | pub trait SuperTrait {
LL | type SubType<'a>: SubTrait where Self: 'a;
| ^^^^^^^ ...because it contains the generic associated type `SubType`
= help: consider moving `SubType` to another trait
= help: only type `SuperStruct` is seen to implement the trait in this crate, consider using it directly instead
= 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

error[E0038]: the trait `SuperTrait` cannot be made into an object
--> $DIR/issue-76535.rs:39:57
Expand All @@ -43,6 +45,8 @@ LL | pub trait SuperTrait {
LL | type SubType<'a>: SubTrait where Self: 'a;
| ^^^^^^^ ...because it contains the generic associated type `SubType`
= help: consider moving `SubType` to another trait
= help: only type `SuperStruct` is seen to implement the trait in this crate, consider using it directly instead
= 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
= note: required for the cast from `Box<SuperStruct>` to `Box<dyn SuperTrait<SubType = SubStruct<'_>>>`

error: aborting due to 3 previous errors
Expand Down
6 changes: 6 additions & 0 deletions tests/ui/generic-associated-types/issue-79422.base.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,9 @@ LL | trait MapLike<K, V> {
LL | type VRefCont<'a>: RefCont<'a, V> where Self: 'a;
| ^^^^^^^^ ...because it contains the generic associated type `VRefCont`
= help: consider moving `VRefCont` to another trait
= 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:
std::collections::BTreeMap<K, V>
Source

error[E0038]: the trait `MapLike` cannot be made into an object
--> $DIR/issue-79422.rs:44:13
Expand All @@ -43,6 +46,9 @@ LL | trait MapLike<K, V> {
LL | type VRefCont<'a>: RefCont<'a, V> where Self: 'a;
| ^^^^^^^^ ...because it contains the generic associated type `VRefCont`
= help: consider moving `VRefCont` to another trait
= 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:
std::collections::BTreeMap<K, V>
Source
= note: required for the cast from `Box<BTreeMap<u8, u8>>` to `Box<dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>>`

error: aborting due to 3 previous errors
Expand Down
1 change: 1 addition & 0 deletions tests/ui/impl-trait/in-trait/foreign-dyn-error.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
|
LL | fn bar(self) -> impl Deref<Target = impl Sized>;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait cannot be made into an object because method `bar` references an `impl Trait` type in its return type
= help: only type `rpitit::Foreign` implements the trait, consider using it directly instead

error: aborting due to previous error

Expand Down
4 changes: 4 additions & 0 deletions tests/ui/impl-trait/in-trait/object-safety.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@ LL | trait Foo {
LL | fn baz(&self) -> impl Debug;
| ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait
= help: only type `u32` implements the trait, consider using it directly instead

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/object-safety.rs:17:15
Expand All @@ -27,6 +28,7 @@ LL | trait Foo {
LL | fn baz(&self) -> impl Debug;
| ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait
= help: only type `u32` implements the trait, consider using it directly instead

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/object-safety.rs:17:13
Expand All @@ -42,6 +44,7 @@ LL | trait Foo {
LL | fn baz(&self) -> impl Debug;
| ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait
= help: only type `u32` implements the trait, consider using it directly instead

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/object-safety.rs:14:13
Expand All @@ -57,6 +60,7 @@ LL | trait Foo {
LL | fn baz(&self) -> impl Debug;
| ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait
= help: only type `u32` implements the trait, consider using it directly instead
= note: required for the cast from `Box<u32>` to `Box<dyn Foo>`

error: aborting due to 4 previous errors
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,9 @@ LL | trait NotObjectSafe {
| ------------- this trait cannot be made into an object...
LL | fn foo() -> Self;
| ^^^ ...because associated function `foo` has no `self` parameter
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `NotObjectSafe` for this new enum and using it instead:
A
B
help: consider turning `foo` into a method by giving it a `&self` argument
|
LL | fn foo(&self) -> Self;
Expand All @@ -33,6 +36,9 @@ LL | trait NotObjectSafe {
| ------------- this trait cannot be made into an object...
LL | fn foo() -> Self;
| ^^^ ...because associated function `foo` has no `self` parameter
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `NotObjectSafe` for this new enum and using it instead:
A
B
help: consider turning `foo` into a method by giving it a `&self` argument
|
LL | fn foo(&self) -> Self;
Expand Down
2 changes: 2 additions & 0 deletions tests/ui/issues/issue-19380.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ LL | trait Qiz {
| --- this trait cannot be made into an object...
LL | fn qiz();
| ^^^ ...because associated function `qiz` has no `self` parameter
= help: only type `Foo` implements the trait, consider using it directly instead
help: consider turning `qiz` into a method by giving it a `&self` argument
|
LL | fn qiz(&self);
Expand All @@ -33,6 +34,7 @@ LL | trait Qiz {
| --- this trait cannot be made into an object...
LL | fn qiz();
| ^^^ ...because associated function `qiz` has no `self` parameter
= help: only type `Foo` implements the trait, consider using it directly instead
= note: required for the cast from `&Foo` to `&'static (dyn Qiz + 'static)`
help: consider turning `qiz` into a method by giving it a `&self` argument
|
Expand Down
2 changes: 2 additions & 0 deletions tests/ui/object-safety/issue-19538.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ LL | fn foo<T>(&self, val: T);
LL | trait Bar: Foo { }
| --- this trait cannot be made into an object...
= help: consider moving `foo` to another trait
= help: only type `Thing` implements the trait, consider using it directly instead

error[E0038]: the trait `Bar` cannot be made into an object
--> $DIR/issue-19538.rs:17:30
Expand All @@ -29,6 +30,7 @@ LL | fn foo<T>(&self, val: T);
LL | trait Bar: Foo { }
| --- this trait cannot be made into an object...
= help: consider moving `foo` to another trait
= help: only type `Thing` implements the trait, consider using it directly instead
= note: required for the cast from `&mut Thing` to `&mut dyn Bar`

error: aborting due to 2 previous errors
Expand Down
1 change: 1 addition & 0 deletions tests/ui/object-safety/object-safety-issue-22040.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ LL | trait Expr: Debug + PartialEq {
| ---- ^^^^^^^^^ ...because it uses `Self` as a type parameter
| |
| this trait cannot be made into an object...
= help: only type `SExpr<'x>` implements the trait, consider using it directly instead

error: aborting due to previous error

Expand Down
1 change: 1 addition & 0 deletions tests/ui/object-safety/object-safety-no-static.curr.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn foo() {}
| ^^^ ...because associated function `foo` has no `self` parameter
= help: only type `Bar` implements the trait, consider using it directly instead
help: consider turning `foo` into a method by giving it a `&self` argument
|
LL | fn foo(&self) {}
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn foo() {}
| ^^^ ...because associated function `foo` has no `self` parameter
= help: only type `Bar` implements the trait, consider using it directly instead
= note: required for the cast from `Box<Bar>` to `Box<dyn Foo>`
help: consider turning `foo` into a method by giving it a `&self` argument
|
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn foo(self: &Rc<Self>) -> usize;
| ^^^^^^^^^ ...because method `foo`'s `self` parameter cannot be dispatched on
= help: only type `usize` implements the trait, consider using it directly instead

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/arbitrary-self-types-not-object-safe.rs:33:13
Expand All @@ -31,6 +32,7 @@ LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn foo(self: &Rc<Self>) -> usize;
| ^^^^^^^^^ ...because method `foo`'s `self` parameter cannot be dispatched on
= help: only type `usize` implements the trait, consider using it directly instead
= note: required for the cast from `Rc<usize>` to `Rc<dyn Foo>`

error: aborting due to 2 previous errors
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn foo(self: &Rc<Self>) -> usize;
| ^^^^^^^^^ ...because method `foo`'s `self` parameter cannot be dispatched on
= help: only type `usize` implements the trait, consider using it directly instead
= note: required for the cast from `Rc<usize>` to `Rc<dyn Foo>`

error: aborting due to previous error
Expand Down
2 changes: 2 additions & 0 deletions tests/ui/traits/issue-38604.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ LL | trait Foo where u32: Q<Self> {
| --- ^^^^^^^ ...because it uses `Self` as a type parameter
| |
| this trait cannot be made into an object...
= help: only type `()` implements the trait, consider using it directly instead

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/issue-38604.rs:15:9
Expand All @@ -25,6 +26,7 @@ LL | trait Foo where u32: Q<Self> {
| --- ^^^^^^^ ...because it uses `Self` as a type parameter
| |
| this trait cannot be made into an object...
= help: only type `()` implements the trait, consider using it directly instead
= note: required for the cast from `Box<()>` to `Box<dyn Foo>`

error: aborting due to 2 previous errors
Expand Down
1 change: 1 addition & 0 deletions tests/ui/traits/item-privacy.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -143,6 +143,7 @@ LL | const C: u8 = 0;
= help: consider moving `C` to another trait
= help: consider moving `A` to another trait
= help: consider moving `B` to another trait
= help: only type `S` implements the trait, consider using it directly instead

error[E0223]: ambiguous associated type
--> $DIR/item-privacy.rs:115:12
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@ LL | trait Foo: for<T> Bar<T> {}
| --- ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
| |
| this trait cannot be made into an object...
= help: only type `()` implements the trait, consider using it directly instead
= note: required for the cast from `&()` to `&dyn Foo`

error[E0038]: the trait `Foo` cannot be made into an object
Expand All @@ -35,6 +36,7 @@ LL | trait Foo: for<T> Bar<T> {}
| --- ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
| |
| this trait cannot be made into an object...
= help: only type `()` implements the trait, consider using it directly instead

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/supertrait-object-safety.rs:22:5
Expand All @@ -49,6 +51,7 @@ LL | trait Foo: for<T> Bar<T> {}
| --- ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
| |
| this trait cannot be made into an object...
= help: only type `()` implements the trait, consider using it directly instead

error: aborting due to 3 previous errors; 1 warning emitted

Expand Down
2 changes: 2 additions & 0 deletions tests/ui/traits/object/safety.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ LL | trait Tr {
| -- this trait cannot be made into an object...
LL | fn foo();
| ^^^ ...because associated function `foo` has no `self` parameter
= help: only type `St` implements the trait, consider using it directly instead
= note: required for the cast from `&St` to `&dyn Tr`
help: consider turning `foo` into a method by giving it a `&self` argument
|
Expand All @@ -34,6 +35,7 @@ LL | trait Tr {
| -- this trait cannot be made into an object...
LL | fn foo();
| ^^^ ...because associated function `foo` has no `self` parameter
= help: only type `St` implements the trait, consider using it directly instead
help: consider turning `foo` into a method by giving it a `&self` argument
|
LL | fn foo(&self);
Expand Down
9 changes: 9 additions & 0 deletions tests/ui/traits/test-2.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,9 @@ LL | trait bar { fn dup(&self) -> Self; fn blah<X>(&self); }
| this trait cannot be made into an object...
= help: consider moving `dup` to another trait
= help: consider moving `blah` to another trait
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `bar` for this new enum and using it instead:
i32
u32

error[E0038]: the trait `bar` cannot be made into an object
--> $DIR/test-2.rs:13:5
Expand All @@ -59,6 +62,9 @@ LL | trait bar { fn dup(&self) -> Self; fn blah<X>(&self); }
| this trait cannot be made into an object...
= help: consider moving `dup` to another trait
= help: consider moving `blah` to another trait
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `bar` for this new enum and using it instead:
i32
u32

error[E0038]: the trait `bar` cannot be made into an object
--> $DIR/test-2.rs:13:6
Expand All @@ -76,6 +82,9 @@ LL | trait bar { fn dup(&self) -> Self; fn blah<X>(&self); }
| this trait cannot be made into an object...
= help: consider moving `dup` to another trait
= help: consider moving `blah` to another trait
= help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `bar` for this new enum and using it instead:
i32
u32
= note: required for the cast from `Box<{integer}>` to `Box<dyn bar>`

error: aborting due to 5 previous errors
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,7 @@ LL | trait MyAdd<Rhs=Self> { fn add(&self, other: &Rhs) -> Self; }
| |
| this trait cannot be made into an object...
= help: consider moving `add` to another trait
= help: only type `i32` implements the trait, consider using it directly instead

error: aborting due to 2 previous errors

Expand Down
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