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rustc suggests dyn SomeTrait for traits which aren't object safe #119525
Description
Code
fn ord_prefer_dot(s: &str) -> Ord {
(s.starts_with("."), s)
}Current output
error[E0038]: the trait `Ord` cannot be made into an object
--> src/lib.rs:1:31
|
1 | fn ord_prefer_dot(s: &str) -> Ord {
| ^^^ `Ord` cannot be made into an object
|
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>
--> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs:305:15
|
305 | pub trait Eq: PartialEq<Self> {
| ^^^^^^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
...
793 | pub trait Ord: Eq + PartialOrd<Self> {
| ^^^^^^^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
error[E0782]: trait objects must include the `dyn` keyword
--> src/lib.rs:1:31
|
1 | fn ord_prefer_dot(s: &str) -> Ord {
| ^^^
|
help: add `dyn` keyword before this trait
|
1 | fn ord_prefer_dot(s: &str) -> dyn Ord {
| +++
Some errors have detailed explanations: E0038, E0782.
For more information about an error, try `rustc --explain E0038`.Desired output
error[E0038]: the trait `Ord` cannot be made into an object
--> src/lib.rs:1:31
|
1 | fn ord_prefer_dot(s: &str) -> Ord {
| ^^^ `Ord` cannot be made into an object
|
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>
--> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs:305:15
|
305 | pub trait Eq: PartialEq<Self> {
| ^^^^^^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
...
793 | pub trait Ord: Eq + PartialOrd<Self> {
| ^^^^^^^^^^^^^^^^ the trait cannot be made into an object because it uses `Self` as a type parameter
help: add `impl` keyword before this trait
|
1 | fn ord_prefer_dot(s: &str) -> impl Ord {
| ++++Rationale and extra context
In this case, a possible valid implementation would be:
fn ord_prefer_dot(s: &str) -> impl Ord + '_ {
(s.starts_with("."), s)
}
It would be an improvement to not suggest dyn Ord, but ideally rustc would suggest impl Ord I think.
Other cases
No response
Anything else?
No response
Activity
fmease commentedon Jan 3, 2024
As far as I can tell, #119148 would slightly help here by also suggesting
impl Ord(next toBox<dyn Ord>).@estebank, do you think you can tweak #119148 to prefer impl-Trait over dyn-Trait if
!tcx.check_is_object_safe(def_id)(or in a follow-up PR)? Unless that extra query call would lead to cycles of course.Account for object unsafe traits
Rollup merge of rust-lang#119148 - estebank:bare-traits, r=davidtwco
Unrolled build for rust-lang#119148
Rollup merge of rust-lang#119148 - estebank:bare-traits, r=davidtwco