Repository navigation
Function with impl trait in argument position does not see all impls of trait on value returned from function with impl trait in return position #59365
Description
struct A {}
struct B {}
trait Marker {}
impl Marker for A {}
impl Marker for B {}
trait Marked<M: Marker> {}
struct Singleton {}
impl<M: Marker> Marked<M> for Singleton {}
fn produce_a() -> impl Marked<A> {
Singleton{}
}
fn produce_b() -> impl Marked<B> {
Singleton{}
}
fn combine<M1: Marker, M2: Marker>(_t1: &impl Marked<M1>, _t2: &impl Marked<M2>) -> impl Marked<M1> + Marked<M2> {
Singleton{}
}
fn consume_a(_: &impl Marked<A>) {}
fn main() {
let an_a = produce_a();
consume_a(&an_a);
consume_a(&an_a);
let a_b = produce_b();
//consume_a(&a_b); // Errors, as hoped.
let combined1 = combine(&a_b, &an_a);
let combined2 = combine(&an_a, &a_b);
consume_a(&combined1);
consume_a(&combined2);
let combined3 = combine(&combined1, &a_b);
let combined4 = combine(&combined2, &a_b);
consume_a(&combined3);
//consume_a(&combined4); // Errors???
println!("Hello, world!");
}Output:
Hello, world!
Errors:
Compiling playground v0.0.1 (/playground)
warning: unused variable: `combined4`
--> src/main.rs:40:9
|
40 | let combined4 = combine(&combined2, &a_b);
| ^^^^^^^^^ help: consider using `_combined4` instead
|
= note: #[warn(unused_variables)] on by default
warning: struct is never constructed: `B`
--> src/main.rs:2:1
|
2 | struct B {}
| ^^^^^^^^
|
= note: #[warn(dead_code)] on by default
Finished dev [unoptimized + debuginfo] target(s) in 0.77s
Running `target/debug/playground`
With the second commented line uncommented:
Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `impl Marked<B>+Marked<B>: Marked<A>` is not satisfied
--> src/main.rs:42:5
|
42 | consume_a(&combined4); // Errors???
| ^^^^^^^^^ the trait `Marked<A>` is not implemented for `impl Marked<B>+Marked<B>`
|
note: required by `consume_a`
--> src/main.rs:27:1
|
27 | fn consume_a(_: &impl Marked<A>) {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.
error: Could not compile `playground`.
To learn more, run the command again with --verbose.
I tested this across Stable, Beta, and Nightly in the playground, same behavior in all of them.
My intent with this code was to have a set of "markers" that could be associated with a type purely at a type-system level, with the actual implementation being a single concrete ZST. As the first commented-out line shows, this works in the simple case, but things get confused if you compose markers together multiple times.
Activity
Centril commentedon Mar 22, 2019
Simplified: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9bb30a29adb5af30e9675b81614f1b87
istankovic commentedon Oct 20, 2023
This seems to have been fixed. Tested with 1.73.