Closed
Description
This is a minimal version of a part from my project (compilable repro here)
pub struct Struct1;
pub struct Struct2;
pub struct Struct3;
// These two from external crate
pub trait ExternalTrait {
type Output;
fn do_something() -> Self::Output;
}
pub trait ExternalUtilityTrait<T> {
type Output;
fn do_something_with_type(&mut self, input: T) -> Self::Output;
}
// These from current crate
// ########## Variances Cycle Error raised here ########
pub struct ProblemStruct<T: HelperTrait> {
inner: T,
}
pub trait HelperTrait: ExternalUtilityTrait<Struct2> {}
impl<T: HelperTrait> ExternalTrait for ProblemStruct<T> {
type Output = ();
fn do_something() -> Self::Output {
todo!()
}
}
impl<T: HelperTrait> ExternalUtilityTrait<Struct1> for ProblemStruct<T> {
type Output = ();
// Commenting this function out correctly shows the
// unstable inherent assoc. types warning
fn do_something_with_type(&mut self, input: Struct1) -> Self::Output {
todo!()
}
}
impl<T: HelperTrait> ProblemStruct<T> {
// Inherent associated type used here
type Output = ();
}
fn main() {}Expected compiler error (reported after commenting out the marked part above):
error[E0658]: inherent associated types are unstable
--> src/main.rs:46:5
|
46 | type Output = ();
| ^^^^^^^^^^^^^^^^^
|
= note: see issue #8995 <https://github.com/rust-lang/rust/issues/8995> for more information
Actual compiler error:
error[E0391]: cycle detected when computing the variances of `ProblemStruct`
--> src/main.rs:20:1
|
20 | pub struct ProblemStruct<T: HelperTrait> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: ...which requires computing the variances for items in this crate...
note: ...which requires computing function signature of `<impl at src/main.rs:34:1: 34:72>::do_something_with_type`...
--> src/main.rs:39:5
|
39 | fn do_something_with_type(&mut self, input: Struct1) -> Self::Output {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: ...which again requires computing the variances of `ProblemStruct`, completing the cycle
note: cycle used when computing function signature of `<impl at src/main.rs:26:1: 26:56>::do_something`
--> src/main.rs:29:5
|
29 | fn do_something() -> Self::Output {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Meta
Bug was initially discovered with a nightly compiler, but I was able to raise it on stable as well. Both versions used from Flatpak's rust-stable and rust-nightly SDKs
Nightly: rustc --version --verbose:
rustc 1.72.0-nightly (6162f6f12 2023-07-01)
binary: rustc
commit-hash: 6162f6f12339aa81fe16b8a64644ead497e411b2
commit-date: 2023-07-01
host: x86_64-unknown-linux-gnu
release: 1.72.0-nightly
LLVM version: 16.0.5
Stable: rustc --version --verbose:
rustc 1.70.0 (90c541806 2023-05-31)
binary: rustc
commit-hash: 90c541806f23a127002de5b4038be731ba1458ca
commit-date: 2023-05-31
host: x86_64-unknown-linux-gnu
release: 1.70.0
LLVM version: 16.0.2