Cycle detected error instead of invalid function signature on trait

[AngelOnFira]

Given the following code: Link to playground

fn main() {}

trait A {
    fn a(aa: B) -> Result<_, B> {
        Ok(())
    }
}

enum B {}

The current output is:

   Compiling s1 v0.1.0 (/Users/forest/Documents/test/s1)
error[E0391]: cycle detected when computing function signature of `A::a`
 --> src/main.rs:3:5
  |
3 |     fn a(aa: B) -> Result<_, B> {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: ...which requires type-checking `A::a`...
 --> src/main.rs:3:5
  |
3 |     fn a(aa: B) -> Result<_, B> {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires computing the variances of `B`...
 --> src/main.rs:8:1
  |
8 | enum B {}
  | ^^^^^^
  = note: ...which requires computing the variances for items in this crate...
  = note: ...which again requires computing function signature of `A::a`, completing the cycle
note: cycle used when collecting item types in top-level module
 --> src/main.rs:1:1
  |
1 | fn main() {}
  | ^^^^^^^^^

For more information about this error, try `rustc --explain E0391`.
error: could not compile `s1` due to previous error

I expected the compiler to let me know that function signature was wrong with the _ in Result<_, B>. Once I changed this, it compiled fine.

[yanchen4791]

@rustbot claim

[yanchen4791]

I've come up a solution with which the output of this test case will be like the following:

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> main.rs:3:27
  |
3 |     fn a(aa: B) -> Result<_, B> {
  |                           ^ not allowed in type signatures

error[E0391]: cycle detected when computing function signature of `A::a`
 --> main.rs:3:5
  |
3 |     fn a(aa: B) -> Result<_, B> {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: ...which requires type-checking `A::a`...
 --> main.rs:3:5
  |
3 |     fn a(aa: B) -> Result<_, B> {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires computing the variances of `B`...
 --> main.rs:8:1
  |
8 | enum B {}
  | ^^^^^^
  = note: ...which requires computing the variances for items in this crate...
  = note: ...which again requires computing function signature of `A::a`, completing the cycle
note: cycle used when collecting item types in top-level module
 --> main.rs:1:1
  |
1 | fn main() {}
  | ^^^^^^^^^

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0121, E0391.
For more information about an error, try `rustc --explain E0121`.

It is cumbersome for still having E0391 printed, but it seems not easy to remove this without a significant code changes. I think the first message E0121 "the placeholder _ is not allowed within types on item signatures for return types" should have told the user enough information how to fix the problem. Any comments/suggestions?

[spastorino]

@yanchen4791 having an early return for when there's no generics on compiler/rustc_typeck/src/variance/mod.rs will solve this problem.

[yanchen4791]

@spastorino Thanks a lot for the advise!

After the following code change, the faulty cycle detected message has gone way, and correct output generated.

--- a/compiler/rustc_typeck/src/variance/mod.rs
+++ b/compiler/rustc_typeck/src/variance/mod.rs
@@ -37,6 +37,12 @@ fn crate_variances(tcx: TyCtxt<'_>, (): ()) -> CrateVariancesMap<'_> {
 }

 fn variances_of(tcx: TyCtxt<'_>, item_def_id: DefId) -> &[ty::Variance] {
+
+    // Skip items with no generics - there's nothing to infer in them.
+    if tcx.generics_of(item_def_id).count() == 0 {
+        return &[];
+    }
+
     match tcx.def_kind(item_def_id) {
         DefKind::Fn
         | DefKind::AssocFn

[yanchen4791]

The output of the test case after the fix is correct:

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> main.rs:3:27
  |
3 |     fn a(aa: B) -> Result<_, B> {
  |                    -------^----
  |                    |      |
  |                    |      not allowed in type signatures
  |                    help: replace with the correct return type: `Result<(), B>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0121`.

[spastorino]

@yanchen4791 great, now you can send a pull request :).

[AngelOnFira]

Thanks for your work @yanchen4791!