add variation of the rust-lang#135246 unsoundness

Author: lcnr

tests/ui/traits/next-solver/cycles/coinduction/item-bound-via-impl-where-clause.current.stderr

@@ -0,0 +1,23 @@
+error[E0275]: overflow evaluating the requirement `Vec<u8>: Trait<String>`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:21
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: required for `Vec<u8>` to implement `Trait<String>`
+  --> $DIR/item-bound-via-impl-where-clause.rs:22:12
+   |
+LL | impl<L, R> Trait<R> for L
+   |            ^^^^^^^^     ^
+LL | where
+LL |     L: Trait<R>,
+   |        -------- unsatisfied trait bound introduced here
+note: required by a bound in `transmute`
+  --> $DIR/item-bound-via-impl-where-clause.rs:29:17
+   |
+LL | fn transmute<L: Trait<R>, R>(r: L) -> <L::Proof as Trait<R>>::Proof { r }
+   |                 ^^^^^^^^ required by this bound in `transmute`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/next-solver/cycles/coinduction/item-bound-via-impl-where-clause.next.stderr

@@ -0,0 +1,49 @@
+error[E0275]: overflow evaluating the requirement `Vec<u8>: Trait<String>`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:33
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                                 ^
+   |
+note: required by a bound in `transmute`
+  --> $DIR/item-bound-via-impl-where-clause.rs:29:17
+   |
+LL | fn transmute<L: Trait<R>, R>(r: L) -> <L::Proof as Trait<R>>::Proof { r }
+   |                 ^^^^^^^^ required by this bound in `transmute`
+
+error[E0275]: overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof == _`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:21
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0275]: overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof == String`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:21
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0275]: overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof: Sized`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:21
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the return type of a function must have a statically known size
+
+error[E0275]: overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof well-formed`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:21
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0275]: overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof == _`
+  --> $DIR/item-bound-via-impl-where-clause.rs:31:21
+   |
+LL |     let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/next-solver/cycles/coinduction/item-bound-via-impl-where-clause.rs

@@ -0,0 +1,39 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+// A variation of #135246 where the cyclic bounds are part of
+// the impl instead of the impl associated item.
+
+trait Trait<R>: Sized {
+    type Proof: Trait<R, Proof = Self>;
+}
+
+// We need to use indirection here as we otherwise normalize
+// `<L::Proof as Trait<R>>::Proof` before recursing into
+// `R: Trait<R, Proof = <L::Proof as Trait<R>>::Proof>`.
+trait Indir<L: Trait<R>, R>: Trait<R, Proof = <L::Proof as Trait<R>>::Proof> {}
+impl<L, R> Indir<L, R> for R
+where
+    L: Trait<R>,
+    R: Trait<R, Proof = <L::Proof as Trait<R>>::Proof>,
+{}
+
+impl<L, R> Trait<R> for L
+where
+    L: Trait<R>,
+    R: Indir<L, R>,
+ {
+    type Proof = R;
+}
+fn transmute<L: Trait<R>, R>(r: L) -> <L::Proof as Trait<R>>::Proof { r }
+fn main() {
+    let s: String = transmute::<_, String>(vec![65_u8, 66, 67]);
+    //~^ ERROR overflow evaluating the requirement `Vec<u8>: Trait<String>` 
+    //[next]~| ERROR overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof == _`
+    //[next]~| ERROR overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof == String`
+    //[next]~| ERROR overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof: Sized`
+    //[next]~| ERROR overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof well-formed`
+    //[next]~| ERROR overflow evaluating the requirement `<<Vec<u8> as Trait<String>>::Proof as Trait<String>>::Proof == _`
+    println!("{}", s); // ABC
+}