add a test for issue rust-lang/rust#81317

tests/ui/type-inference/regression-issue-81317.rs

@@ -0,0 +1,71 @@
+// Regression test for #81317: type can no longer be infered as of 1.49
+//
+// The problem is that the xor operator and the index.into() call
+// each have two candidate impls that could apply
+// { S as BitXor<S>, S as BitXor<&'a S> } for xor and
+// { T::I as Into<u64>, T::I as Into<S> } for index.into()
+// previously inference was able to infer that the only valid combination was
+// S as BitXor<S> and T::I as Into<S>
+//
+// after rust-lang/rust#73905 this is no longer infered
+//
+// the error message could be better e.g.
+// when iv is unused or has an an explicitly specified type S
+// there is currently the following help message
+//
+// error[E0284]: type annotations needed
+//   --> src/main.rs:13:24
+//    |
+// 44 |     let iv = S ^ index.into();
+//    |                -       ^^^^
+//    |                |
+//    |                type must be known at this point
+//    |
+//    = note: cannot satisfy `<S as BitXor<_>>::Output == _`
+// help: try using a fully qualified path to specify the expected types
+//    |
+// 44 -     let iv = S ^ index.into();
+// 44 +     let iv = S ^ <<T as P>::I as Into<T>>::into(index);
+//
+// this is better as it's actually sufficent to fix the problem,
+// while just specifying the type of iv as currently suggested is insufficent
+//
+//@ check-fail
+
+use std::ops::BitXor;
+
+pub struct S;
+
+pub trait P {
+    type I: Into<u64> + Into<S>;
+}
+
+pub fn decrypt_portion<T: P>(index: T::I) {
+    let iv = S ^ index.into();
+    //~^ ERROR type annotations needed
+    &iv.to_bytes_be();
+}
+
+impl S {
+    fn to_bytes_be(&self) -> &[u8] {
+        &[]
+    }
+}
+
+impl BitXor for S {
+    type Output = S;
+
+    fn bitxor(self, _rhs: Self) -> Self::Output {
+        S
+    }
+}
+
+impl<'a> BitXor<&'a S> for S {
+    type Output = S;
+
+    fn bitxor(self, _rhs: &'a S) -> Self::Output {
+        S
+    }
+}
+
+fn main() {}

tests/ui/type-inference/regression-issue-81317.stderr

@@ -0,0 +1,17 @@
+error[E0282]: type annotations needed
+  --> $DIR/regression-issue-81317.rs:44:9
+   |
+LL |     let iv = S ^ index.into();
+   |         ^^
+LL |
+LL |     &iv.to_bytes_be();
+   |      -- type must be known at this point
+   |
+help: consider giving `iv` an explicit type
+   |
+LL |     let iv: /* Type */ = S ^ index.into();
+   |           ++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.