test: add test suite for the 85681 issue

tests/ui/traits/bad-error-message-impl-trait-double-ref-issue-85681.rs

@@ -0,0 +1,13 @@
+fn foo(_x: u32, a: impl Into<String>, _y: u32, b: impl Into<String>) {
+    println!("fox: a={}, b={}", a.into(), b.into());
+}
+
+fn main() {
+    let bar: String = "bar".to_string();
+    let baz: &str = "baz";
+
+    for (a, b) in &[(&bar, baz)] {
+        let a: &String = a;
+        foo(42, a, 43, b); //~ ERROR: the trait bound `String: From<&&str>` is not satisfied [E0277]
+    }
+}

tests/ui/traits/bad-error-message-impl-trait-double-ref-issue-85681.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the trait bound `String: From<&&str>` is not satisfied
+  --> $DIR/bad-error-message-impl-trait-double-ref-issue-85681.rs:11:24
+   |
+LL |         foo(42, a, 43, b);
+   |         ---            ^ the trait `From<&&str>` is not implemented for `String`
+   |         |
+   |         required by a bound introduced by this call
+   |
+   = help: consider casting the `&&str` value to `&str`
+   = note: required for `&&str` to implement `Into<String>`
+note: required by a bound in `foo`
+  --> $DIR/bad-error-message-impl-trait-double-ref-issue-85681.rs:1:56
+   |
+LL | fn foo(_x: u32, a: impl Into<String>, _y: u32, b: impl Into<String>) {
+   |                                                        ^^^^^^^^^^^^ required by this bound in `foo`
+help: consider dereferencing here
+   |
+LL |         foo(42, a, 43, *b);
+   |                        +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.