Add regression test for rust-lang#71394

Author: ecstatic-morse

src/test/ui/suggestions/issue-71394-no-from-impl.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let data: &[u8] = &[0; 10];
+    let _: &[i8] = data.into();
+    //~^ ERROR the trait bound `&[i8]: std::convert::From<&[u8]>` is not satisfied
+}

src/test/ui/suggestions/issue-71394-no-from-impl.stderr

@@ -0,0 +1,11 @@
+error[E0277]: the trait bound `&[i8]: std::convert::From<&[u8]>` is not satisfied
+  --> $DIR/issue-71394-no-from-impl.rs:3:25
+   |
+LL |     let _: &[i8] = data.into();
+   |                         ^^^^ the trait `std::convert::From<&[u8]>` is not implemented for `&[i8]`
+   |
+   = note: required because of the requirements on the impl of `std::convert::Into<&[i8]>` for `&[u8]`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.