Add test cases for #64945

This also tests that `&&[]` no longer causes an ICE in this PR (although
the test fails the borrow checker). This could be more complete.

Author: ecstatic-morse

src/test/ui/consts/const-eval/generic-slice.rs

@@ -0,0 +1,31 @@
+// Several variants of #64945.
+
+// This struct is not important, we just use it to put `T` and `'a` in scope for our associated
+// consts.
+struct Generic<'a, T>(std::marker::PhantomData<&'a T>);
+
+impl<'a, T: 'static> Generic<'a, T> {
+    const EMPTY_SLICE: &'a [T] = {
+        let x: &'a [T] = &[];
+        x
+    };
+
+    const EMPTY_SLICE_REF: &'a &'static [T] = {
+        let x: &'static [T] = &[];
+        &x
+        //~^ ERROR `x` does not live long enough
+    };
+}
+
+static mut INTERIOR_MUT_AND_DROP: &'static [std::cell::RefCell<Vec<i32>>] = {
+    let x: &[_] = &[];
+    x
+};
+
+static mut INTERIOR_MUT_AND_DROP_REF: &'static &'static [std::cell::RefCell<Vec<i32>>] = {
+    let x: &[_] = &[];
+    &x
+    //~^ ERROR `x` does not live long enough
+};
+
+fn main() {}

src/test/ui/consts/const-eval/generic-slice.stderr

@@ -0,0 +1,30 @@
+error[E0597]: `x` does not live long enough
+  --> $DIR/generic-slice.rs:15:9
+   |
+LL | impl<'a, T: 'static> Generic<'a, T> {
+   |      -- lifetime `'a` defined here
+...
+LL |         &x
+   |         ^^
+   |         |
+   |         borrowed value does not live long enough
+   |         using this value as a constant requires that `x` is borrowed for `'a`
+LL |
+LL |     };
+   |     - `x` dropped here while still borrowed
+
+error[E0597]: `x` does not live long enough
+  --> $DIR/generic-slice.rs:27:5
+   |
+LL |     &x
+   |     ^^
+   |     |
+   |     borrowed value does not live long enough
+   |     using this value as a static requires that `x` is borrowed for `'static`
+LL |
+LL | };
+   | - `x` dropped here while still borrowed
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0597`.