Add test.

tests/mir-opt/issue_120925_unsafefncast.rs

@@ -0,0 +1,25 @@
+// Verify that we do not ICE when attempting to interpret casts between fn types.
+// skip-filecheck
+
+static FOO: fn() = || assert_ne!(42, 43);
+static BAR: fn(i32, i32) = |a, b| assert_ne!(a, b);
+
+fn main() {
+    FOO();
+
+    let bar: unsafe fn(i32, i32) = BAR;
+
+    let f: fn() = || {};
+    f();
+
+    f();
+
+    f();
+
+    let g: fn(i32) = |i| assert_eq!(i, 2);
+    g(2);
+
+    g(2);
+
+    g(2);
+}