Add a codegen test for a missed optimisation with spare niches

tests/codegen-llvm/enum/enum-array-index-spare-niche.rs

@@ -0,0 +1,28 @@
+//! Regression test for https://github.com/rust-lang/rust/issues/113899.
+//! When indexing into an array of an enum type with spare niches, the compiler
+//! used to emit a superfluous branch checking whether the loaded value was
+//! a niche value. Every element in the array is a valid variant, so this check
+//! is unnecessary and should be optimised away.
+
+//@ compile-flags: -Copt-level=3
+#![crate_type = "lib"]
+
+#[derive(Clone, Copy)]
+pub enum Outer {
+    A([u8; 8]),
+    B([u8; 8]),
+}
+
+pub struct Error(u8);
+
+// CHECK-LABEL: @test
+#[no_mangle]
+pub fn test(x: usize) -> Result<Outer, Error> {
+    // There should be exactly one comparison: the bounds check on `x`.
+    // There must be no second comparison checking the discriminant
+    // against the niche value used by `Option<Outer>` (from `get()`).
+    // CHECK: icmp ult
+    // CHECK-NOT: icmp
+    // CHECK: ret void
+    [Outer::A([10; 8]), Outer::B([20; 8])].get(x).copied().ok_or(Error(5))
+}