Rollup merge of #122572 - the8472:test-const-deadness, r=RalfJung

add test for #122301 to cover behavior that's on stable

If this ought to be broken it should at least happen intentionally

See #122301

tests/ui/consts/control-flow/dead_branches_dont_eval.rs

@@ -0,0 +1,46 @@
+//@ build-pass
+
+// issue 122301 - currently the only way to supress
+// const eval and codegen of code conditional on some other const
+
+struct Foo<T, const N: usize>(T);
+
+impl<T, const N: usize> Foo<T, N> {
+    const BAR: () = if N == 0 {
+        panic!()
+    };
+}
+
+struct Invoke<T, const N: usize>(T);
+
+impl<T, const N: usize> Invoke<T, N> {
+    const FUN: fn() = if N != 0 {
+        || Foo::<T, N>::BAR
+    } else {
+        || {}
+    };
+}
+
+// without closures
+
+struct S<T>(T);
+impl<T> S<T> {
+    const C: () = panic!();
+}
+
+const fn bar<T>() { S::<T>::C }
+
+struct ConstIf<T, const N: usize>(T);
+
+impl<T, const N: usize> ConstIf<T, N> {
+    const VAL: () = if N != 0 {
+        bar::<T>() // not called for N == 0, and hence not monomorphized
+    } else {
+        ()
+    };
+}
+
+fn main() {
+    let _val = Invoke::<(), 0>::FUN();
+    let _val = ConstIf::<(), 0>::VAL;
+}