Rollup merge of #81426 - BoxyUwU:boxychangesv2, r=oli-obk

const_evaluatable: expand abstract consts in try_unify

See this [zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/combining.20const.20bounds) for more info

cc ```@lcnr```
r? ```@oli-obk```

src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-1.rs

@@ -0,0 +1,24 @@
+// run-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+fn callee<const M2: usize>() -> usize
+where
+    [u8; M2 + 1]: Sized,
+{
+    M2
+}
+
+fn caller<const N1: usize>() -> usize
+where
+    [u8; N1 + 1]: Sized,
+    [u8; (N1 + 1) + 1]: Sized,
+{
+    callee::<{ N1 + 1 }>()
+}
+
+fn main() {
+    assert_eq!(caller::<4>(), 5);
+}
+
+// Test that the ``(N1 + 1) + 1`` bound on ``caller`` satisfies the ``M2 + 1`` bound on ``callee``

src/test/ui/const-generics/const_evaluatable_checked/nested-abstract-consts-2.rs

@@ -0,0 +1,35 @@
+// run-pass
+#![feature(const_evaluatable_checked, const_generics)]
+#![allow(incomplete_features)]
+
+struct Generic<const K: u64>;
+
+struct ConstU64<const K: u64>;
+
+impl<const K: u64> Generic<K>
+where
+    ConstU64<{ K - 1 }>: ,
+{
+    fn foo(self) -> u64 {
+        K
+    }
+}
+
+impl<const K: u64> Generic<K>
+where
+    ConstU64<{ K - 1 }>: ,
+    ConstU64<{ K + 1 }>: ,
+    ConstU64<{ K + 1 - 1 }>: ,
+{
+    fn bar(self) -> u64 {
+        let x: Generic<{ K + 1 }> = Generic;
+        x.foo()
+    }
+}
+
+fn main() {
+    assert_eq!((Generic::<10>).bar(), 11);
+}
+
+// Test that the ``ConstU64<{ K + 1 - 1}>`` bound on ``bar``'s impl block satisfies the
+// ``ConstU64<{K - 1}>`` bound on ``foo``'s impl block

src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs

@@ -0,0 +1,35 @@
+// run-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+fn zero_init<const N: usize>() -> Substs1<N>
+where
+    [u8; N + 1]: ,
+{
+    Substs1([0; N + 1])
+}
+struct Substs1<const N: usize>([u8; N + 1])
+where
+    [(); N + 1]: ;
+
+fn substs2<const M: usize>() -> Substs1<{ M * 2 }>
+where
+    [(); { M * 2 } + 1]: ,
+{
+    zero_init::<{ M * 2 }>()
+}
+
+fn substs3<const L: usize>() -> Substs1<{ (L - 1) * 2 }>
+where
+    [(); (L - 1)]: ,
+    [(); (L - 1) * 2 + 1]: ,
+{
+    substs2::<{ L - 1 }>()
+}
+
+fn main() {
+    assert_eq!(substs3::<2>().0, [0; 3]);
+}
+
+// Test that the ``{ (L - 1) * 2 + 1 }`` bound on ``substs3`` satisfies the
+// ``{ N + 1 }`` bound on ``Substs1``

src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-2.rs

@@ -0,0 +1,29 @@
+// run-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features, unused_parens, unused_braces)]
+
+fn zero_init<const N: usize>() -> Substs1<{ (N) }>
+where
+    [u8; { (N) }]: ,
+{
+    Substs1([0; { (N) }])
+}
+
+struct Substs1<const N: usize>([u8; { (N) }])
+where
+    [(); { (N) }]: ;
+
+fn substs2<const M: usize>() -> Substs1<{ (M) }> {
+    zero_init::<{ (M) }>()
+}
+
+fn substs3<const L: usize>() -> Substs1<{ (L) }> {
+    substs2::<{ (L) }>()
+}
+
+fn main() {
+    assert_eq!(substs3::<2>().0, [0; 2]);
+}
+
+// Test that the implicit ``{ (L) }`` bound on ``substs3`` satisfies the
+// ``{ (N) }`` bound on ``Substs1``