Rollup merge of #81577 - BoxyUwU:subexpr_const_evaluatable, r=oli-obk

const_evaluatable: consider sub-expressions to be evaluatable

see [zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/const_evaluatable.3A.20subexpressions) for more info

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

2 files changed, 17 insertions, 1 deletions

diff --git a/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs b/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs
index 1428f774b0d..4d0b87efc77 100644
--- a/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs
+++ b/src/test/ui/const-generics/const_evaluatable_checked/nested_uneval_unification-1.rs
@@ -21,7 +21,6 @@ where
 
 fn substs3<const L: usize>() -> Substs1<{ (L - 1) * 2 }>
 where
-    [(); (L - 1)]: ,
     [(); (L - 1) * 2 + 1]: ,
 {
     substs2::<{ L - 1 }>()
diff --git a/src/test/ui/const-generics/const_evaluatable_checked/subexprs_are_const_evalutable.rs b/src/test/ui/const-generics/const_evaluatable_checked/subexprs_are_const_evalutable.rs
new file mode 100644
index 00000000000..11c0760cdfe
--- /dev/null
+++ b/src/test/ui/const-generics/const_evaluatable_checked/subexprs_are_const_evalutable.rs
@@ -0,0 +1,17 @@
+// run-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+fn make_array<const M: usize>() -> [(); M + 1] {
+    [(); M + 1]
+}
+
+fn foo<const N: usize>() -> [(); (N * 2) + 1] {
+    make_array::<{ N * 2 }>()
+}
+
+fn main() {
+    assert_eq!(foo::<10>(), [(); 10 * 2 + 1])
+}
+
+// Tests that N * 2 is considered const_evalutable by appearing as part of the (N * 2) + 1 const