fix tidy, small cleanup

Author: lcnr

compiler/rustc_feature/src/active.rs

@@ -586,7 +586,7 @@ declare_features! (
     (active, if_let_guard, "1.47.0", Some(51114), None),
 
     /// Allows non trivial generic constants which have to be manually propageted upwards.
-    (active, const_evaluatable_checked, "1.48.0", Some(0), None),
+    (active, const_evaluatable_checked, "1.48.0", Some(76560), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -73,10 +73,6 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
         }
     }
 
-    if concrete.is_ok() {
-        debug!("is_const_evaluatable: concrete ~~> ok");
-    } else {
-        debug!("is_const_evaluatable: concrete ~~> err");
-    }
+    debug!(?concrete, "is_const_evaluatable");
     concrete.map(drop)
 }

compiler/rustc_typeck/src/collect.rs

@@ -1678,13 +1678,8 @@ fn predicates_defined_on(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericPredicate
 
     if tcx.features().const_evaluatable_checked {
         let const_evaluatable = const_evaluatable_predicates_of(tcx, def_id, &result);
-        if result.predicates.is_empty() {
-            result.predicates = tcx.arena.alloc_from_iter(const_evaluatable);
-        } else {
-            result.predicates = tcx
-                .arena
-                .alloc_from_iter(result.predicates.iter().copied().chain(const_evaluatable));
-        }
+        result.predicates =
+            tcx.arena.alloc_from_iter(result.predicates.iter().copied().chain(const_evaluatable));
     }
 
     debug!("predicates_defined_on({:?}) = {:?}", def_id, result);

src/test/ui/const-generics/const_evaluatable_checked/feature-gate-const_evaluatable_checked.rs

@@ -0,0 +1,14 @@
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+type Arr<const N: usize> = [u8; N - 1];
+
+fn test<const N: usize>() -> Arr<N> where Arr<N>: Default {
+    //~^ ERROR constant expression depends
+    Default::default()
+}
+
+fn main() {
+    let x = test::<33>();
+    assert_eq!(x, [0; 32]);
+}

src/test/ui/const-generics/const_evaluatable_checked/feature-gate-const_evaluatable_checked.stderr

@@ -0,0 +1,10 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/feature-gate-const_evaluatable_checked.rs:6:30
+   |
+LL | fn test<const N: usize>() -> Arr<N> where Arr<N>: Default {
+   |                              ^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error
+