Ensure param-env is const before calling eval_to_valtree

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -569,7 +569,9 @@ fn eval_ty_constant(
             ty::ConstKind::Unevaluated(uv) => {
                 let instance = self.resolve(uv.def, uv.substs)?;
                 let cid = GlobalId { instance, promoted: None };
-                self.ctfe_query(span, |tcx| tcx.eval_to_valtree(self.param_env.and(cid)))?
+                self.ctfe_query(span, |tcx| {
+                    tcx.eval_to_valtree(self.param_env.with_const().and(cid))
+                })?
                 .unwrap_or_else(|| bug!("unable to create ValTree for {uv:?}"))
             }
             ty::ConstKind::Bound(..) | ty::ConstKind::Infer(..) => {

src/test/ui/consts/issue-104396.rs

@@ -0,0 +1,36 @@
+// compile-flags: -Zmir-opt-level=3
+// check-pass
+
+#![feature(generic_const_exprs)]
+//~^ WARN the feature `generic_const_exprs` is incomplete
+
+#[inline(always)]
+fn from_fn_1<const N: usize, F: FnMut(usize) -> f32>(mut f: F) -> [f32; N] {
+    let mut result = [0.0; N];
+    let mut i = 0;
+    while i < N {
+        result[i] = f(i);
+        i += 1;
+    }
+    result
+}
+
+pub struct TestArray<const N: usize>
+where
+    [(); N / 2]:,
+{
+    array: [f32; N / 2],
+}
+
+impl<const N: usize> TestArray<N>
+where
+    [(); N / 2]:,
+{
+    fn from_fn_2<F: FnMut(usize) -> f32>(f: F) -> Self {
+        Self { array: from_fn_1(f) }
+    }
+}
+
+fn main() {
+    TestArray::<4>::from_fn_2(|i| 0.0);
+}

src/test/ui/consts/issue-104396.stderr

@@ -0,0 +1,11 @@
+warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-104396.rs:4:12
+   |
+LL | #![feature(generic_const_exprs)]
+   |            ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+