Rollup merge of #105847 - compiler-errors:issue-104396, r=oli-obk

Ensure param-env is const before calling `eval_to_valtree`

Other queries call `ParamEnv::with_const` *inside* of the query itself (e.g. `const_eval_global_id_for_typeck`), so this could alternatively be moved into the provider of `eval_to_valtree` instead. I don't have a particularly strong opinion, though *theoretically* caching is better if we make the query keys more constrained.

I'm not exactly sure how this is an effect of the `-Zmir-opt-level=3` flag. Maybe something about the inliner causes us to inline an unevaluated const into a body where it can be evaluated, but where it has not yet been normalized.

This seems likely, since we're inlining `from_fn_1::<{ N / 2 }, _>` in `from_fn_2`, which means that we will need to evaluate that constant during the const prop pass after inlining.

Fixes #104396

2 files changed, 47 insertions, 0 deletions

diff --git a/src/test/ui/consts/issue-104396.rs b/src/test/ui/consts/issue-104396.rs
new file mode 100644
index 00000000000..315b0cf0fd6
--- /dev/null
+++ b/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);
+}
diff --git a/src/test/ui/consts/issue-104396.stderr b/src/test/ui/consts/issue-104396.stderr
new file mode 100644
index 00000000000..5856bee09a3
--- /dev/null
+++ b/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
+