Fix Crash when indexing into a generic slice with computed length in const fns

Author: eval-exec

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -347,18 +347,21 @@ where
 
     // Check the qualifs of the value of `const` items.
     let uneval = match constant.const_ {
-        Const::Ty(_, ct)
-            if matches!(
-                ct.kind(),
-                ty::ConstKind::Param(_) | ty::ConstKind::Error(_) | ty::ConstKind::Value(_)
-            ) =>
-        {
-            None
-        }
-        Const::Ty(_, c) => {
-            bug!("expected ConstKind::Param or ConstKind::Value here, found {:?}", c)
-        }
+        // Type-level constants that are already concrete or are params/errors: fall back to
+        // type-based qualifs below.
+        Const::Ty(_, ct) => match ct.kind() {
+            ty::ConstKind::Param(_) | ty::ConstKind::Error(_) | ty::ConstKind::Value(_) => None,
+            // A type-level unevaluated const (e.g., from generic const exprs). Treat like a
+            // MIR unevaluated const so we can look up qualifs on the referenced def.
+            ty::ConstKind::Unevaluated(uv) => Some(mir::UnevaluatedConst::new(uv.def, uv.args)),
+            // Any other kind should not reach this path in MIR const-checking.
+            other => {
+                bug!("expected ConstKind::Param or ConstKind::Value here, found {:?}", other)
+            }
+        },
+        // A MIR unevaluated const: inspect its definition's qualifs below.
         Const::Unevaluated(uv, _) => Some(uv),
+        // Already a value: rely on type-based qualifs below.
         Const::Val(..) => None,
     };
 

tests/ui/const-generics/generic_const_exprs/ice-qualifs-unevaluated-operand.rs

@@ -0,0 +1,35 @@
+// check-pass
+// This is a regression test for an ICE in const qualifs where
+// `Const::Ty` containing `ty::ConstKind::Unevaluated` was not handled.
+// The pattern arises with `generic_const_exprs` and const fn using
+// array lengths like `LEN * LEN` and repeat expressions.
+
+#![feature(generic_const_exprs)]
+
+trait One: Sized + Copy {
+    const ONE: Self;
+}
+
+const fn noop<T: One>(a: &mut T, b: &mut T) {
+    let _ = (a, b);
+}
+
+struct Test<T: One, const LEN: usize>([T; LEN * LEN])
+where
+    [u8; LEN * LEN]:;
+
+impl<T: One, const LEN: usize> Test<T, LEN>
+where
+    [u8; LEN * LEN]:,
+{
+    const fn test() -> Self {
+        let mut a = Self([T::ONE; LEN * LEN]);
+        let mut i = 0;
+        while i < LEN {
+            let mut one = T::ONE;
+            noop(&mut one, &mut a.0[i * i + 1]);
+            i += 1;
+        }
+        a
+    }
+}