Fix ICE checking for feature gated const fn

Author: Jarcho

clippy_utils/src/qualify_min_const_fn.rs

@@ -375,8 +375,8 @@ fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: Option<&RustcVersion>) -> b
                         .expect("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted"),
                 )
             } else {
-                // `rustc_mir::const_eval::is_const_fn` should return false for unstably const functions.
-                unreachable!();
+                // Unstable const fn with the feature enabled.
+                msrv.is_none()
             }
         } else {
             true

tests/ui/crashes/ice-7126.rs

@@ -0,0 +1,14 @@
+// This test requires a feature gated const fn and will stop working in the future.
+
+#![feature(const_btree_new)]
+
+use std::collections::BTreeMap;
+
+struct Foo(BTreeMap<i32, i32>);
+impl Foo {
+    fn new() -> Self {
+        Self(BTreeMap::new())
+    }
+}
+
+fn main() {}