Coerce const FnDefs to implement const Fn traits

compiler/rustc_middle/src/traits/select.rs

@@ -120,7 +120,9 @@ pub enum SelectionCandidate<'tcx> {
 
     /// Implementation of a `Fn`-family trait by one of the anonymous
     /// types generated for a fn pointer type (e.g., `fn(int) -> int`)
-    FnPointerCandidate,
+    FnPointerCandidate {
+        is_const: bool,
+    },
 
     /// Builtin implementation of `DiscriminantKind`.
     DiscriminantKindCandidate,