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,

compiler/rustc_middle/src/ty/context.rs

@@ -2750,7 +2750,10 @@ impl<'tcx> TyCtxt<'tcx> {
                 Some(stability) if stability.level.is_unstable() => {
                     // has a `rustc_const_unstable` attribute, check whether the user enabled the
                     // corresponding feature gate.
-                    self.features().declared_lib_features.iter().any(|&(sym, _)| sym == stability.feature)
+                    self.features()
+                        .declared_lib_features
+                        .iter()
+                        .any(|&(sym, _)| sym == stability.feature)
                 }
                 // functions without const stability are either stable user written
                 // const fn or the user is using feature gates and we thus don't