Add method to Candidate that determines its promotability rules

src/librustc_mir/transform/promote_consts.rs

@@ -80,6 +80,17 @@ pub enum Candidate {
     Argument { bb: BasicBlock, index: usize },
 }
 
+impl Candidate {
+    /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
+    fn forces_explicit_promotion(&self) -> bool {
+        match self {
+            Candidate::Ref(_) |
+            Candidate::Repeat(_) => false,
+            Candidate::Argument { .. } => true,
+        }
+    }
+}
+
 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
     let attrs = tcx.get_attrs(def_id);
     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
@@ -727,11 +738,7 @@ pub fn validate_candidates(
     };
 
     candidates.iter().copied().filter(|&candidate| {
-        validator.explicit = match candidate {
-            Candidate::Ref(_) |
-            Candidate::Repeat(_) => false,
-            Candidate::Argument { .. } => true,
-        };
+        validator.explicit = candidate.forces_explicit_promotion();
 
         // FIXME(eddyb) also emit the errors for shuffle indices
         // and `#[rustc_args_required_const]` arguments here.