Add utility function to iterate over all cfg in an expr

Author: Urgau

crates/cargo-platform/src/cfg.rs

@@ -89,6 +89,19 @@ impl CfgExpr {
             CfgExpr::Value(ref e) => cfg.contains(e),
         }
     }
+
+    pub fn fold<E>(&self, f: &impl Fn(&Cfg) -> Result<(), E>) -> Result<(), E> {
+        match *self {
+            CfgExpr::Not(ref e) => CfgExpr::fold(e, f),
+            CfgExpr::All(ref e) | CfgExpr::Any(ref e) => {
+                for e in e {
+                    let _ = CfgExpr::fold(e, f)?;
+                }
+                Ok(())
+            }
+            CfgExpr::Value(ref e) => f(e),
+        }
+    }
 }
 
 impl FromStr for CfgExpr {