minor: Add some debug traces for cfg fetching

crates/project_model/src/cfg_flag.rs

@@ -1,7 +1,7 @@
 //! Parsing of CfgFlags as command line arguments, as in
 //!
 //! rustc main.rs --cfg foo --cfg 'feature="bar"'
-use std::str::FromStr;
+use std::{fmt, str::FromStr};
 
 use cfg::CfgOptions;
 
@@ -48,3 +48,16 @@ impl Extend<CfgFlag> for CfgOptions {
         }
     }
 }
+
+impl fmt::Display for CfgFlag {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            CfgFlag::Atom(atom) => f.write_str(atom),
+            CfgFlag::KeyValue { key, value } => {
+                f.write_str(key)?;
+                f.write_str("=")?;
+                f.write_str(value)
+            }
+        }
+    }
+}