Add delayed warning to manifest. Pass test

Author: fbunt

src/cargo/util/toml/mod.rs

@@ -1021,6 +1021,8 @@ impl TomlManifest {
             None => false,
         };
 
+        let warn_default_feat = summary.features().contains_key("default-features");
+
         let custom_metadata = project.metadata.clone();
         let mut manifest = Manifest::new(
             summary,
@@ -1056,6 +1058,12 @@ impl TomlManifest {
         for error in errors {
             manifest.warnings_mut().add_critical_warning(error);
         }
+        if warn_default_feat {
+            manifest.warnings_mut().add_warning(
+                "`default-features = [\"..\"]` was found in [features]. \
+                Did you mean to use `default = [\"..\"]`?".to_string()
+            );
+        }
 
         manifest.feature_gate()?;