More helpful error for invalid cargo-features = []

Author: kornelski

src/cargo/core/features.rs

@@ -620,7 +620,30 @@ impl Features {
     ) -> CargoResult<()> {
         let nightly_features_allowed = self.nightly_features_allowed;
         let Some((slot, feature)) = self.status(feature_name) else {
-            bail!("unknown cargo feature `{}`", feature_name)
+            let mut msg = format!(
+                "feature `{feature_name}` is not a valid unstable feature for Cargo.toml\n\n"
+            );
+
+            if feature_name.contains('_') {
+                let _ = writeln!(msg, "Feature names must use '-' instead of '_'.");
+            } else {
+                let underscore_name = feature_name.replace('-', "_");
+                if CliUnstable::help()
+                    .iter()
+                    .any(|(option, _)| *option == underscore_name)
+                {
+                    let _ = writeln!(
+                        msg,
+                        "This feature can be enabled via -Z{feature_name} or the `[unstable]` section in config.toml."
+                    );
+                }
+            }
+
+            let _ = writeln!(
+                msg,
+                "See https://doc.rust-lang.org/cargo/reference/unstable.html for more information."
+            );
+            bail!(msg)
         };
 
         if *slot {

tests/testsuite/cargo_features.rs

@@ -173,8 +173,9 @@ fn unknown_feature() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  unknown cargo feature `foo`
+  feature `foo` is not a valid unstable feature for Cargo.toml
 
+  See https://doc.rust-lang.org/cargo/reference/unstable.html for more information.
 "#]])
         .run();
 }