More helpful error for invalid cargo-features = []

Author: kornelski

src/cargo/core/features.rs

@@ -620,7 +620,32 @@ 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!("unknown Cargo.toml feature `{feature_name}`\n\n");
+            let mut append_see_docs = true;
+
+            if feature_name.contains('_') {
+                let _ = writeln!(msg, "Feature names must use '-' instead of '_'.");
+                append_see_docs = false;
+            } 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."
+                    );
+                }
+            }
+
+            if append_see_docs {
+                let _ = writeln!(
+                    msg,
+                    "See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html for more information."
+                );
+            }
+            bail!(msg)
         };
 
         if *slot {

tests/testsuite/cargo_features.rs

@@ -204,7 +204,10 @@ fn wrong_kind_of_feature() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  unknown cargo feature `build-dir`
+  unknown Cargo.toml feature `build-dir`
+
+  This feature can be enabled via -Zbuild-dir or the `[unstable]` section in config.toml.
+  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html for more information.
 
 "#]])
         .run();
@@ -233,7 +236,9 @@ fn feature_syntax() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  unknown cargo feature `bad_feature`
+  unknown Cargo.toml feature `bad_feature`
+
+  Feature names must use '-' instead of '_'.
 
 "#]])
         .run();