More helpful error for invalid cargo-features = [] (#15781)

The error message for unknown values of `cargo-features` was unhelpful:

```text
Caused by:
  unknown cargo feature `build-dir`
```

I've made it more explanatory, and matching the style of the other
errors:

```text
Caused by:
  feature `build-dir` is not a valid unstable feature for Cargo.toml

  This feature can be enabled via -Zbuild-dir or the `[unstable]` section in config.toml.
  See https://doc.rust-lang.org/cargo/reference/unstable.html for more information.
```

Author: epage

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

@@ -173,7 +173,72 @@ fn unknown_feature() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  unknown cargo feature `foo`
+  unknown Cargo.toml feature `foo`
+
+  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html for more information.
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn wrong_kind_of_feature() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                cargo-features = ["build-dir"]
+
+                [package]
+                name = "a"
+                version = "0.0.1"
+                edition = "2015"
+                authors = []
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    p.cargo("check")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
+
+Caused by:
+  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();
+}
+
+#[cargo_test]
+fn feature_syntax() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                cargo-features = ["bad_feature"]
+
+                [package]
+                name = "a"
+                version = "0.0.1"
+                edition = "2015"
+                authors = []
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    p.cargo("check")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
+
+Caused by:
+  unknown Cargo.toml feature `bad_feature`
+
+  Feature names must use '-' instead of '_'.
 
 "#]])
         .run();