Add error message for wrong cargo-features placement.

This is intended to help if the user puts cargo-features in the
wrong place in Cargo.toml.

src/cargo/util/toml/mod.rs

@@ -69,6 +69,17 @@ fn do_read_manifest(
         parse(contents, pretty_filename, config)?
     };
 
+    // Provide a helpful error message for a common user error.
+    if let Some(package) = toml.get("package").or_else(|| toml.get("project")) {
+        if let Some(feats) = package.get("cargo-features") {
+            bail!(
+                "cargo-features = {} was found in the wrong location, it \
+                 should be set at the top of Cargo.toml before any tables",
+                toml::to_string(feats).unwrap()
+            );
+        }
+    }
+
     let mut unused = BTreeSet::new();
     let manifest: TomlManifest = serde_ignored::deserialize(toml, |path| {
         let mut key = String::new();

tests/testsuite/cargo_features.rs

@@ -327,3 +327,32 @@ fn publish_allowed() {
         .masquerade_as_nightly_cargo()
         .run();
 }
+
+#[cargo_test]
+fn wrong_position() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                cargo-features = ["test-dummy-unstable"]
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    p.cargo("check")
+        .masquerade_as_nightly_cargo()
+        .with_status(101)
+        .with_stderr(
+            "\
+error: failed to parse manifest at [..]
+
+Caused by:
+  cargo-features = [\"test-dummy-unstable\"] was found in the wrong location, it \
+  should be set at the top of Cargo.toml before any tables
+",
+        )
+        .run();
+}