Improve diagnostic for unexpected feature config name from Cargo

compiler/rustc_lint/src/context.rs

@@ -739,6 +739,8 @@ pub trait LintContext {
                         } else {
                             db.span_suggestion(name_span, "there is a config with a similar name", best_match, Applicability::MaybeIncorrect);
                         }
+                    } else if name == sym::feature && std::env::var_os("CARGO").is_some() {
+                        db.help("consider defining some features in `Cargo.toml`");
                     } else if !possibilities.is_empty() {
                         let mut possibilities = possibilities.iter()
                             .map(Symbol::as_str)

tests/ui/check-cfg/cargo-feature.rs

@@ -0,0 +1,14 @@
+// This test checks that when no features are passed by Cargo we
+// suggest adding some in the Cargo.toml instead of vomitting a
+// list of all the expected names
+//
+// check-pass
+// rustc-env:CARGO=/usr/bin/cargo
+// compile-flags: --check-cfg=cfg() -Z unstable-options
+// error-pattern:Cargo.toml
+
+#[cfg(feature = "serde")]
+//~^ WARNING unexpected `cfg` condition name
+fn ser() {}
+
+fn main() {}

tests/ui/check-cfg/cargo-feature.stderr

@@ -0,0 +1,11 @@
+warning: unexpected `cfg` condition name: `feature`
+  --> $DIR/cargo-feature.rs:10:7
+   |
+LL | #[cfg(feature = "serde")]
+   |       ^^^^^^^^^^^^^^^^^
+   |
+   = help: consider defining some features in `Cargo.toml`
+   = note: `#[warn(unexpected_cfgs)]` on by default
+
+warning: 1 warning emitted
+