Make incomplete features part of delcaration

This prevents mistakes where the feature is in the list of incomplete
features but not actually a feature by making the incompleteness a part
of the declaration.

src/test/ui/feature-gates/rustc_insignificant_dtor.rs

@@ -1,2 +0,0 @@
-#![feature(rustc_insignificant_dtor)] //~ ERROR unknown feature `rustc_insignificant_dtor`
-fn main() {}

src/test/ui/feature-gates/rustc_insignificant_dtor.stderr

@@ -1,9 +0,0 @@
-error[E0635]: unknown feature `rustc_insignificant_dtor`
-  --> $DIR/rustc_insignificant_dtor.rs:1:12
-   |
-LL | #![feature(rustc_insignificant_dtor)]
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0635`.

src/tools/tidy/src/features.rs

@@ -301,6 +301,7 @@ fn collect_lang_features_in(base: &Path, file: &str, bad: &mut bool) -> Features
             let mut parts = line.split(',');
             let level = match parts.next().map(|l| l.trim().trim_start_matches('(')) {
                 Some("active") => Status::Unstable,
+                Some("incomplete") => Status::Unstable,
                 Some("removed") => Status::Removed,
                 Some("accepted") => Status::Stable,
                 _ => return None,