Undeprecate lint unstable_features and make use of it in the compiler

compiler/rustc_lexer/src/lib.rs

@@ -20,8 +20,9 @@
 //! [`rustc_parse::lexer`]: ../rustc_parse/lexer/index.html
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
-// We want to be able to build this crate with a stable compiler, so no
-// `#![feature]` attributes should be added.
+// We want to be able to build this crate with a stable compiler,
+// so no `#![feature]` attributes should be added.
+#![deny(unstable_features)]
 
 mod cursor;
 pub mod unescape;

compiler/rustc_lint/messages.ftl

@@ -148,7 +148,7 @@ lint_builtin_unsafe_impl = implementation of an `unsafe` trait
 
 lint_builtin_unsafe_trait = declaration of an `unsafe` trait
 
-lint_builtin_unstable_features = unstable feature
+lint_builtin_unstable_features = use of an unstable feature
 
 lint_builtin_unused_doc_comment = unused doc comment
     .label = rustdoc does not generate documentation for {$kind}

compiler/rustc_lint/src/builtin.rs

@@ -1233,10 +1233,30 @@ impl<'tcx> LateLintPass<'tcx> for MutableTransmutes {
 }
 
 declare_lint! {
-    /// The `unstable_features` is deprecated and should no longer be used.
+    /// The `unstable_features` lint detects uses of `#![feature]`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(unstable_features)]
+    /// #![feature(test)]
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In larger nightly-based projects which
+    ///
+    /// * consist of a multitude of crates where a subset of crates has to compile on
+    ///   stable either unconditionally or depending on a `cfg` flag to for example
+    ///   allow stable users to depend on them,
+    /// * don't use nightly for experimental features but for, e.g., unstable options only,
+    ///
+    /// this lint may come in handy to enforce policies of these kinds.
     UNSTABLE_FEATURES,
     Allow,
-    "enabling unstable features (deprecated. do not use)"
+    "enabling unstable features"
 }
 
 declare_lint_pass!(
@@ -1246,11 +1266,11 @@ declare_lint_pass!(
 
 impl<'tcx> LateLintPass<'tcx> for UnstableFeatures {
     fn check_attribute(&mut self, cx: &LateContext<'_>, attr: &ast::Attribute) {
-        if attr.has_name(sym::feature) {
-            if let Some(items) = attr.meta_item_list() {
-                for item in items {
-                    cx.emit_spanned_lint(UNSTABLE_FEATURES, item.span(), BuiltinUnstableFeatures);
-                }
+        if attr.has_name(sym::feature)
+            && let Some(items) = attr.meta_item_list()
+        {
+            for item in items {
+                cx.emit_spanned_lint(UNSTABLE_FEATURES, item.span(), BuiltinUnstableFeatures);
             }
         }
     }

compiler/rustc_parse_format/src/lib.rs

@@ -11,8 +11,9 @@
 )]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
-// WARNING: We want to be able to build this crate with a stable compiler,
-//          so no `#![feature]` attributes should be added!
+// We want to be able to build this crate with a stable compiler,
+// so no `#![feature]` attributes should be added.
+#![deny(unstable_features)]
 
 use rustc_lexer::unescape;
 pub use Alignment::*;

tests/ui/feature-gates/feature-gate-feature-gate.stderr

@@ -1,4 +1,4 @@
-error: unstable feature
+error: use of an unstable feature
   --> $DIR/feature-gate-feature-gate.rs:2:12
    |
 LL | #![feature(intrinsics)]