Rollup merge of #118639 - fmease:deny-features-in-stable-rustc-crates, r=WaffleLapkin

Undeprecate lint `unstable_features` and make use of it in the compiler

See also #117937.

r? compiler

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::*;