Turn soft_unstable into a future-compatibility lint

src/librustc_lint/lib.rs

@@ -436,7 +436,12 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(INDIRECT_STRUCTURAL_MATCH),
             reference: "issue #62411 <https://github.com/rust-lang/rust/issues/62411>",
             edition: None,
-        }
+        },
+        FutureIncompatibleInfo {
+            id: LintId::of(SOFT_UNSTABLE),
+            reference: "issue #64266 <https://github.com/rust-lang/rust/issues/64266>",
+            edition: None,
+        },
         ]);
 
     // Register renamed and removed lints.

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

@@ -1,4 +1,5 @@
 #[bench] //~ ERROR use of unstable library feature 'test'
+         //~| WARN this was previously accepted
 fn bench() {}
 
 fn main() {}

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

@@ -5,6 +5,8 @@ LL | #[bench]
    |   ^^^^^
    |
    = note: `#[deny(soft_unstable)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
 
 error: aborting due to previous error