Rollup merge of #94274 - djkoloski:unknown_unstable_lints, r=tmandry

Treat unstable lints as unknown

This change causes unstable lints to be ignored if the `unknown_lints`
lint is allowed. To achieve this, it also changes lints to apply as soon
as they are processed. Previously, lints in the same set were processed
as a batch and then all simultaneously applied.

Implementation of https://github.com/rust-lang/compiler-team/issues/469

compiler/rustc_lint_defs/src/builtin.rs

@@ -3128,6 +3128,7 @@ declare_lint_pass! {
         SUSPICIOUS_AUTO_TRAIT_IMPLS,
         UNEXPECTED_CFGS,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
+        TEST_UNSTABLE_LINT,
     ]
 }
 
@@ -3771,3 +3772,25 @@ declare_lint! {
     Warn,
     "deprecated where clause location"
 }
+
+declare_lint! {
+    /// The `test_unstable_lint` lint tests unstable lints and is perma-unstable.
+    ///
+    /// ### Example
+    ///
+    /// ```
+    /// #![allow(test_unstable_lint)]
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In order to test the behavior of unstable lints, a permanently-unstable
+    /// lint is required. This lint can be used to trigger warnings and errors
+    /// from the compiler related to unstable lints.
+    pub TEST_UNSTABLE_LINT,
+    Deny,
+    "this unstable lint is only for testing",
+    @feature_gate = sym::test_unstable_lint;
+}