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_session/src/parse.rs

@@ -98,7 +98,26 @@ pub fn feature_err_issue<'a>(
     explain: &str,
 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
     let mut err = sess.span_diagnostic.struct_span_err_with_code(span, explain, error_code!(E0658));
+    add_feature_diagnostics_for_issue(&mut err, sess, feature, issue);
+    err
+}
 
+/// Adds the diagnostics for a feature to an existing error.
+pub fn add_feature_diagnostics<'a>(err: &mut Diagnostic, sess: &'a ParseSess, feature: Symbol) {
+    add_feature_diagnostics_for_issue(err, sess, feature, GateIssue::Language);
+}
+
+/// Adds the diagnostics for a feature to an existing error.
+///
+/// This variant allows you to control whether it is a library or language feature.
+/// Almost always, you want to use this for a language feature. If so, prefer
+/// `add_feature_diagnostics`.
+pub fn add_feature_diagnostics_for_issue<'a>(
+    err: &mut Diagnostic,
+    sess: &'a ParseSess,
+    feature: Symbol,
+    issue: GateIssue,
+) {
     if let Some(n) = find_feature_issue(feature, issue) {
         err.note(&format!(
             "see issue #{} <https://github.com/rust-lang/rust/issues/{}> for more information",
@@ -110,8 +129,6 @@ pub fn feature_err_issue<'a>(
     if sess.unstable_features.is_nightly_build() {
         err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feature));
     }
-
-    err
 }
 
 /// Info about a parsing session.