Make extern blocks without unsafe warn in edition 2024

compiler/rustc_lint/messages.ftl

@@ -462,6 +462,8 @@ lint_metavariable_wrong_operator = meta-variable repeats with different Kleene o
 
 lint_missing_fragment_specifier = missing fragment specifier
 
+lint_missing_unsafe_on_extern = extern blocks should be unsafe
+
 lint_mixed_script_confusables =
     the usage of Script Group `{$set}` in this crate consists solely of mixed script confusables
     .includes_note = the usage includes {$includes}

compiler/rustc_lint/src/context/diagnostics.rs

@@ -205,6 +205,9 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
             };
             lints::DeprecatedWhereClauseLocation { suggestion }.decorate_lint(diag);
         }
+        BuiltinLintDiag::MissingUnsafeOnExtern => {
+            lints::MissingUnsafeOnExtern.decorate_lint(diag);
+        }
         BuiltinLintDiag::SingleUseLifetime {
             param_span,
             use_span: Some((use_span, elide)),

compiler/rustc_lint/src/lints.rs

@@ -2738,6 +2738,10 @@ pub enum DeprecatedWhereClauseLocationSugg {
     },
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_missing_unsafe_on_extern)]
+pub struct MissingUnsafeOnExtern;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_single_use_lifetime)]
 pub struct SingleUseLifetime {