Add -Zfuture-incompat-test to assist with testing future-incompat reports.

compiler/rustc_middle/src/lint.rs

@@ -8,7 +8,7 @@ use rustc_hir::HirId;
 use rustc_index::vec::IndexVec;
 use rustc_session::lint::{
     builtin::{self, FORBIDDEN_LINT_GROUPS},
-    FutureIncompatibilityReason, FutureIncompatibleInfo, Level, Lint, LintId,
+    FutureIncompatibilityReason, Level, Lint, LintId,
 };
 use rustc_session::{DiagnosticMessageId, Session};
 use rustc_span::hygiene::MacroKind;
@@ -223,12 +223,12 @@ pub fn struct_lint_level<'s, 'd>(
         let lint_id = LintId::of(lint);
         let future_incompatible = lint.future_incompatible;
 
-        let has_future_breakage = matches!(
-            future_incompatible,
-            Some(FutureIncompatibleInfo {
-                reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
-                ..
-            })
+        let has_future_breakage = future_incompatible.map_or(
+            // Default allow lints trigger too often for testing.
+            sess.opts.debugging_opts.future_incompat_test && lint.default_level != Level::Allow,
+            |incompat| {
+                matches!(incompat.reason, FutureIncompatibilityReason::FutureReleaseErrorReportNow)
+            },
         );
 
         let mut err = match (level, span) {