Prevent ICE from expected future breakage

compiler/rustc_errors/src/json.rs

@@ -135,7 +135,12 @@ impl Emitter for JsonEmitter {
         let data: Vec<FutureBreakageItem<'_>> = diags
             .into_iter()
             .map(|mut diag| {
-                if diag.level == crate::Level::Allow {
+                // The `FutureBreakageItem` is collected and serialized.
+                // However, the `allow` and `expect` lint levels can't usually
+                // be serialized. The lint level is overwritten to allow the
+                // serialization again and force a lint emission.
+                // (This is an educated guess. I didn't originally add this)
+                if matches!(diag.level, crate::Level::Allow | crate::Level::Expect(..)) {
                     diag.level = crate::Level::Warning;
                 }
                 FutureBreakageItem {

tests/ui/lint/expect-future_breakage-crash-issue-126521.rs

@@ -1,3 +1,5 @@
+//@ check-pass
+
 // This test covers similar crashes from both #126521 and #126751.
 
 macro_rules! foo {
@@ -12,12 +14,25 @@ macro_rules! bar {
     };
 }
 
-fn main() {
+fn allow() {
+    #[allow(semicolon_in_expressions_from_macros)]
+    let _ = foo!(x);
+
+    #[allow(semicolon_in_expressions_from_macros)]
+    let _ = bar!(x);
+}
+
+// The `semicolon_in_expressions_from_macros` lint seems to be emitted even if the
+// lint level is `allow` as shown in the function above. The behavior of `expect`
+// should mirror this behavior. However, no `unfulfilled_lint_expectation` lint
+// is emitted, since the expectation is theoretically fulfilled.
+fn expect() {
     #[expect(semicolon_in_expressions_from_macros)]
-    //~^ ERROR the `#[expect]` attribute is an experimental feature
     let _ = foo!(x);
 
     #[expect(semicolon_in_expressions_from_macros)]
-    //~^ ERROR the `#[expect]` attribute is an experimental feature
     let _ = bar!(x);
 }
+
+fn main() {
+}

tests/ui/lint/expect-future_breakage-crash-issue-126521.stderr

@@ -1,23 +1,52 @@
-error[E0658]: the `#[expect]` attribute is an experimental feature
-  --> $DIR/expect-future_breakage-crash-issue-126521.rs:16:5
+Future incompatibility report: Future breakage diagnostic:
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/expect-future_breakage-crash-issue-126521.rs:7:13
    |
-LL |     #[expect(semicolon_in_expressions_from_macros)]
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |         true;
+   |             ^
+...
+LL |     let _ = foo!(x);
+   |             ------- in this macro invocation
    |
-   = note: see issue #54503 <https://github.com/rust-lang/rust/issues/54503> for more information
-   = help: add `#![feature(lint_reasons)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = 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 #79813 <https://github.com/rust-lang/rust/issues/79813>
+   = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0658]: the `#[expect]` attribute is an experimental feature
-  --> $DIR/expect-future_breakage-crash-issue-126521.rs:20:5
+Future breakage diagnostic:
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/expect-future_breakage-crash-issue-126521.rs:13:35
    |
-LL |     #[expect(semicolon_in_expressions_from_macros)]
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |         (5_i32.overflowing_sub(3));
+   |                                   ^
+...
+LL |     let _ = bar!(x);
+   |             ------- in this macro invocation
    |
-   = note: see issue #54503 <https://github.com/rust-lang/rust/issues/54503> for more information
-   = help: add `#![feature(lint_reasons)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = 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 #79813 <https://github.com/rust-lang/rust/issues/79813>
+   = note: this warning originates in the macro `bar` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 2 previous errors
+Future breakage diagnostic:
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/expect-future_breakage-crash-issue-126521.rs:7:13
+   |
+LL |         true;
+   |             ^
+...
+LL |     let _ = foo!(x);
+   |             ------- in this macro invocation
+   |
+   = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+Future breakage diagnostic:
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/expect-future_breakage-crash-issue-126521.rs:13:35
+   |
+LL |         (5_i32.overflowing_sub(3));
+   |                                   ^
+...
+LL |     let _ = bar!(x);
+   |             ------- in this macro invocation
+   |
+   = note: this warning originates in the macro `bar` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-For more information about this error, try `rustc --explain E0658`.