Fix assertion failure for some Expect diagnostics.

In #120699 I moved some code dealing with `has_future_breakage` earlier
in `emit_diagnostic`. Issue #126521 identified a case where that
reordering was invalid (leading to an assertion failure) for some `Expect`
diagnostics.

This commit partially undoes the change, by moving the handling of
unstable `Expect` diagnostics earlier again. This makes
`emit_diagnostic` a bit uglier, but is necessary to fix the problem.

Fixes #126521.

compiler/rustc_errors/src/lib.rs

@@ -1437,6 +1437,24 @@ impl DiagCtxtInner {
 
     // Return value is only `Some` if the level is `Error` or `DelayedBug`.
     fn emit_diagnostic(&mut self, mut diagnostic: DiagInner) -> Option<ErrorGuaranteed> {
+        match diagnostic.level {
+            Expect(expect_id) | ForceWarning(Some(expect_id)) => {
+                // The `LintExpectationId` can be stable or unstable depending on when it was
+                // created. Diagnostics created before the definition of `HirId`s are unstable and
+                // can not yet be stored. Instead, they are buffered until the `LintExpectationId`
+                // is replaced by a stable one by the `LintLevelsBuilder`.
+                if let LintExpectationId::Unstable { .. } = expect_id {
+                    // We don't call TRACK_DIAGNOSTIC because we wait for the
+                    // unstable ID to be updated, whereupon the diagnostic will be
+                    // passed into this method again.
+                    self.unstable_expect_diagnostics.push(diagnostic);
+                    return None;
+                }
+                // Continue through to the `Expect`/`ForceWarning` case below.
+            }
+            _ => {}
+        }
+
         if diagnostic.has_future_breakage() {
             // Future breakages aren't emitted if they're `Level::Allow`,
             // but they still need to be constructed and stashed below,
@@ -1512,16 +1530,8 @@ impl DiagCtxtInner {
                 return None;
             }
             Expect(expect_id) | ForceWarning(Some(expect_id)) => {
-                // Diagnostics created before the definition of `HirId`s are
-                // unstable and can not yet be stored. Instead, they are
-                // buffered until the `LintExpectationId` is replaced by a
-                // stable one by the `LintLevelsBuilder`.
                 if let LintExpectationId::Unstable { .. } = expect_id {
-                    // We don't call TRACK_DIAGNOSTIC because we wait for the
+                    unreachable!(); // this case was handled at the top of this function
-                    // unstable ID to be updated, whereupon the diagnostic will
-                    // be passed into this method again.
-                    self.unstable_expect_diagnostics.push(diagnostic);
-                    return None;
                 }
                 self.fulfilled_expectations.insert(expect_id.normalize());
                 if let Expect(_) = diagnostic.level {

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

@@ -1,4 +1,3 @@
-//@ known-bug: rust-lang/rust#126521
 macro_rules! foo {
     ($val:ident) => {
         true;
@@ -7,5 +6,6 @@ macro_rules! foo {
 
 fn main() {
     #[expect(semicolon_in_expressions_from_macros)]
+    //~^ ERROR the `#[expect]` attribute is an experimental feature
     let _ = foo!(x);
 }

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

@@ -0,0 +1,13 @@
+error[E0658]: the `#[expect]` attribute is an experimental feature
+  --> $DIR/expect-future_breakage-crash-issue-126521.rs:8:5
+   |
+LL |     #[expect(semicolon_in_expressions_from_macros)]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.