Avoid ICE in non-check builds with bad #[coverage(..)] attributes

This code can sometimes witness malformed coverage attributes in builds that
are going to fail, so use `span_delayed_bug` to avoid an inappropriate ICE in
that case.

compiler/rustc_mir_transform/src/coverage/query.rs

@@ -63,7 +63,8 @@ fn coverage_attr_on(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
             Some([item]) if item.has_name(sym::on) => return true,
             Some(_) | None => {
                 // Other possibilities should have been rejected by `rustc_parse::validate_attr`.
-                tcx.dcx().span_bug(attr.span, "unexpected value of coverage attribute");
+                // Use `span_delayed_bug` to avoid an ICE in failing builds (#127880).
+                tcx.dcx().span_delayed_bug(attr.span, "unexpected value of coverage attribute");
             }
         }
     }

tests/run-make/coverage-attr-ice/127880.rs

@@ -0,0 +1,2 @@
+#[coverage]
+fn main() {}

tests/run-make/coverage-attr-ice/rmake.rs

@@ -0,0 +1,19 @@
+use run_make_support::rustc;
+
+// Illegal `#[coverage(..)]` attributes should not cause an ICE in non-check
+// builds with `-Cinstrument-coverage`.
+// Regression test for <https://github.com/rust-lang/rust/issues/127880>.
+//
+// This currently can't be a UI test, because there's no way to force a
+// non-check build of a UI test that would also fail a check build.
+
+fn main() {
+    rustc()
+        .edition("2021")
+        .arg("-Cinstrument-coverage")
+        .input("127880.rs")
+        .run_fail()
+        // The build should fail cleanly, but not ICE.
+        .assert_exit_code(1)
+        .assert_stderr_contains("error: malformed `coverage` attribute input");
+}