Rollup merge of #104751 - nnethercote:fix-104620, r=petrochenkov

Fix an ICE parsing a malformed attribute.

Fixes #104620.

r? `@petrochenkov`

src/test/ui/parser/issue-104620.rs

@@ -0,0 +1,4 @@
+#![feature(rustc_attrs)]
+
+#![rustc_dummy=5z] //~ ERROR unexpected expression: `5z`
+fn main() {}

src/test/ui/parser/issue-104620.stderr

@@ -0,0 +1,8 @@
+error: unexpected expression: `5z`
+  --> $DIR/issue-104620.rs:3:16
+   |
+LL | #![rustc_dummy=5z]
+   |                ^^
+
+error: aborting due to previous error
+