Fix: ICE when formatting builtins

Replace `unreachable!` with `None`. Now rustfmt won't
panic when it comes across a `builtin # offset_of` or any other builtin

src/expr.rs

@@ -404,8 +404,11 @@ pub(crate) fn format_expr(
         ast::ExprKind::FormatArgs(..)
         | ast::ExprKind::IncludedBytes(..)
         | ast::ExprKind::OffsetOf(..) => {
-            // These do not occur in the AST because macros aren't expanded.
-            unreachable!()
+            // These don't normally occur in the AST because macros aren't expanded. However,
+            // rustfmt tries to parse macro arguments when formatting macros, so it's not totally
+            // impossible for rustfmt to come across one of these nodes when formatting a file.
+            // Also, rustfmt might get passed the output from `-Zunpretty=expanded`.
+            None
         }
         ast::ExprKind::Err => None,
     };

tests/rustfmt/main.rs

@@ -176,8 +176,14 @@ fn rustfmt_emits_error_on_line_overflow_true() {
 #[test]
 #[allow(non_snake_case)]
 fn dont_emit_ICE() {
-    let files = ["tests/target/issue_5728.rs", "tests/target/issue_5729.rs", "tests/target/issue_6069.rs"];
-
+    let files = [
+        "tests/target/issue_5728.rs",
+        "tests/target/issue_5729.rs",
+        "tests/target/issue-5885.rs",
+        "tests/target/issue_6069.rs",
+        "tests/target/issue-6105.rs",
+    ];
+
     for file in files {
         let args = [file];
         let (_stdout, stderr) = rustfmt(&args);

tests/target/issue-5885.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("{}", builtin # offset_of(A, 0. 1.1.1));
+}

tests/target/issue-6105.rs

@@ -0,0 +1 @@
+const _: () = builtin # offset_of(x, x);