Prevent rustfmt from touching inside verus!{...}

Fixes #33

src/lib.rs

@@ -1431,11 +1431,12 @@ fn strip_whitespace(s: String) -> String {
 pub const VERUS_PREFIX: &str = "verus! {\n\n";
 pub const VERUS_SUFFIX: &str = "\n} // verus!\n";
 
-/// Run rustfmt
+/// Run rustfmt, only on code outside the `verus!` macro
 pub fn rustfmt(value: &str) -> Option<String> {
     if let Ok(mut proc) = Command::new("rustfmt")
         .arg("--emit=stdout")
         .arg("--edition=2021")
+        .arg(r#"--config=skip_macro_invocations=["verus"]"#)
         .stdin(Stdio::piped())
         .stdout(Stdio::piped())
         .stderr(Stdio::piped())

tests/rustfmt-does-not-touch-verus.rs

@@ -0,0 +1,18 @@
+#[test]
+/// Regression test for https://github.com/verus-lang/verusfmt/issues/33
+fn mod_macro_item_idempotent() {
+    let file = r#"
+  mod foo {
+    verus! {
+
+        bar! {
+            baz
+        }
+
+    }
+  }
+"#;
+    let formatted1 = verusfmt::rustfmt(&verusfmt::parse_and_format(file).unwrap()).unwrap();
+    let formatted2 = verusfmt::rustfmt(&verusfmt::parse_and_format(&formatted1).unwrap()).unwrap();
+    assert_eq!(formatted1, formatted2);
+}