correct to get wrong BytePos

src/types.rs

@@ -320,7 +320,7 @@ fn format_function_type<'a, I>(
     let list_lo = context.snippet_provider.span_after(span, "(");
     let (list_str, tactic) = if inputs.len() == 0 {
         let tactic = get_tactics(&[], &output, shape);
-        let list_hi = context.snippet_provider.span_before_last(span, ")");
+        let list_hi = context.snippet_provider.span_before(span, ")");
         let comment = context
             .snippet_provider
             .span_to_snippet(mk_sp(list_lo, list_hi))?

tests/target/issue-3539.rs

@@ -0,0 +1,8 @@
+use std::io::Error;
+
+fn main() {
+    let _read_num: fn() -> Result<(i32), Error> = || -> Result<(i32), Error> {
+        let a = 1;
+        Ok(a)
+    };
+}