chore: inspect span.find_oldest_ancestor_in_same_ctxt()

cc #31 (comment)

$ cargo r -- --json=false tests/proofs/gen_by_macros.rs
...
(stable_mir span to internal span) [tests/proofs/gen_by_macros.rs:4:9: 4:26 (#4)]
fn $name() $block
[src/functions/mod.rs:114:5] span.from_expansion() = true
[src/functions/mod.rs:114:5] ancestor_span.from_expansion() = false
(find_oldest_ancestor_in_same_ctxt) [tests/proofs/gen_by_macros.rs:10:5: 10:57 (#0)]
gen! { proof1, { assert_eq!(kani::any::<u8>(), 0) }}

(stable_mir span to internal span) [/home/zjp/rust/distributed-verification/kani/library/kani_core/src/arbitrary.rs:38:21: 41:22 (#32)]
fn any() -> Self {
                        // This size_of call does not use generic_const_exprs feature. It's inside a macro, and Self isn't generic.
                        unsafe { crate::kani::any_raw_internal::<Self>() }
                    }
[src/functions/mod.rs:114:5] span.from_expansion() = true
[src/functions/mod.rs:114:5] ancestor_span.from_expansion() = true
(find_oldest_ancestor_in_same_ctxt) [/home/zjp/rust/distributed-verification/kani/library/kani_core/src/arbitrary.rs:66:9: 66:31 (#5)]
trivial_arbitrary!(u8)
...

Author: zjp-CN

src/functions/mod.rs

@@ -105,6 +105,18 @@ fn ty_to_fndef(ty: Ty) -> Option<FnDef> {
 
 /// Source code for a span.
 fn source_code(span: Span, src_map: &SourceMap) -> String {
+    // println!("{span:?}\n{:?}\n\n", span.find_oldest_ancestor_in_same_ctxt());
+    _ = src_map.span_to_source(span, |text, x, y| {
+        println!("(stable_mir span to internal span) [{span:?}]\n{}", &text[x..y]);
+        Ok(())
+    });
+    let ancestor_span = span.find_oldest_ancestor_in_same_ctxt();
+    dbg!(span.from_expansion(), ancestor_span.from_expansion());
+    _ = src_map.span_to_source(ancestor_span, |text, x, y| {
+        println!("(find_oldest_ancestor_in_same_ctxt) [{ancestor_span:?}]\n{}\n\n", &text[x..y]);
+        Ok(())
+    });
+
     src_map
         .span_to_source(span, |text, x, y| {
             let src = &text[x..y];