I’m studying the code of rupta, a pointer analysis framework for rust. I tried to add some codes to rupta, in order to learn where certain function is defined, given the DefId of that function, as follows:

let cur_tcx: rustc_middle::ty::context::tyCtxt = /* snipped */;
let def_id_of_func: rustc_span::def_id::DefId = /* snipped */;

let cur_session: rustc_session::session::Session = /* snipped */;
let source_map = cur_session.source_map();
let span = cur_tcx.def_span(def_id_of_func);
let file = source_map.lookup_source_file(span.lo());

With the variable file, I wrote these codes in addition, trying to figure out the exact path that the file locates in the file system:

let filename = file.name.clone();
let filename_cloned = filename.clone();
let path_of_file = match filename {
    FileName::Real(real_file_name) => match real_file_name {
        RealFileName::LocalPath(path_buf) => Ok(path_buf),
        RealFileName::Remapped {
            local_path: path_buf_optional,
            virtual_name: virtual_path_buf,
        } => {
            if let Some(path_buf) = path_buf_optional {
                Ok(path_buf)
            } else {
                Err(format!("Virtual: {}", virtual_path_buf.to_string_lossy()))
            }
        }
    },
    _ => Err(format!("Other: {:?}", filename_cloned)),
};

It works just fine in most cases, however something weird happened. For example, when analyzing config.rs from the rustc_session crate, the codes above indicate that the file is located in /home/<my-user-name>/.rustup/toolchains/nightly-2024-02-03-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/rustc_session/src/config.rs. However the actual path is /home/<my-user-name>/.rustup/toolchains/nightly-2024-02-03-x86_64-unknown-linux-gnu/lib/rustlib/rustc-src/rust/compiler/rustc_session/config.rs instead.

This phenomenon also happens when rupta analyzes rustc_driver_impl, rustc_errors and rustc_error_messages, which are all from rustc-dev.

In short, I want to figure out two questions:

1. Why does rustc_span::source_map::SourceMap::lookup_source_file gives non-existent file paths?
2. Did I do wrong when fetching the file path that certain function is defined in? If so, is there any other way to do the same task?

Sadly I have little experience developing inside rustc. Being not familiar with the rustc_* libraries, I really don’t have any idea what I should try next.

All I am expecting is that I could know the real file path of the functions in the crates mentioned above. I am alse curious about the reason that rustc_span::source_map::SourceMap::lookup_source_file gives false file paths.