[Rust]: Fix calltree discovery logic

[arthurscchan]

This PR fixes the calltree discovery logic to correctly interpret function calls initiated from fuzzing harness for rust frontend.

[arthurscchan]

The newest calltree in .data format for some fuzzing harnesses of project httparse.
parse_header

Call tree
fuzz_target /src/httparse/fuzz/fuzz_targets/parse_headers.rs linenumber=-1
 parse_headers /src/httparse/fuzz/fuzz_targets/parse_headers.rs linenumber=6
   Bytes::new /src/httparse/src/lib.rs linenumber=1012
     a[u8]::as_ptr /src/httparse/src/iter.rs linenumber=17
   parse_headers_iter /src/httparse/src/lib.rs linenumber=1013
     parse_headers_iter_uninit /src/httparse/src/lib.rs linenumber=1023
       Err /src/httparse/src/lib.rs linenumber=1091
       iter_mut /src/httparse/src/lib.rs linenumber=1093
   Ok /src/httparse/src/lib.rs linenumber=1014
   Status::Complete /src/httparse/src/lib.rs linenumber=1014

parse_response_multspaces

Call tree
fuzz_target /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=-1
 Response::new /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=7
 httparse::ParserConfig::default /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=8
 ParserConfig::allow_multiple_spaces_in_response_status_delimiters /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=9
 ParserConfig::parse_response /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=10
   Response::parse_with_config /src/httparse/src/lib.rs linenumber=469
     core::mem::replace /src/httparse/src/lib.rs linenumber=686

parse_chunk_size

Call tree
fuzz_target /src/httparse/fuzz/fuzz_targets/parse_chunk_size.rs linenumber=-1
 parse_chunk_size /src/httparse/fuzz/fuzz_targets/parse_chunk_size.rs linenumber=5
   Bytes::new /src/httparse/src/lib.rs linenumber=1313
     a[u8]::as_ptr /src/httparse/src/iter.rs linenumber=17
   Ok /src/httparse/src/lib.rs linenumber=1385
   Status::Complete /src/httparse/src/lib.rs linenumber=1385
   Bytes::pos /src/httparse/src/lib.rs linenumber=1385

[arthurscchan]

The newest logic successfully identified most of the call, with some exceptions that is known to be limitations. for example, in the call tree for parse_response_multspaces, it does not identify any function calls within the unsafe block, it seems that the syn approach failed to identify those blocked code. Will be working on finding alternative ways to identify those call.

    fn parse_with_config(&mut self, buf: &'b [u8], config: &ParserConfig) -> Result<usize> {
        let headers = core::mem::replace(&mut self.headers, &mut []);

        /* SAFETY: see `parse_headers_iter_uninit` guarantees */
        unsafe {
            let headers: *mut [Header<'_>] = headers;
            let headers = headers as *mut [MaybeUninit<Header<'_>>];
            match self.parse_with_config_and_uninit_headers(buf, config, &mut *headers) {
                Ok(Status::Complete(idx)) => Ok(Status::Complete(idx)),
                other => {
                    // put the original headers back
                    self.headers = &mut *(headers as *mut [Header<'_>]);
                    other
                },
            }
        }
    }