RStan yields incorrect line numbers in compiler and runtime errors

[WardBrian]

Description:

It has come up several times on the forums that the line numbers reported by RStan are incorrect. The most recent is here: https://discourse.mc-stan.org/t/stan-error-messages/34820

The reportee (@cdriveraus) provided a script which shows this by inserting a bad line at a known position in the source and showing what the compiler reports:

library(curl)

# URL of the raw file on GitHub
github_raw_url <- "https://raw.githubusercontent.com/cdriveraus/ctsem/master/inst/stan/ctsm.stan"

# Read the file content directly from the URL
stanmodeltext <- readLines(curl(github_raw_url))
line_number <- c()
# Split the code into lines
for(linei in 800:1000){
  # code_lines <- strsplit(stanmodeltext, "\n")[[1]]
  
  # Generate a random line of code
  random_line <- "function(x);"
  
  # Insert the random line at a random position
  insert_position <- linei#sample(length(code_lines), 1)
  modified_code <- append(stanmodeltext, random_line, after = insert_position - 1)
  
  # Join the modified lines back into a single string
  modified_stan_code <- paste(modified_code, collapse = "\n")
  Sys.sleep(.02)
  error_string=try(rstan::stan_model(model_code = modified_stan_code))
  #extract the string " , line xxx' from the error message
  line_number <- c(line_number,as.numeric(gsub(".*line (\\d+).*", "\\1", error_string)))
    
    plot(1:length(line_number),line_number)
    abline(min(line_number),1)
}

I have modified the above code to use both cmdstanr and raw V8 calls to stancjs, and both times have gotten a plot which was a straight line:

Possible causes

Rstan does a lot of preprocessing on files before passing them to stanc3js: https://github.com/stan-dev/rstan/blob/50c3c6f4e44a849b17af7e9e2b8656cd93238a18/rstan/rstan/R/stanc.R#L62-L90

Oddly enough, the ctsm file which is used in the above example contains no #includes, so I would not expect it to be running.

I believe the same preprocessing is what leads to https://github.com/stan-dev/rstan/issues/1067