String literals concatenation not optimized anymore in compiler output

[cknitt]

Another small regression in compiler output where an optimization for string literals is not applied anymore. Consider the following code:

let mediaSubDirectory = "media"

let f = filename => "../Documents/" ++ mediaSubDirectory ++ "/" ++ filename

In ReScript 9, this compiles to:

var mediaSubDirectory = "media";

function f(filename) {
  return "../Documents/media/" + filename;
}

In current master, I get

var mediaSubDirectory = "media";

function f(filename) {
  return "../Documents/" + mediaSubDirectory + "/" + filename;
}

(The same behavior can be reproduced in ReScript 9.1.4 by making mediaSubdirectory a backtick string.)

[cristianoc]

Is concatenation safe perhaps?
Length is not.
This might be different.

[cristianoc]

@cknitt is it safe to just concatenate the raw strings?

[cknitt]

I think so, but I am not 100% sure as I am not an expert on the UTF-8 encoding.
(I did some reading on it and found that UTF-8 files may have a BOM mark as a header that can interfere with concatenation, but that should not be an issue here.)

[cristianoc]

This is already supported in master.