Unnecessary variables generated for string literals in record fields

[cknitt]

This is item 5 from my list of observations in #5418 (changes in compiler output from 9.1.4 to current master).

On current master,

type t = {
  a: string,
  b: string,
  c: Belt.Set.String.t,
}

let empty = {
  a: "A",
  b: "B",
  c: Belt.Set.String.empty,
}

compiles to

var empty_a = "A";

var empty_b = "B";

var empty = {
  a: empty_a,
  b: empty_b,
  c: undefined
};

whereas in 9.1.4, I get

var empty = {
  a: "A",
  b: "B",
  c: undefined
};

[cknitt]

@cristianoc @IwanKaramazow I researched this and found that

1. it is caused by the unicode support in the parser, i.e., it only happens after rescript-lang/syntax@79a4bef.
2. The same behavior can be reproduced in 9.1.4 by using backquotes:

   let empty = {
     a: `A`,
     b: `B`,
     c: Belt.Set.String.empty,
   }

3. It only occurs when the third field with Belt.Set.String.empty (or, as far as I can see, an instance of any other abstract type) is present.

[cristianoc]

Looks like inlining gets disabled in the compiler when one uses the other strings.

Likely somewhere to do with:

  | Str of bool * string
  (* A string is UTF-8 encoded, and may contain
     escape sequences.
     First argument: used to mark it as non-pure.
     Please treat it carefully as it affects optimization.
     Second argument: the string "j" in `{j||j}`.
  *)