Elm 0.19.2: U+2028 and U+2029 are no longer escaped in char literals

[lydell]

SSCCE:

module Main exposing (main)
import Html
main = Html.text (String.fromChar '\u{2028}' ++ String.fromChar '\u{2029}')

make --output=elm.js src/Main.elm

tail elm.js

Elm 0.19.1:

};
var $elm$virtual_dom$VirtualDom$text = _VirtualDom_text;
var $elm$html$Html$text = $elm$virtual_dom$VirtualDom$text;
var $author$project$Main$main = $elm$html$Html$text(
	_Utils_ap(
		$elm$core$String$fromChar(
			_Utils_chr('\u2028')),
		$elm$core$String$fromChar(
			_Utils_chr('\u2029'))));
_Platform_export({'Main':{'init':_VirtualDom_init($author$project$Main$main)(0)(0)}});}(this));

Elm 0.19.2:

};
var $elm$virtual_dom$VirtualDom$text = _VirtualDom_text;
var $elm$html$Html$text = $elm$virtual_dom$VirtualDom$text;
var $author$project$Main$main = $elm$html$Html$text(
	_Utils_ap(
		$elm$core$String$fromChar(
			_Utils_chr('
')),
		$elm$core$String$fromChar(
			_Utils_chr('
'))));
_Platform_export({'Main':{'init':_VirtualDom_init($author$project$Main$main)(0)(0)}});}(this));

Note _Utils_chr('\u2029') vs _Utils_chr('
'). Elm 0.19.1 used an escape in the output, while Elm 0.19.2 used the literal character. (The character is not visible on GitHub, but if you copy the text to a text editor you can inspect it.)

The thing with U+2028 and U+2029 is that they used to not be allowed in string literals in JavaScript, until ES2019:

ECMAScript 2019 … allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON.

Elm now outputting the literal character means:

• The generated Elm JavaScript is no longer ES5 compatible.
• (Very) old browsers and (very) old Node.js can no longer run all Elm code.
• Tools used on Elm’s JavaScript that use a parser that does not allow those two characters now fail.

Example of code that deals with U+2028 and U+2029: https://github.com/dillonkearns/elm-markdown/blob/6b8d7e50745812bab0f2db8ab73de59c31c9a33b/src/Markdown/InlineParser.elm#L655-L659

Example of a tool that now breaks: https://github.com/hmsk/vite-plugin-elm/blob/b527ad9821fdb2933b7f1e7d29213422ae6da746/src/assetsInjector.ts#L57
(This tool should not arbitrarily require ES2015, but it is an example of what can happen in the wild from a small thing like this.)

(Issue found by @simonh1000 on Slack – I helped diagnose the problem and write the issue.)

[github-actions]

Thanks for reporting this! To set expectations:

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• Ask questions in a community forum. You will get an answer quicker that way!
• If you experience something similar, open a new issue. We like duplicates.

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