Fix writing multi-line text without semicolons

[andrewbranch]

Fixes #18780

I have just learned that my fix in #31801, in the cases that it does work, works entirely by accident. The writer returned by getTrailingSemicolonOmittingWriter was never intended to be a writer that actually drops semicolons; it simply doesn’t write them until you start writing something else. The places where it was used to omit a semicolon were simply single-statement (or even less than a single statement) writes, like printing a signature for quick info. If the writer had continued on to print another statement, the semicolon would then be added.

The reason #31801 worked at all is that the emitter ultimately ended up with a semicolon-omitting writer wrapped inside a semicolon-omitting writer, and as a result would never write a trailing semicolon, even where it’s necessary by the rules of ASI. Technically the current state is pretty wrong, but I think that the bad effects of being so wrong will never actually manifest, because I think the emitter always emits list elements on separate lines.

This fix renames getTrailingSemicolonOmittingWriter to getTrailingSemicolonDeferringWriter, and adds a proper getTrailingSemicolonOmittingWriter which will commit trailing semicolons only if they are not followed by a newline.

It should be noted that, because the writer is generally not syntax-aware and can’t perform lookahead, it fails to omit semicolons where it technically could in two scenarios:

1. Closing parens of do/while statements:

do {} while (false) console.log()
//                 ^ don’t need one here but we print it anyway

2. Where the offending token is }:

if (true) {
  1
  2
  3 }
// ^ don’t need one here but we print it anyway

I think these are ok.

Additional note: I didn’t change the behavior of PrinterOptions['omitTrailingSemicolons'] because it’s public API. Everything I changed is internal and applies only to changes made with TextChanges, but would be open to discussing other options.

[rbuckton]

@andrewbranch I should point out that I investigated a more thorough semicolon-eliding writer when I was looking into removing all insignificant whitespace during emit in #25492, specifically here: https://github.com/microsoft/TypeScript/pull/25492/files#diff-942ae3cd2a8bbd85ed86a60cd7c43307R3248

[andrewbranch]

@rbuckton does this cause conflicts for your work there?

[rbuckton]

Not presently. If we do end up introducing the whitespace-removing text writer I will address any conflicts at that point.

[sandersn]

Seems good? I'm not an expert here so if there is one, they should sign off too.

[sandersn]

I don't know much about our emit with respect to formatting. Did we detect that the input didn't have semicolons, so the output shouldn't either?

[andrewbranch]

Yeah. That change was in #31801, but this was a case that didn’t work until now.

[andrewbranch]

Note that this only applies to edits made with TextChanges—JS emit to file always uses semicolons.

[andrewbranch]

This is a prerequisite for some of my other 3.7 work, so I’m gonna go ahead and merge. If an expert comes along and finds I’ve done something horrible here... 🙈🤷‍♀