Fix formatter forcing doc-commented inline-record exceptions onto multiple lines

[MavenRain]

Summary

Fixes #7947. A doc comment on an exception with an inline-record argument forced the record onto multiple lines. One line changes: the inline-record branch of print_constructor_arguments now wraps its argument doc in a Doc.group, exactly as the tuple branch already did.

Problem

With a doc comment, the record is exploded:

/** foo */
exception Foo({name: string, msg: string})

printed as:

/** foo */
exception Foo({
  name: string,
  msg: string,
})

Nothing else in the same neighbourhood behaves this way. All four of these stay on one line today:

// same declaration, no doc comment
exception Bar({name: string, msg: string})

// doc comment, tuple argument instead of an inline record
/** foo */
exception Baz(string, string)

// attribute instead of a doc comment
@attr exception Qux({name: string, msg: string})

// doc comment, inline record in a variant constructor
/** foo */
type t = Foo({name: string, msg: string})

That asymmetry matches the issue thread. @nojaf asked:

Is this also the case with type (records, inline in variant)?

and the reporter answered:

no, just exception

The cause is in compiler/syntax/src/res_printer.ml, in print_constructor_arguments. The Pcstr_tuple branch ends with:

Doc.group (if indent then Doc.indent args else args)

while the Pcstr_record branch ended with a bare:

if indent then Doc.indent args else args

A doc comment is printed as a hard line break by print_exception_def (and by print_extension_constructor). Because the record argument had no group of its own, its soft_lines sat in the same group as that hard break, so the forced break propagated into the record. The tuple branch was immune only because its group isolated it.

Fix

           Doc.rbrace;
           Doc.rparen;
         ]
     in
-    if indent then Doc.indent args else args
+    Doc.group (if indent then Doc.indent args else args)

The two branches are now identical in this respect. Breaking still happens when the record genuinely does not fit in 100 columns, since the group breaks on width as usual.

The same code path serves extension constructors, so type t += /** doc */ Ext({name: string, msg: string}) keeps its record on one line for free.

Two things are deliberately left alone:

• formatter won't keep multiline for exceptions #7946 (a multiline inline-record exception collapsing onto one line) is a policy question about honouring source line breaks. It would need a force_break from source locations, in the style of print_record_declaration. Separate change.
• In a type extension the doc comment is still printed after the |, on its own line. That is a different defect, tracked as switch-branch with multi-match branch is formatted weirdly #7962, and this PR does not touch it. The added fixture records today's behaviour so the change shows up plainly when switch-branch with multi-match branch is formatted weirdly #7962 is fixed.

Testing

Printer fixtures added, with the expected files regenerated:

• tests/syntax_tests/data/printer/structure/exception.res: doc-commented inline-record exceptions with one field and with two fields, plus a four-field case that is over 100 columns and still breaks.
• tests/syntax_tests/data/printer/signature/exception.resi: the same one-field and two-field cases in a signature.
• tests/syntax_tests/data/printer/structure/typeExtension.res: a doc-commented inline-record extension constructor.

No existing expected file changed. The new group only takes effect when an enclosing group is already broken, which no previous fixture exercised.

Gates, all green:

• ROUNDTRIP_TEST=1 ./scripts/test_syntax.sh: no unstaged test differences, roundtrip tests succeeded.
• bsc -format over the CI file list: 1813 files checked, 0 need formatting.
• ocamlformat 0.29.0 on res_printer.ml: clean.

Manual check on the reported snippet: it prints on one line after the fix, and formatting the output a second time is idempotent.

_{AI-assisted; I reviewed and tested every change.}

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.32%. Comparing base (bc382a4) to head (b4daa71).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8622      +/-   ##
==========================================
- Coverage   77.32%   77.32%   -0.01%     
==========================================
  Files         467      467              
  Lines       63342    63313      -29     
==========================================
- Hits        48982    48957      -25     
+ Misses      14360    14356       -4     

Files with missing lines	Coverage Δ
compiler/syntax/src/res_printer.ml	92.95% <100.00%> (+0.01%)	⬆️

... and 2 files with indirect coverage changes

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commit: b4daa71

[cknitt]

@MavenRain Thanks a lot for your contribution! Want to get in @cristianoc's 6-PRs stack first, will get back to this afterwards.

[cknitt]

Looks good and no conflicts with the other PRs that were merged on master. 🎉