[davesnx]

styled-ppx's next snapshot (davesnx/styled-ppx#614 and the stack below it) turns the CSS.styles carrier into (className, style, label), where the label is the binding name, and renders that label as the element's part attribute so styles stay findable in DevTools ([part~="Button"]) now that class names carry no label suffix. Styled components do that themselves; lowercase elements with styles= are expanded here, so the ppx has to emit it too.

Styles_attribute.expand_attributes now adds ?part next to className and style:

<div styles=x />
/* becomes */
div ~className:(CSS.className x) ~style:(CSS.styles x)
  ?part:(match CSS.label x with "" -> None | part -> Some part) ...

• Only a non-empty label produces the attribute; unlabeled carriers render exactly as before.
• ?styles keeps part optional through the None case.
• An explicit part prop merges with the label the way className merges (part="Button host"); merge_className became merge_string_attribute name so both share one implementation.
• Native and Melange mode share the expansion. On the Melange side the emitted ?part reaches ReactDOM.domProps, which does not accept ~part in reason-react 0.17 yet, so that side needs a reason-react change before this can be released. Draft until that lands or until we decide to gate the Melange emission.

Requires a styled-ppx runtime that defines CSS.label.

Tests

At c3eae9a, OCaml 5.4.0, quickjs 0.5.1, melange 7.0.1, ocamlformat 0.28.1:

• dune build @packages/styles-attribute/runtest: 7 tests, including the new expand_styles_with_previous_part.
• dune build @packages/server-reason-react-ppx/runtest: 72 tests; the CSS stub is a triple now, with styles_attribute_with_label, styles_attribute_with_label_and_part and styles_attribute_optional_some_with_label asserting part="Button", part="Button host" and part="Card" in the rendered HTML. Cram expectations for styles, styles-native, styles-js and optional-prop-shadowed-none were promoted and reviewed; the only change is the added part branch.
• dune build @runtest: everything passes except packages/router/test/browser, which needs npm install in that checkout.
• dune build @fmt on the touched directories: clean.

Risk: two-way door. Reverting restores the className/style-only expansion. Merging as is breaks Melange builds that use styles= on lowercase elements until reason-react's domProps accepts ~part.

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Project	Deployment	Actions	Updated
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Benchmarks

No baseline from main available yet, showing absolute values only.

Benchmark	ops/sec
trivial/renderToStaticMarkup	2,498,245
trivial/renderToString	2,259,862
depth/10	62,602
depth/25	24,662
depth/50	12,579
depth/100	6,350
width/10	38,015
width/100	3,913
width/500	713.14
width/1000	359.02
table/10	25,105
table/50	5,410
table/100	2,318
table/500	505.58
props/small	10,033
props/medium	3,469
props/large	1,129
realworld/ecommerce24	6,295
realworld/ecommerce48	3,374
realworld/dashboard	26,500
realworld/blog50	4,104
realworld/form	13,897
primitive/React.string	17,637,948
primitive/React.int	8,495,653
primitive/React.null	26,749,388
primitive/createElement_empty	11,765,229
primitive/createElement_children	854,185
primitive/React.array_10	797,578
primitive/React.array_100	87,312
primitive/React.list_10	848,740
primitive/React.list_100	94,989
rsc/trivial	312,007
rsc/depth/50	4,818
rsc/width/100	520.18
rsc/width/500	95.75
rsc/width/1000	45.47
rsc/table/100	267.6
rsc/table/500	50.81
streaming/renderToStream/wide100	3,552
streaming/renderToStream/suspense-drained	36,986
rsc/render_html/wide100	522.8
rsc/render_html/suspense	15,374
rsc/render_model/wide100	1,055
router/match/static-small-first	18,779,677
router/match/static-small-last	27,877,377
router/match/static-large-first	27,405,232
router/match/static-large-last	26,727,559
router/match/static-large-miss	2,565,807
router/match/dynamic-large-first	1,725,625
router/match/dynamic-large-last	1,956,616
router/endpoint/static-large-last	15,204,466
router/endpoint/dynamic-large-last	1,798,458
router/parse/path-plain	1,846,274
router/parse/path-encoded	1,217,019
router/parse/search	2,422,913