Key React.Children.map results like React

[davesnx]

React.Children.map and mapWithIndex returned mapped children without keys. A React 19.1 development client therefore reported Each child in a list should have a unique "key" prop for a mapped React.list or React.array, where React keys the same children silently. Split out of #399, which adds the Flight validation states this fix is visible through.

• Every mapped element with a key slot (Lower_case_element, Client_component, Suspense) gets the key React's mapChildren assigns: .<index in base 36> for an unkeyed member, .$<key> for a keyed member with = and : escaped as =0 and =2, and a <mapped key>/ prefix when the callback returns an element with a different key.
• Results without a key slot, such as text, pass through unchanged. A single element input is still mapped directly, without a key.
• React.Children.toArray is unchanged.

The rules are copied from mapIntoArray, getElementKey, escape, and escapeUserProvidedKey in react@19.1.0.

Evidence at 01360e0: dune build @packages/react/runtest runs the new Children.map keys case, which checks index keys, base 36 at the eleventh member, keyed and escaped input, callback-key prefixing, slash doubling, text pass-through, mapWithIndex, and the single-element path.

Test Successful in 0.001s. 50 tests run.

Risk: two-way door. A revert restores unkeyed mapped children and the false missing-key warning. Mapped children that already had a key keep it under the .$ prefix, as in React; nothing else reads these keys.

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Benchmarks

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

Benchmark	ops/sec
trivial/renderToStaticMarkup	3,262,780
trivial/renderToString	2,942,132
depth/10	81,305
depth/25	31,904
depth/50	16,323
depth/100	8,193
width/10	49,317
width/100	5,108
width/500	937.96
width/1000	475.22
table/10	32,345
table/50	6,949
table/100	2,964
table/500	647.97
props/small	12,939
props/medium	4,496
props/large	1,530
realworld/ecommerce24	8,129
realworld/ecommerce48	4,342
realworld/dashboard	33,950
realworld/blog50	5,237
realworld/form	17,658
primitive/React.string	20,370,588
primitive/React.int	10,835,195
primitive/React.null	33,447,400
primitive/createElement_empty	14,344,405
primitive/createElement_children	1,174,250
primitive/React.array_10	1,164,147
primitive/React.array_100	128,388
primitive/React.list_10	1,176,754
primitive/React.list_100	126,884
rsc/trivial	417,551
rsc/depth/50	6,267
rsc/width/100	655.37
rsc/width/500	119.05
rsc/width/1000	55.32
rsc/table/100	332.76
rsc/table/500	61.17
streaming/renderToStream/wide100	4,600
streaming/renderToStream/suspense-drained	47,902
rsc/render_html/wide100	660.32
rsc/render_html/suspense	19,786
rsc/render_model/wide100	1,373
router/match/static-small-first	24,059,335
router/match/static-small-last	35,277,222
router/match/static-large-first	36,329,177
router/match/static-large-last	35,879,575
router/match/static-large-miss	3,485,475
router/match/dynamic-large-first	2,349,213
router/match/dynamic-large-last	2,616,316
router/endpoint/static-large-last	19,807,630
router/endpoint/dynamic-large-last	2,398,984
router/parse/path-plain	2,401,010
router/parse/path-encoded	1,751,677
router/parse/search	3,074,765