perf(stdune): use memchr for string searches - #16224
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Removes avoidable temporary allocations from frequently used string and path helpers. It adds option-free internal character searches, uses them in path decomposition, and constructs `Filename.concat` results in one pass. The dune-action-plugin path implementation now reuses the allocation-conscious `Stdune.Filename.concat`. This preparatory change was split from #16224 so allocation reductions can be reviewed independently from the C-backed search optimization. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
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Replace the pure unchecked forward string scan with a memchr primitive, use it for Stdune string searches, splitting, and validation, and generalize the spawn-specific NUL-byte search throughout Dune. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
Override the inherited scalar split_on_char implementation with Stdune.String.split so both public splitting APIs use the shared memchr-backed search. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
Skip spans without percent signs using the shared unchecked string search instead of examining every source byte in OCaml. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
Add a bounded memchr-backed Bytes search and use it to skip placeholder-free spans in the artifact substitution scanner. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
Scan Unix path components forward with the shared memchr primitive while retaining the multi-separator scanner on Windows. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
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Generalizes the spawn-specific NUL search from #16117 into a reusable forward string-search primitive backed by
memchr.Stdune.Stringuses it for filename validation, splitting, spawn arguments, Unix path decomposition, and other forward character searches. Reverse searches and unrelated string transformations are deliberately left unchanged.Follow-up commits use the shared search while scanning
dune substinputs and Unix path components, and add a bounded bytes search for the performance-sensitive artifact-substitution scanner. Lower-level standalone libraries that cannot depend on Stdune are also left unchanged.