[quinnj]

Follow-up to #80. normpath still took time quadratic in the path length when a path had many segments and dot segments. The cause is append!(output, codeunits(segment)): append! checks the destination and source for aliasing via objectid, which hashes the segment's whole parent string. So copying each kept segment cost the length of the entire path. The allocation-growth test from #80 could not see this, because no extra memory is allocated. resolvereference and joinpath both run their result through normpath.

Segments are now copied byte by byte (_append_codeunits!).

normpath input (Julia 1.12)	before	after
"/a/b/c/./../../g"	496 ns	329 ns
"/a/b/../c/./d/../e/f/g/../h"	917 ns	597 ns
"name.ext/./"^512 (5.6 KB)	586 µs	53 µs
"/" * "a/"^2_500 * "../"^2_500 (12.5 KB)	6.5 ms	0.23 ms
"/" * "a/"^25_000 * "../"^25_000 (125 KB)	1.82 s	2.3 ms
"/" * "a/"^250_000 * "../"^250_000 (1.25 MB)	46.7 s	23 ms

Paths without dot segments take the existing no-copy fast path and are unchanged (18 → 15 ns, within noise). Julia 1.10 shows the same per-append! cost (measured: a 1-byte append! from a SubString of a 10 MB string takes ~4 ms).

Verification. New test in "Path normalization boundaries and allocation growth": a 500 KB path of segments and dot segments normalizes to "/" in under 1 s. With this change it takes milliseconds. Without it, the same test took 30.2 s and failed. Full suite passes on Julia 1.12 and 1.10.

Review.

• normpath(::AbstractString) is the only caller of the new helper. Its two former append! sites both receive SubString{String} views of str.
• UTF-8 is copied byte for byte, so invalid sequences pass through unchanged; the existing 0xff/0xfe case covers that.
• push! after sizehint! doesn't reallocate.
• The method is concrete, so the --trim=safe native job's path stays resolvable.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.89%. Comparing base (d9689ac) to head (74304b3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   93.75%   93.89%   +0.14%     
==========================================
  Files           5        5              
  Lines         416      426      +10     
==========================================
+ Hits          390      400      +10     
  Misses         26       26              

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