Moving-GC witness gates misparse current copying-minor diagnostics as zero relocation #8049

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1. proggeramlug

   changed the title [-]Moving-GC witness gates collect and reclaim but relocate zero objects[/-] [+]Moving-GC witness gates misparse current copying-minor diagnostics as zero relocation[/+] on Aug 13, 2026
2. proggeramlug commented on Aug 13, 2026

   

   proggeramlug

   on Aug 13, 2026

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   Source attribution changes the diagnosis: the collector's reported zero is a harness parse result, not evidence that current copying minors truly moved zero objects.

   #7744 (b2aef5055) changed the diagnostic from the legacy shape

   [gc-copy-minor] ran copied_objects=...
   

   to the current key/value record

   [gc-copy-minor] ran in_place=... untraced=... ... copied_objects=... promoted_objects=...
   

   but scripts/gc_repsel_matrix.sh still uses this exact-prefix regex:

   grep -oE '\[gc-copy-minor\] ran copied_objects=[0-9]+'

   That expression cannot match any current-format ran line, whatever its object counts, so scavenged is forced to zero and every requires-move arm becomes UNVER. This exactly brackets the transition: the last successful main run was dc6721c7f before #7744; the first complete post-batch run was red.

   The correct liveness quantity is not copied_objects alone. gc/instruments.rs::note_copying_minor_moved already documents and implements copied + promoted, because object-by-object promotion relocates too (#7657). The parser must nevertheless exclude in_place=true, whose whole-block promotion moves nothing.

   The pending fix parses the record by field name, sums copied+promoted only for non-in-place cycles, and includes sabotage/self-tests for legacy format, current field ordering, ordinary promotion, and in-place promotion. The liveness requirement remains nonzero relocation; it is not weakened.
3. proggeramlug

   closed this as completedin #8056on Aug 14, 2026