A `[[test]] harness = false` target supplies its own `main`, so cargo
invokes rustc with neither `--test` nor `--crate-type`. Its extensionless
output therefore classifies as `Other("rustc:unknown")`, whose link
strategy is `Hardlink`, and the restore path derived the output's
permissions from that strategy alone — so no `0o755` was applied. The
restored test binary comes back non-executable and cargo fails the run
with "Permission denied (os error 13)".

The resulting mode depends on which materialization path runs, and only
one of them preserves the bit by accident:

  reflink   cache+target on one CoW filesystem   fresh file at the umask
  hardlink  one filesystem, copy-ingested blob   blob's 0o555 — survives
  hardlink  one filesystem, reflink-ingested     blob's 0o444
  copy      cache and target on different fs     0o644, hardcoded

So this is not specific to any filesystem: reflink-capable volumes (btrfs,
XFS with reflink=1, ZFS >= 2.2, bcachefs, APFS) hit it, and so does any
setup whose cache and build directories live on separate filesystems, ext4
included. The one configuration that works is a same-filesystem hardlink
from a copy-ingested blob, which is what a plain ext4 checkout produces —
and why this went unnoticed.

The executable bit recorded at insert time is the reliable signal, and the
insert side already trusts it over the filename (`hardlink_eligible`
refuses to hardlink anything carrying a mode bit). Restore now trusts it
the same way, which also keeps executables on the independent-inode path
so a post-build `strip` or codesign cannot reach back into the shared blob.

Covered by unit tests in `wrapper::tests`, which assert the contract
directly without depending on filesystem behaviour, and by a new
end-to-end `custom_harness_test` that builds the
`test-projects/custom-harness` fixture through the real wrapper, drops the
target dir, and lets cargo execute the restored binary. That test runs on
the repository's filesystem rather than TMPDIR, which would otherwise land
in the one blind spot above.