... exposed a new bug on 1.10:
$ ./julia --project=test/project/Extensions/Parent.jl -q
(Parent) pkg> precompile
Precompiling project...
✓ DepWithParentExt
✓ Parent
2 dependencies successfully precompiled in 2 seconds
julia> using Parent
[ Info: Precompiling ParentExt [3290166e-fa45-5082-a21d-a36e4f051b06]
] precompile doesn't realize it needs to compile the extension because it doesn't have Parent in the old manifest format.
Originally posted by @topolarity in #56666 (comment)