Currently, when there is both an expandlibs descriptor and an actual static
library, expandlibs picks the static library. This has the side effect that
if there are object files in the static library that aren't directly used,
they're dropped when linking, even when they export symbols that would be
exported in the final linked binary.
In most cases in the code base, files are not dropped that way. The most
notable counter-example is xpcomglue, where actually not dropping files
leads to link failure because of missing symbols those files reference
(yes, that would tend to say the glue is broken in some way).
On the opposite side, there is mozglue, which does have both a descriptor
and a static library (the latter being necessary for the SDK), and that
linking as a static library drops files that shouldn't be dropped (like
jemalloc). We're currently relying on -Wl,--whole-archive for those files
not to be dropped, but that won't really be possible without much hassle
in a world where mozglue dependencies live in moz.build land.
Switching expandlibs to use descriptors when they exist, even when there
is a static library (so, the opposite of the current behavior) allows to
drop -Wl,--whole-archive and prepare for a better future. However, as
mentioned, xpcomglue does still require to be linked through the static
library, so we need to make it a static library only.
To achieve that, we make NO_EXPAND_LIBS now actually mean no expandlibs
and use that to build the various different xpcomglues.
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