Asked whether a defconst could be redefined, probed it, and got ":status ok"
for a change that did nothing at all - the module was built, delivered,
installed, and the program went on using the old value. That is the
silent-wrongness class the house rule exists to prevent, so it is now four
refusals and a fix.
A defconst's value is folded into its call sites - into an array length at
worst, which is decided before any type resolves - so it lives in the running
program's code and not only in its storage. Refused. A defenum member is the
same thing: :space is erased to an i32 literal in the caller. Refused, and
compared over declarations rather than over Tast.program, which carries no
enums at all for exactly that reason.
A defvar's initial value is deliberately not refused. Its storage holds live
state the program moved past long ago, and refusing to change the initialiser
would be refusing "edit the code, keep the sand". Same Tast.global record as a
defconst, opposite answers, told apart by gconst.
The value comparison is structural and conservative - anything it does not
recognise counts as changed. Comparing emitted text would be wrong, since
Emit.const on a string allocates a name off a per-module counter and two
different strings in two throwaway modules both come out as @".str.0".
Third: a new global's declared initial value was being dropped. flan_dev_global
callocs, so (defvar n i64 42) added at run time was silently zero. It now takes
the initial value as a blob, copies it on the allocation and ignores it
afterwards - the second half being where "a reload must not reset the program's
state" lives. In the allocation path rather than a branch at the call site, so
it cannot be got wrong at one of them.
Fourth: a change with no body to publish and no storage to allocate now answers
"nothing to install" instead of shipping an empty module. That is what the
defconst probe actually did, and it cost the program a frame's worth of reload
it did not need.
Joseph Ferano
52d3898116