The JIT wants to detect calls to functions bound to module-level
variables that are always instantiated to a procedure of a particular
arity, in which case it can avoid some checks. The problem is that
bytecode and JITted code can be shared via the module cache across
namespaces that use different modules to implement a particular module
path or that enable the JIT differently. In particular, starting with
one that has a procedure binding and then using one (in a different
namespace) that has a non-procedure binding could lead to a crash.
Defend against this possibility by never treating imported variables
as constant in that sense. The JIT detects imported variables through
a new import map in the prefix.
This change may slow code. My guess is that it will have no
performance impact in practice, due to cross-module inlining,
although I can construct a microbenchmark that slows by 20%.
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