Commits on May 21, 2024

1. restore (debug-level 2) behavior, but reduce its guarantees …

   When `debug-level` is 2 or more, optimization is supposed to refrain
   from moving a call to an error function into tail position of the
   enclosing function. (That's a stronger guarantee than the usual one of
   not moving an expression into tail position if there's potentially a
   way to detect the movement through continuation marks.) An earlier
   commit 6a73b9e strengthened movement of error calls in a way that
   did not preserve this `(debug-level 2)` constraint, so this commit
   primarily adjusts those changes. Also, the earlier commit 19af32e
   allowed the compiler to move an error call out of tail position, and
   that transformation is now suppressed when `debug-level` is 2 or more.
   
   These changes are meant to help with debugging, but they're not enough
   to implement the previously specified behavior of `debug-level`. It
   turns out that Chez Scheme v9.x didn't implement the specified
   behavior, either, because it would convert `(let ([x (error ...)]) x)`
   to just `(error ...)`, for example. The old specification seems too
   strong, and so part of the revision here is to change the specificaton
   to be encouraging a particular interaction of errors and
   continuaton-inspection results, but not guaranteeing it. Meanwhile,
   when well-defined reflection on continuations is needed, continuation
   marks provide that functionality.

   

   mflatt committed May 21, 2024
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