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fix: kill must respect the exceptional postconditions - #1176
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On a hoare goal, `EcLowPhlGoal.t_code_transform` handed the code
transformations the main postcondition only. `kill` uses it to check
that the killed code writes no variable read by the postcondition, so a
variable read only by an exceptional postcondition could be killed,
which proves false judgements, e.g.
hoare [N.f : N.x = 0 ==> true | oops _ => N.x = 0]
for `N.f = { N.x <- 1; raise (oops 0); }`, by `kill 1`. As for #1129
(`swap`), no derivation of `false` is known: no rule relates exceptional
postconditions to `phoare` / `Pr`.
The transformations now get the conjunction of the main and exceptional
postconditions (they only use it for the variables it reads; `kill` is
the only one that does). The issue dates from the introduction of
exceptions (bba1f1b, first released in r2026.03).
tests/kill-exn.ec checks that `kill` is rejected in that case and still
applies when the exceptional postconditions do not read the killed
variables. The stdlib, the unit tests and the examples pass.
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On a hoare goal,
EcLowPhlGoal.t_code_transformhanded the codetransformations the main postcondition only.
killuses it to checkthat the killed code writes no variable read by the postcondition, so a
variable read only by an exceptional postcondition could be killed,
which proves false judgements, e.g.
hoare [N.f : N.x = 0 ==> true | oops _ => N.x = 0]
for
N.f = { N.x <- 1; raise (oops 0); }, bykill 1. As for #1129(
swap), no derivation offalseis known: no rule relates exceptionalpostconditions to
phoare/Pr.The transformations now get the conjunction of the main and exceptional
postconditions (they only use it for the variables it reads;
killisthe only one that does). The issue dates from the introduction of
exceptions (bba1f1b, first released in r2026.03).
tests/kill-exn.ec checks that
killis rejected in that case and stillapplies when the exceptional postconditions do not read the killed
variables. The stdlib, the unit tests and the examples pass.