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clean up checks for constant promotion of integer division/remainder operations #151773

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Merged
rust-bors merged 1 commit into from
Jan 29, 2026
+24 −38
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clean up checks for constant promotion of integer division/remainder operations #151773

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62 changes: 24 additions & 38 deletions compiler/rustc_mir_transform/src/promote_consts.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -485,47 +485,33 @@ impl<'tcx> Validator<'_, 'tcx> {
if lhs_ty.is_integral() {
let sz = lhs_ty.primitive_size(self.tcx);
// Integer division: the RHS must be a non-zero const.
let rhs_val = match rhs {
Operand::Constant(c)
if self.should_evaluate_for_promotion_checks(c.const_) =>
{
c.const_.try_eval_scalar_int(self.tcx, self.typing_env)
}
_ => None,
};
match rhs_val.map(|x| x.to_uint(sz)) {
let rhs_val = if let Operand::Constant(rhs_c) = rhs
&& self.should_evaluate_for_promotion_checks(rhs_c.const_)
&& let Some(rhs_val) =
rhs_c.const_.try_eval_scalar_int(self.tcx, self.typing_env)
// for the zero test, int vs uint does not matter
Some(x) if x != 0 => {} // okay
_ => return Err(Unpromotable), // value not known or 0 -- not okay
}
&& rhs_val.to_uint(sz) != 0
{
rhs_val
} else {
// value not known or 0 -- not okay
return Err(Unpromotable);
};
// Furthermore, for signed division, we also have to exclude `int::MIN /
// -1`.
if lhs_ty.is_signed() {
match rhs_val.map(|x| x.to_int(sz)) {
Some(-1) | None => {
// The RHS is -1 or unknown, so we have to be careful.
// But is the LHS int::MIN?
let lhs_val = match lhs {
Operand::Constant(c)
if self.should_evaluate_for_promotion_checks(
c.const_,
) =>
{
c.const_
.try_eval_scalar_int(self.tcx, self.typing_env)
}
_ => None,
};
let lhs_min = sz.signed_int_min();
match lhs_val.map(|x| x.to_int(sz)) {
// okay
Some(x) if x != lhs_min => {}

// value not known or int::MIN -- not okay
_ => return Err(Unpromotable),
}
}
_ => {}
if lhs_ty.is_signed() && rhs_val.to_int(sz) == -1 {
// The RHS is -1, so we have to be careful. But is the LHS int::MIN?
if let Operand::Constant(lhs_c) = lhs
&& self.should_evaluate_for_promotion_checks(lhs_c.const_)
&& let Some(lhs_val) =
lhs_c.const_.try_eval_scalar_int(self.tcx, self.typing_env)
&& let lhs_min = sz.signed_int_min()
&& lhs_val.to_int(sz) != lhs_min
{
// okay
} else {
// value not known or int::MIN -- not okay
return Err(Unpromotable);
}
}
}
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