fix(mir): constant propagation folds 0^0 to 1 #509
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Leo-Besancon
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Dec 2, 2025
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Leo-Besancon
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Leo-Besancon
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Thank you for the PR, good catch! I've left a few comments inline.
| use crate::ir::{Enf, Exp, Link, Mir, MirValue, Op, SpannedMirValue, Value, ConstantValue}; | ||
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| #[test] | ||
| fn constant_propagation_folds_zero_pow_zero_to_one() { |
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Could you remove the test (it does not build as is, is not included when running cargo test and it does not seem needed to me)
| } else if let Some(0) = get_inner_const(&lhs) { | ||
| // 0^k = 0, but only when k is known and non-zero | ||
| if let Some(k) = get_inner_const(&rhs) | ||
| && k != 0 |
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The case "k == 0" is already handled in the check "rhs = 0" above, you do not need it here.
| span: exp_ref.span, | ||
| }))); | ||
| } | ||
| try_fold_const_binary_op(lhs, rhs, exp.clone(), exp_ref.span()) |
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If k is not constant, this function call will always return Ok(None), so we should return it directly.
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Describe your changes
The issue was in the MIR constant propagation pass where exponentiation folding handled the base-equals-zero case before checking for a zero exponent, causing 0^0 to fold to 0. This contradicted both the AST constant folding, which uses checked_pow and yields 1 for 0^0, and the MIR evaluator, which evaluates x^0 as 1 in the field. The fix changes the folding order in visit_exp_bis so that we fold x^0 to 1 first and only fold 0^k to 0 when k is a known, nonzero constant, delegating to generic constant folding otherwise. A MIR-level unit test was added to assert that 0^0 folds to a constant 1, guarding against regressions even if AST behavior changes.