Problem
stream/transformer.rs:411 accumulates a \uXXXX escape by bit-packing:
let value = value << 4 | u16::from(digit);
The |→^ mutation of this line cannot be observed. value << 4 leaves the low four bits zero and digit is a hex nibble in 0..=15, so the operands never share a set bit and OR and XOR agree on every reachable input.
That is a property of the spelling, not of the computation. The | is doing the work of an addition over disjoint bit ranges, and any of |, ^ or + would compute the same thing — which is precisely why mutating between them is invisible. An operator whose choice cannot be observed is an operator whose choice was arbitrary.
Required change
Rewrite the accumulation so every operator carries meaning:
let value = value * 16 + u16::from(digit);
This is the same computation for disjoint operands, and both of its mutations are observable: *→/ and +→- each change the result, so a test over \uXXXX decoding kills them. The |/^ ambiguity disappears along with the |.
Check the rewrite is faithful before taking it. A \uXXXX escape carries at most four hex digits, so the accumulated value stays within u16 and the multiply cannot overflow where the shift would have wrapped. Confirm that bound rather than assuming it, and confirm the surrounding loop cannot feed a fifth digit.
Acceptance criteria
decode_key_byte has no mutation that survives a test.- The new operators are killed by a test over
\uXXXX key decoding, each verified by counterfactual.
- No
.cargo/mutants.toml exclusion is added for this line.
- Behaviour is unchanged for every four-digit escape, including the maximum
�.
Boundary
Only the accumulation expression and the tests that cover it.
History
Filed first as an exclusion request, on the reasoning that an equivalent mutant cannot be killed by any test. That is true and beside the point: the mutant is only equivalent because of how the line is written, and rewriting it removes the equivalence. The same revision wrongly claimed emit_file's two > mutants at transformer.rs:661 needed a page of 500,000 files — files_seen is a field, so a test seeds it to the boundary and calls emit_file once. Those are ordinary #1893 work.
Problem
stream/transformer.rs:411accumulates a\uXXXXescape by bit-packing:The
|→^mutation of this line cannot be observed.value << 4leaves the low four bits zero anddigitis a hex nibble in0..=15, so the operands never share a set bit and OR and XOR agree on every reachable input.That is a property of the spelling, not of the computation. The
|is doing the work of an addition over disjoint bit ranges, and any of|,^or+would compute the same thing — which is precisely why mutating between them is invisible. An operator whose choice cannot be observed is an operator whose choice was arbitrary.Required change
Rewrite the accumulation so every operator carries meaning:
This is the same computation for disjoint operands, and both of its mutations are observable:
*→/and+→-each change the result, so a test over\uXXXXdecoding kills them. The|/^ambiguity disappears along with the|.Check the rewrite is faithful before taking it. A
\uXXXXescape carries at most four hex digits, so the accumulated value stays withinu16and the multiply cannot overflow where the shift would have wrapped. Confirm that bound rather than assuming it, and confirm the surrounding loop cannot feed a fifth digit.Acceptance criteria
decode_key_bytehas no mutation that survives a test.\uXXXXkey decoding, each verified by counterfactual..cargo/mutants.tomlexclusion is added for this line.�.Boundary
Only the accumulation expression and the tests that cover it.
History
Filed first as an exclusion request, on the reasoning that an equivalent mutant cannot be killed by any test. That is true and beside the point: the mutant is only equivalent because of how the line is written, and rewriting it removes the equivalence. The same revision wrongly claimed
emit_file's two>mutants attransformer.rs:661needed a page of 500,000 files —files_seenis a field, so a test seeds it to the boundary and callsemit_fileonce. Those are ordinary #1893 work.