[perfloop-agent]

Repeated-character string append now reads the target length before borrowing the receiver for resizing. This avoids a checked-borrow panic for strings whose data pointer is used, including calls that ignore the result.

• Save the post-pop length plus the append count in a local before resizing in rules/string/tgt_refcount.rs.
• Keep the trailing-zero update and the existing result clone.
• Use the size type explicitly in the fixture's modulo expressions so its generated-source expectations do not depend on the standard library's type spelling for sizeof.

Runtime borrow checking and the unsafe-model rule are unchanged. This repairs the receiver's own length read; it does not change append's return-reference semantics or operand evaluation.

Testing

tests/unit/string_repeat_append.cpp checks size, receiver bytes and the trailing zero over a bounded grid of initial lengths, counts and byte values, with and without shrinking first. It includes empty strings, zero counts, embedded NULs and growth, plus unboxed-local, reference and pointer receivers. The scoped lit runner passes four tests with no failures or skips, comparing native C++ with both output models and checking their generated-source expectations. The existing string fixture checks the surrounding string rules.

Coverage is bounded; application frequency remains unmeasured.

4 recorded checks pass with this change.

• Scoped string lit tests
• Rules clippy
• String rule and expected-output rustfmt
• Append fixture clang-format

Case and evidence

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