Two rules the AST makes possible, plus the mutation analysis behind them.
const-local reports a local that is never written. It is restricted to SCALARS
— integers, bools, enums, floating types — and that restriction is what makes
the answer exact rather than a guess: a scalar has no member functions, so the
only ways to write one are assignment, ++/--, having its address taken, or
binding to a non-const reference. All four are now tracked in the walk:
- assignment and compound assignment visit their LEFT operand in a write
context, the right one normally;
- ++/-- and & write their operand;
- a call argument is checked against the callee's parameter type, so passing
to `const int&` or by value is a read while `int&` is a write;
- initialising a non-const reference writes what it binds to.
For a class type a non-const method call could mutate it, and deciding that is
the whole-program analysis clang-tidy does, so those are simply out of scope
rather than guessed at.
constexpr-constant promotes a const constant whose initialiser is made only of
literals and operators, so `const int A = 1 << 4;` qualifies and
`const int B = Compute();` does not.
On this repository const-local found 103 candidates, which was too many to be
useful, and the reason was informative: most were range-for bindings and
pointer locals. `for (T* const x : …)` and `T* const p` are not spellings
anybody writes, and the useful constness for a pointer is on the pointee, which
this rule cannot advise on. Excluding both leaves 36, all plain bool or enum
locals worth fixing — isWasm, isPe, exists, writes, isC and so on. Those 36 are
fixed in this commit; the compiler verified every one.
Both rules are report-only. The analysis is exact, but adding const is a
judgement about intent as much as mechanics, and a wrong suggestion should cost
a glance rather than a build. const-local also deliberately does not become a
transform: inserting `const` before a shared type would apply it to every
declarator in a multi-declarator statement, including any that IS written.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Jorijn van der Graaf
651720e494