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feat(lint): const-local and constexpr-constant rules

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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>
This commit is contained in:
Jorijn van der Graaf 2026-07-31 00:50:48 +02:00
commit 651720e494
10 changed files with 370 additions and 39 deletions
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6
implementations/Crafter.Build-Clang.cpp
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@ -204,7 +204,7 @@ void Configuration::GetInterfacesAndImplementations(std::span<fs::path> interfac
fileCopy.replace_extension("");
Implementation& implementation = this->implementations.emplace_back(std::move(fileCopy));
if (std::regex_search(fileContent, match, std::regex(R"(module ([a-zA-Z0-9_\.\-]+)(:[a-zA-Z0-9_\.\-]+)?\s*;)"))) {
bool isPartitionImpl = match[2].length() > 0;
const bool isPartitionImpl = match[2].length() > 0;
for(const std::unique_ptr<Module>& interface : this->interfaces) {
if(interface->name == match[1]) {
if (!isPartitionImpl) {
@ -275,7 +275,7 @@ CompileCommand Crafter::GetCompileCommand(const Configuration& config) {
// wasm32 targets reject -march and silently ignore -mtune (clang errors on
// the former). Skip both for any wasm32-* triple.
bool isWasm = config.target.starts_with("wasm32");
const bool isWasm = config.target.starts_with("wasm32");
std::string archFlags = isWasm
? std::string()
: std::format(" -march={} -mtune={}", config.march, config.mtune);
@ -1777,7 +1777,7 @@ std::int32_t Crafter::Run(std::int32_t argc, char** argv) {
// server (browser build with index.html). std::system on the
// .wasm path goes nowhere useful — replace with detection.
if (config.target.starts_with("wasm32")) {
bool browserBuild = fs::exists(absDir / "index.html");
const bool browserBuild = fs::exists(absDir / "index.html");
auto have = [](std::string_view exe) {
#ifdef _WIN32
std::string probe = std::format("where {} > NUL 2>&1", exe);