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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-External.cpp
View file

@ -279,7 +279,7 @@ ExternalBuildResult Crafter::BuildExternal(const ExternalDependency& dep, std::s
// LD_LIBRARY_PATH gymnastics. Static deps (.a) ignore the rpath
// harmlessly. PE/COFF targets (mingw, msvc) resolve DLLs via PATH
// or adjacency rather than rpath, so lld warns if we pass it.
bool isPe = target == "x86_64-w64-mingw32" || target == "x86_64-pc-windows-msvc";
const bool isPe = target == "x86_64-w64-mingw32" || target == "x86_64-pc-windows-msvc";
std::string buildDirAbs = fs::absolute(cmakeBuildDir).string();
result.linkFlags.push_back(std::format("-L{}", buildDirAbs));
if (!isPe) result.linkFlags.push_back(std::format("-Wl,-rpath,{}", buildDirAbs));
@ -375,7 +375,7 @@ Configuration* Crafter::GitProject(const GitProjectSpec& spec) {
fs::path cloneDir = externalRoot / std::format("{}-{}", name, srcHash);
{
bool exists = fs::exists(cloneDir);
const bool exists = fs::exists(cloneDir);
Progress::Task task(std::format("{} {}", exists ? "Updating" : "Cloning", name));
if (std::string err = FetchGit(spec.source, cloneDir); !err.empty()) {
throw std::runtime_error(std::format("GitProject({}): {}", spec.source.url, err));
@ -416,7 +416,7 @@ fs::path Crafter::GitFetch(const GitSource& source) {
fs::path cloneDir = externalRoot / std::format("{}-{:016x}", name, key);
{
bool exists = fs::exists(cloneDir);
const bool exists = fs::exists(cloneDir);
Progress::Task task(std::format("{} {}", exists ? "Updating" : "Cloning", name));
if (std::string err = FetchGit(source, cloneDir); !err.empty()) {
throw std::runtime_error(std::format("GitFetch({}): {}", source.url, err));