cuda.core: eliminate build warnings; set option to build with warnings as errors

[Andy-Jost]

Summary

The wheel builds print no Cython errors or warnings, since build_hooks.py already sets warning_errors, but each build prints about 150 Cython performance hints, a dozen gcc warnings, and several hundred MSVC narrowing warnings. All of them trace back to a handful of source lines. This PR fixes the ones that are ours, silences the ones we can't fix, and treats warnings as errors to avoid future drift.

Changes

• build_hooks.py, build-wheel.yml, AGENTS.md: the CUDA_PYTHON_WERROR switch described below, set for the CI wheel builds.

• _rt.pxd / _rt.pyx: sm_resource_split and memcpy_with_attributes_async return a CUresult and never raise, but were declared nogil without noexcept. Cython cannot pick an error sentinel for an enum return type, so every call from a nogil block acquired the GIL to run PyErr_Occurred(). Both are now noexcept nogil, which removes the two hints Cython raised at their call sites in _device_resources.pyx and _memory/_buffer.pyx.

• _rt.pyx: # cython: show_performance_hints=False, with a comment explaining why. Every except+ nogil handle factory trips the same generic hint, but for except+ the GIL is taken only inside the C++ catch handler, so the success path pays nothing. That accounts for 54 of the hints.

• _memoryview.pyx: initialize three locals that gcc could not prove set on every path.

• graph/_graph_builder.pyx: a guarded #pragma GCC diagnostic ignored "-Wunused-function" in a cdef extern from * block. Each @overload stub of Graph.__getitem__ compiles to a static wrapper that nothing references, because only the final definition is bound, and the stubs stay because stubgen reads them. The pragma covers only that translation unit and sits next to the comment that explains the stubs.

• system/_device_utils.pxi, _device_resources.pyx: loop index types now match their bounds (sign-compare).

• _layout.pxd / _layout.pyx, _tensor_map.pyx, _launch_config.pyx, _stream.pyx, _device_resources.pyx: explicit casts where a Py_ssize_t, size_t, int64_t, or long feeds an int or unsigned int that only ever holds a rank, a count, or a gcd bounded by its int input, plus an int return type for _init_dense, which returns a status. These are the MSVC C4244/C4267 sites in hand-written code.

What stays

• MSVC C4551 (643 per build) and the remaining C4244 come from Cython's own utility code and from the overflow-check helpers that @cython.overflowcheck(True) instantiates in _layout.pxd. They cannot be fixed in .pyx sources.
• The remaining Cython hints are in generated cuda.bindings files.

Warnings as errors

Cython warnings are already errors, since build_hooks.py sets warning_errors. This PR adds the compiler side as an opt-in: CUDA_PYTHON_WERROR=1 appends -Werror on gcc/clang and /WX /wd4551 /wd4244 on MSVC. The two MSVC exemptions are the families above that Cython's utility code produces in every module; gcc and clang need none. The CI wheel builds set the variable in build-wheel.yml; source builds and local builds do not, because they run on compilers we do not control. A debug build with the variable set trips the glibc _FORTIFY_SOURCE warning at -O0, so it is meant for optimized builds.

If a future Cython or compiler version introduces a new warning class in generated code, the CI build fails and the exemption list or the source gets the fix. That is the point of the switch.

🤖 Generated with Claude Code

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[juenglin]

Nice, thanks!