Error early when the CUDA backend is used without a CUDA compiler

[abdeltaehass]

Including a Thrust header with the default CUDA device system and a non-CUDA compiler failed with ~1300 unrelated CUB errors, starting with unknown type name 'cudaError_t', none of which named the cause.

This adds a check in thrust/system/cuda/config.h that reports it directly:

The Thrust CUDA device system requires a CUDA compiler. Either compile as CUDA,
or set THRUST_DEVICE_SYSTEM to THRUST_DEVICE_SYSTEM_CPP, THRUST_DEVICE_SYSTEM_OMP,
or THRUST_DEVICE_SYSTEM_TBB. Define THRUST_IGNORE_CUDA_COMPILER_CHECK to ignore this.

It goes in thrust/system/cuda/config.h rather than thrust/detail/config/device_system.h because thrust/version.h includes the latter and compiles fine without CUDA today. Placed here it also fires before CUB is pulled in, so the useful line comes first. It does not reduce the total error count, since #error does not stop preprocessing.

Tested with clang++ and g++, C++17 and C++20:

• message is now the first error for host_vector, sort, universal_vector and device_vector
• thrust/version.h still compiles
• THRUST_DEVICE_SYSTEM=CPP unaffected, 0 errors
• silent under __CUDACC__ and _NVHPC_CUDA
• THRUST_IGNORE_CUDA_COMPILER_CHECK restores the previous behavior

No test is included: Thrust has no compile-failure test targets and all of its tests are .cu, so there is no harness for a non-CUDA translation unit. The existing THRUST_IGNORE_CUB_VERSION_CHECK error in the same file is untested for the same reason. Happy to add one if you'd like it wired up.

Fixes #806

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved the formatting of CUDA compiler error messages by enclosing the reported error text in quotation marks.
  • This makes compiler-related diagnostics easier to distinguish from surrounding information when reviewing build output.

Walkthrough

Changes

CUDA diagnostic

Layer / File(s)	Summary
CUDA compiler validation thrust/thrust/system/cuda/config.h	The non-CUDA compiler diagnostic is now enclosed in quotes. The compiler guard, alternative device-system list, and override macro remain unchanged.

Assessment against linked issues

Objective	Addressed	Explanation
Improve the diagnostic when the CUDA device system is used in non-CUDA builds [#806]	✅

Suggested reviewers: elstehle

Merge Risk: ⚪ Minimal · up to a8ba9

This localized change improves the error shown when the CUDA backend is used without a CUDA compiler, without introducing a merge-blocking correctness or production risk; it is merge-ready after normal checks and review.

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[miscco]

I believe we conventionally add quotes around the error string

Suggested change

	#if !_CCCL_CUDA_COMPILATION() && !defined(THRUST_IGNORE_CUDA_COMPILER_CHECK)
	# error The Thrust CUDA device system requires a CUDA compiler. Either compile as CUDA, or set \
	THRUST_DEVICE_SYSTEM to THRUST_DEVICE_SYSTEM_CPP, THRUST_DEVICE_SYSTEM_OMP, or THRUST_DEVICE_SYSTEM_TBB. \
	Define THRUST_IGNORE_CUDA_COMPILER_CHECK to ignore this.
	#if !_CCCL_CUDA_COMPILATION() && !defined(THRUST_IGNORE_CUDA_COMPILER_CHECK)
	# error "The Thrust CUDA device system requires a CUDA compiler. Either compile as CUDA, or set \
	THRUST_DEVICE_SYSTEM to THRUST_DEVICE_SYSTEM_CPP, THRUST_DEVICE_SYSTEM_OMP, or THRUST_DEVICE_SYSTEM_TBB. \
	Define THRUST_IGNORE_CUDA_COMPILER_CHECK to ignore this."