llvm-project

2d31fc85 - Reland [clang][modules-driver] Add scanner to detect C++20 module presence (#147630)

Reland [clang][modules-driver] Add scanner to detect C++20 module presence (#147630) This patch is part of a series to natively support C++20 module usage from the Clang driver (without requiring an external build system). This introduces a new scanner that detects C++20 module usage in source files without using the preprocessor or lexer. For now, it is enabled only with the `-fmodules-driver` flag and serves solely diagnostic purposes. In the future, the scanner will be enabled for any (modules-driver compatible) compilation with two or more inputs, and will help the driver determine whether to implicitly enable the modules driver. Since the scanner adds very little overhead, we are also exploring enabling it for compilations with only a single input. This approach could allow us to detect `import std` usage in a single-file compilation, which would then activate the modules driver. For performance measurements on this, see https://github.com/naveen-seth/llvm-dev-cxx-modules-check-benchmark. RFC: https://discourse.llvm.org/t/rfc-modules-support-simple-c-20-modules-use-from-the-clang-driver-without-a-build-system This patch relands commit ded1426. The CI failure is resolved by removing the compatibility warning for using the `-fmodules-driver` flag with pre-C++20 standards, which also better aligns its behavior with other features/flags supported only in newer standards.