[all-commits] [llvm/llvm-project] 2d31fc: Reland [clang][modules-driver] Add scanner to dete...

[Naveen Seth Hanig via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2d31fc85a847759a5ea1fb39718a3851d57913c3
      https://github.com/llvm/llvm-project/commit/2d31fc85a847759a5ea1fb39718a3851d57913c3
  Author: Naveen Seth Hanig <naveen.hanig at outlook.com>
  Date:   2025-07-21 (Mon, 21 Jul 2025)

  Changed paths:
    M clang/include/clang/Basic/DiagnosticDriverKinds.td
    M clang/include/clang/Basic/DiagnosticGroups.td
    M clang/include/clang/Driver/Driver.h
    M clang/include/clang/Driver/Options.td
    M clang/include/clang/Lex/DependencyDirectivesScanner.h
    M clang/lib/Driver/Driver.cpp
    M clang/lib/Lex/DependencyDirectivesScanner.cpp
    A clang/test/Driver/modules-driver-cxx20-module-usage-scanner.cpp

  Log Message:
  -----------
  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.



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