[clang-tools-extra] [clangd] [C++20] [Modules] Add modules suffix for 'Header' Source Switch (PR #131591)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 created https://github.com/llvm/llvm-project/pull/131591

Support the trivial "header"/source switch for module interfaces.

I initially thought the naming are bad and we should rename it. But later I feel it is better to split patches as much as possible. 

>From the codes it looks like there are problems. e.g., `isHeaderFile`. But let's try to fix them in different patches.

>From be78b07d83704ead5ce13e8f696f8ab45d1a1add Mon Sep 17 00:00:00 2001
From: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
Date: Mon, 17 Mar 2025 17:45:58 +0800
Subject: [PATCH] [clangd] [C++20] [Modules] Add modules suffix for 'Header'
 Source Switch

---
 .../clangd/HeaderSourceSwitch.cpp             |  4 +-
 .../unittests/HeaderSourceSwitchTests.cpp     | 38 +++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clangd/HeaderSourceSwitch.cpp b/clang-tools-extra/clangd/HeaderSourceSwitch.cpp
index 2351858cc6297..d54c3668570eb 100644
--- a/clang-tools-extra/clangd/HeaderSourceSwitch.cpp
+++ b/clang-tools-extra/clangd/HeaderSourceSwitch.cpp
@@ -22,7 +22,9 @@ std::optional<Path> getCorrespondingHeaderOrSource(
     PathRef OriginalFile, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) {
   llvm::StringRef SourceExtensions[] = {".cpp", ".c", ".cc", ".cxx",
                                         ".c++", ".m", ".mm"};
-  llvm::StringRef HeaderExtensions[] = {".h", ".hh", ".hpp", ".hxx", ".inc"};
+  llvm::StringRef HeaderExtensions[] = {".h",    ".hh",   ".hpp", ".hxx",
+                                        ".inc",  ".cppm", ".ccm", ".cxxm",
+                                        ".c++m", ".ixx"};
 
   llvm::StringRef PathExt = llvm::sys::path::extension(OriginalFile);
 
diff --git a/clang-tools-extra/clangd/unittests/HeaderSourceSwitchTests.cpp b/clang-tools-extra/clangd/unittests/HeaderSourceSwitchTests.cpp
index 6e280f5b085a9..e600207de458a 100644
--- a/clang-tools-extra/clangd/unittests/HeaderSourceSwitchTests.cpp
+++ b/clang-tools-extra/clangd/unittests/HeaderSourceSwitchTests.cpp
@@ -76,6 +76,44 @@ TEST(HeaderSourceSwitchTest, FileHeuristic) {
   EXPECT_FALSE(PathResult.has_value());
 }
 
+TEST(HeaderSourceSwitchTest, ModuleInterfaces) {
+  MockFS FS;
+
+  auto FooCC = testPath("foo.cc");
+  auto FooCPPM = testPath("foo.cppm");
+  FS.Files[FooCC];
+  FS.Files[FooCPPM];
+  std::optional<Path> PathResult =
+      getCorrespondingHeaderOrSource(FooCC, FS.view(std::nullopt));
+  EXPECT_TRUE(PathResult.has_value());
+  ASSERT_EQ(*PathResult, FooCPPM);
+
+  auto Foo2CPP = testPath("foo2.cpp");
+  auto Foo2CCM = testPath("foo2.ccm");
+  FS.Files[Foo2CPP];
+  FS.Files[Foo2CCM];
+  PathResult = getCorrespondingHeaderOrSource(Foo2CPP, FS.view(std::nullopt));
+  EXPECT_TRUE(PathResult.has_value());
+  ASSERT_EQ(*PathResult, Foo2CCM);
+
+  auto Foo3CXX = testPath("foo3.cxx");
+  auto Foo3CXXM = testPath("foo3.cxxm");
+  FS.Files[Foo3CXX];
+  FS.Files[Foo3CXXM];
+  PathResult = getCorrespondingHeaderOrSource(Foo3CXX, FS.view(std::nullopt));
+  EXPECT_TRUE(PathResult.has_value());
+  ASSERT_EQ(*PathResult, Foo3CXXM);
+
+  auto Foo4CPLUSPLUS = testPath("foo4.c++");
+  auto Foo4CPLUSPLUSM = testPath("foo4.c++m");
+  FS.Files[Foo4CPLUSPLUS];
+  FS.Files[Foo4CPLUSPLUSM];
+  PathResult =
+      getCorrespondingHeaderOrSource(Foo4CPLUSPLUS, FS.view(std::nullopt));
+  EXPECT_TRUE(PathResult.has_value());
+  ASSERT_EQ(*PathResult, Foo4CPLUSPLUSM);
+}
+
 MATCHER_P(declNamed, Name, "") {
   if (const NamedDecl *ND = dyn_cast<NamedDecl>(arg))
     if (ND->getQualifiedNameAsString() == Name)