[clangd] Support -stdlib flags in SystemIncludeExtractor. (#69283)

The `--stdlib` flag can affect the system headers used by `clang` during
compilation. By default, `clang` will use the platform-installed C++
standard headers, but with `--stdlib=libc++`, `clang` can use headers
included in the distribution for its `libc++` implementation.

Prior to this patch, if `compile_commands.json` specified
`-stdlib=libc++` or an equivalent form and `--query-driver` took effect,
`clangd` would ignore `stdlib` and index based on the platform's
headers. When these mismatch, e.g. due to version differences,
`clangd`'s completions and the actual compilation can differ.

fixes clangd/clangd#1784

---------

Co-authored-by: Chris Carlon <cjc25@cjc25.com>

clang-tools-extra/clangd/SystemIncludeExtractor.cpp

@@ -88,12 +88,14 @@ struct DriverArgs {
   std::string Sysroot;
   std::string ISysroot;
   std::string Target;
+  std::string Stdlib;
 
   bool operator==(const DriverArgs &RHS) const {
     return std::tie(Driver, StandardIncludes, StandardCXXIncludes, Lang,
-                    Sysroot, ISysroot, Target) ==
+                    Sysroot, ISysroot, Target, Stdlib) ==
            std::tie(RHS.Driver, RHS.StandardIncludes, RHS.StandardCXXIncludes,
-                    RHS.Lang, RHS.Sysroot, RHS.ISysroot, RHS.Target);
+                    RHS.Lang, RHS.Sysroot, RHS.ISysroot, RHS.Target,
+                    RHS.Stdlib);
   }
 
   DriverArgs(const tooling::CompileCommand &Cmd, llvm::StringRef File) {
@@ -136,6 +138,13 @@ struct DriverArgs {
       } else if (Arg.consume_front("-target")) {
         if (Arg.empty() && I + 1 < E)
           Target = Cmd.CommandLine[I + 1];
+      } else if (Arg.consume_front("--stdlib")) {
+        if (Arg.consume_front("="))
+          Stdlib = Arg.str();
+        else if (Arg.empty() && I + 1 < E)
+          Stdlib = Cmd.CommandLine[I + 1];
+      } else if (Arg.consume_front("-stdlib=")) {
+        Stdlib = Arg.str();
       }
     }
 
@@ -175,6 +184,8 @@ struct DriverArgs {
       Args.append({"-isysroot", ISysroot});
     if (!Target.empty())
       Args.append({"-target", Target});
+    if (!Stdlib.empty())
+      Args.append({"--stdlib", Stdlib});
     return Args;
   }
 
@@ -206,6 +217,8 @@ template <> struct DenseMapInfo<DriverArgs> {
         Val.Lang,
         Val.Sysroot,
         Val.ISysroot,
+        Val.Target,
+        Val.Stdlib,
     });
   }
   static bool isEqual(const DriverArgs &LHS, const DriverArgs &RHS) {

clang-tools-extra/clangd/test/system-include-extractor.test

@@ -18,6 +18,7 @@
 # RUN: echo '[ -z "${args##*"--sysroot /my/sysroot/path"*}" ] || exit' >> %t.dir/bin/my_driver.sh
 # RUN: echo '[ -z "${args##*"-isysroot /isysroot"*}" ] || exit' >> %t.dir/bin/my_driver.sh
 # RUN: echo '[ -z "${args##*"-target arm-linux-gnueabihf"*}" ] || exit' >> %t.dir/bin/my_driver.sh
+# RUN: echo '[ -z "${args##*"--stdlib libc++"*}" ] || exit' >> %t.dir/bin/my_driver.sh
 # RUN: echo 'echo line to ignore >&2' >> %t.dir/bin/my_driver.sh
 # RUN: echo 'printf "Target: arm-linux-gnueabihf\r\n" >&2' >> %t.dir/bin/my_driver.sh
 # RUN: echo 'printf "#include <...> search starts here:\r\n" >&2' >> %t.dir/bin/my_driver.sh
@@ -37,7 +38,7 @@
 
 # Generate a compile_commands.json that will query the mock driver we've
 # created. Which should add a.h and b.h into include search path.
-# RUN: echo '[{"directory": "%/t.dir", "command": "my_driver.sh the-file.cpp --target=arm-linux-gnueabihf -nostdinc --sysroot /my/sysroot/path -isysroot/isysroot", "file": "the-file.cpp"}]' > %t.dir/compile_commands.json
+# RUN: echo '[{"directory": "%/t.dir", "command": "my_driver.sh the-file.cpp --target=arm-linux-gnueabihf -nostdinc --sysroot /my/sysroot/path -isysroot/isysroot -stdlib=libc++", "file": "the-file.cpp"}]' > %t.dir/compile_commands.json
 
 # RUN: sed -e "s|INPUT_DIR|%/t.dir|g" %s > %t.test.1
 # On Windows, we need the URI in didOpen to look like "uri":"file:///C:/..."
@@ -75,7 +76,7 @@
 {"jsonrpc":"2.0","method":"exit"}
 
 # Generate a different compile_commands.json which does not point to the mock driver
-# RUN: echo '[{"directory": "%/t.dir", "command": "gcc the-file.cpp --target=arm-linux-gnueabihf -nostdinc --sysroot /my/sysroot/path -isysroot/isysroot", "file": "the-file.cpp"}]' > %t.dir/compile_commands.json
+# RUN: echo '[{"directory": "%/t.dir", "command": "gcc the-file.cpp --target=arm-linux-gnueabihf -nostdinc --sysroot /my/sysroot/path -isysroot/isysroot -stdlib=libc++", "file": "the-file.cpp"}]' > %t.dir/compile_commands.json
 
 # Generate a clangd config file which points to the mock driver instead
 # RUN: echo 'CompileFlags:' > %t.dir/.clangd