[lldb][ClangASTImporter] Don't ASTImport LambdaExpr nodes

[Michael137]

This patch works around an assertion that we hit in the LambdaExpr constructor when we call it from ASTNodeImporter::VisitLambdaExpr (see #149477). The lambda that we imported doesn't have the NumCaptures field accurately set to the one on the source decl. This is because in MinimalImport mode, we skip importing of lambda definitions:

llvm-project/clang/lib/AST/ASTImporter.cpp

Line 2499 in e21b0dd

(To->isLambda() && shouldForceImportDeclContext(Kind))) {

In practice we have seen this assertion occur in our import-std-module test-suite when libc++ headers decide to use lambdas inside inline function bodies (the latest failure being caused by #144602).

To avoid running into this whenever libc++ decides to use lambdas in headers, this patch skips ASTImport of lambdas alltogether. Ideally this would bubble up to the user or log as an error, but we swallow the ASTImportErrors currently. The only way this codepath is hit is when lambdas are used inside functions in defined in the expression evaluator, or when importing AST nodes from Clang modules. Both of these are very niche use-cases (for now), so a workaround seemed appropriate.

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Michael Buch (Michael137)

Changes

Works around #149477

---

Full diff: https://github.com/llvm/llvm-project/pull/154962.diff

2 Files Affected:

• (modified) lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp (+10)
• (added) lldb/test/Shell/Expr/TestLambdaExprImport.test (+26)

diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
index 08e2d0f1b4011..92094c01e2a54 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp
@@ -1048,6 +1048,16 @@ ClangASTImporter::MapCompleter::~MapCompleter() = default;
 
 llvm::Expected<Decl *>
 ClangASTImporter::ASTImporterDelegate::ImportImpl(Decl *From) {
+  // FIXME: The Minimal import mode of clang::ASTImporter does not correctly
+  // import Lambda definitions. Work around this for now by not importing
+  // lambdas at all. This is most likely encountered when importing decls from
+  // the `std` module (not from debug-info), where lambdas can be defined in
+  // inline function bodies. Those will be imported by LLDB.
+  if (const auto *CXX = llvm::dyn_cast<clang::CXXRecordDecl>(From))
+    if (CXX->isLambda())
+      return llvm::make_error<ASTImportError>(
+          ASTImportError::UnsupportedConstruct);
+
   if (m_std_handler) {
     std::optional<Decl *> D = m_std_handler->Import(From);
     if (D) {
diff --git a/lldb/test/Shell/Expr/TestLambdaExprImport.test b/lldb/test/Shell/Expr/TestLambdaExprImport.test
new file mode 100644
index 0000000000000..c57ce06453fe2
--- /dev/null
+++ b/lldb/test/Shell/Expr/TestLambdaExprImport.test
@@ -0,0 +1,26 @@
+# Test that we can successfully ASTImport clang::LambdaExpr nodes.
+# Currently this is not supported in MinimalImport mode (which LLDB
+# uses always).
+
+# RUN: split-file %s %t
+# RUN: %clang_host -g -gdwarf %t/main.cpp -o %t.out
+# RUN: %lldb -o "settings set interpreter.stop-command-source-on-error false" \
+# RUN:       -x -b -s %t/commands.input %t.out 2>&1 \
+# RUN:       | FileCheck %s
+
+#--- main.cpp
+
+int main() {
+  __builtin_debugtrap();
+}
+
+#--- commands.input
+
+run
+expression --top-level -- void method(int x) { [x=x] { ; }; }
+target dump typesystem
+
+# CHECK:      expression
+# CHECK:      target dump typesystem
+# CHECK-NOT:  FunctionDecl
+# CHECK-NOT:  LambdaExpr

[adrian-prantl]

I fit unbreaks the test suite, let's do this for now.