[lldb] Adjust default target.max-children-depth (#149282)

Deeply nested structs can be noisy, so Apple's LLDB fork sets the
default to `4`:
https://github.com/swiftlang/llvm-project/blob/9c93adbb283005ab416fd155b75fd43e6a8288ca/lldb/source/Target/TargetProperties.td#L134-L136

Thought it would be useful to upstream this. Though happy to pick a
different default or keep it as-is.

Author: Michael137

lldb/source/Target/TargetProperties.td

@@ -99,7 +99,7 @@ let Definition = "target" in {
     DefaultUnsignedValue<24>,
     Desc<"Maximum number of children to expand in any level of depth.">;
   def MaxChildrenDepth: Property<"max-children-depth", "UInt64">,
-    DefaultUnsignedValue<0xFFFFFFFF>,
+    DefaultUnsignedValue<4>,
     Desc<"Maximum depth to expand children.">;
   def MaxSummaryLength: Property<"max-string-summary-length", "UInt64">,
     DefaultUnsignedValue<1024>,

lldb/test/Shell/Settings/TestChildDepthTruncation.test

@@ -0,0 +1,84 @@
+# Test that we warn the user about truncated output
+# when target.max-children-depth wasn't explicitly set.
+
+# RUN: split-file %s %t
+# RUN: %clang_host -g -gdwarf %t/main.cpp -o %t.out
+# RUN: %lldb -x -b -s %t/dwim-commands.input %t.out -o exit 2>&1 \
+# RUN:       | FileCheck %s --check-prefix=DWIM
+#
+# RUN: %lldb -x -b -s %t/expr-commands.input %t.out -o exit 2>&1 \
+# RUN:       | FileCheck %s --check-prefix=EXPR
+#
+# RUN: %lldb -x -b -s %t/frame-var-commands.input %t.out -o exit 2>&1 \
+# RUN:       | FileCheck %s --check-prefix=VAR
+#
+# RUN: %lldb -x -b -s %t/with-setting-commands.input %t.out -o exit 2>&1 \
+# RUN:       | FileCheck %s --check-prefix=SETTING
+
+#--- main.cpp
+
+struct L1 {
+    int w;
+    struct L2 {
+        int x;
+        struct L3 {
+            int y;
+            struct L4 {
+              int z;
+                struct L5 {
+                  int a;
+                } l5;
+            } l4;
+        } l3;
+    } l2;
+};
+
+int main() {
+  L1 nested;
+  __builtin_debugtrap();
+}
+
+#--- dwim-commands.input
+
+run
+dwim-print nested
+frame variable nested
+
+DWIM:      (lldb) dwim-print nested
+DWIM:      *** Some of the displayed variables have a greater depth of members
+DWIM-SAME: use the --depth option to dwim-print
+DWIM:      (lldb) frame variable nested
+DWIM-NOT:  *** Some of the displayed variables have a greater depth of members
+
+#--- expr-commands.input
+
+run
+expression nested
+frame variable nested
+
+EXPR:      (lldb) expression nested
+EXPR:      *** Some of the displayed variables have a greater depth of members
+EXPR-SAME: use the --depth option to expression
+EXPR:      (lldb) frame variable nested
+EXPR-NOT:  *** Some of the displayed variables have a greater depth of members
+
+#--- frame-var-commands.input
+
+run
+frame variable nested
+frame variable nested
+
+VAR:      (lldb) frame variable nested
+VAR:      *** Some of the displayed variables have a greater depth of members
+VAR-SAME: use the --depth option to frame variable
+VAR:      (lldb) frame variable nested
+VAR-NOT:  *** Some of the displayed variables have a greater depth of members
+
+#--- with-setting-commands.input
+
+run
+settings set target.max-children-depth 1
+frame variable nested
+
+SETTING:      (lldb) frame variable nested
+SETTING-NOT:  *** Some of the displayed variables have a greater depth of members

lldb/test/Shell/SymbolFile/NativePDB/Inputs/class_layout.lldbinit

@@ -1,3 +1,4 @@
+settings set target.max-children-depth 10
 expr a
 expr b.c
 expr b.u.c