Add appropriate LLVM module flags for debug info.

Set "Dwarf Version" to 2 on OS X to avoid toolchain incompatibility, and
set "Debug Info Version" to prevent debug info from being stripped from
bitcode.

Fixes #11352.

src/librustc/lib/llvm.rs

@@ -1493,6 +1493,11 @@ pub mod llvm {
                              Dialect: c_uint)
                              -> ValueRef;
 
+        pub static LLVMRustDebugMetadataVersion: u32;
+
+        pub fn LLVMRustAddModuleFlag(M: ModuleRef,
+                                     name: *c_char,
+                                     value: u32);
 
         pub fn LLVMDIBuilderCreate(M: ModuleRef) -> DIBuilderRef;
 

src/librustc/middle/trans/debuginfo.rs

@@ -149,7 +149,7 @@ use std::ptr;
 use std::sync::atomics;
 use std::vec;
 use syntax::codemap::{Span, Pos};
-use syntax::{ast, codemap, ast_util, ast_map, opt_vec};
+use syntax::{abi, ast, codemap, ast_util, ast_map, opt_vec};
 use syntax::parse::token;
 use syntax::parse::token::special_idents;
 
@@ -263,6 +263,20 @@ pub fn finalize(cx: @CrateContext) {
     unsafe {
         llvm::LLVMDIBuilderFinalize(DIB(cx));
         llvm::LLVMDIBuilderDispose(DIB(cx));
+        // Debuginfo generation in LLVM by default uses a higher
+        // version of dwarf than OS X currently understands. We can
+        // instruct LLVM to emit an older version of dwarf, however,
+        // for OS X to understand. For more info see #11352
+        // This can be overridden using --llvm-opts -dwarf-version,N.
+        if cx.sess.targ_cfg.os == abi::OsMacos {
+            "Dwarf Version".with_c_str(
+                |s| llvm::LLVMRustAddModuleFlag(cx.llmod, s, 2));
+        }
+
+        // Prevent bitcode readers from deleting the debug info.
+        "Debug Info Version".with_c_str(
+            |s| llvm::LLVMRustAddModuleFlag(cx.llmod, s,
+                                            llvm::LLVMRustDebugMetadataVersion));
     };
 }
 

src/rustllvm/RustWrapper.cpp

@@ -156,6 +156,14 @@ DIT unwrapDI(LLVMValueRef ref) {
     return DIT(ref ? unwrap<MDNode>(ref) : NULL);
 }
 
+extern "C" const uint32_t LLVMRustDebugMetadataVersion = DEBUG_METADATA_VERSION;
+
+extern "C" void LLVMRustAddModuleFlag(LLVMModuleRef M,
+                                      const char *name,
+                                      uint32_t value) {
+    unwrap(M)->addModuleFlag(Module::Warning, name, value);
+}
+
 extern "C" DIBuilderRef LLVMDIBuilderCreate(LLVMModuleRef M) {
     return new DIBuilder(*unwrap(M));
 }