Fix debuginfo for machO

This only fixes it when using object::write as backend, and not when using faerie.

There were two problems:

* object::write doesn't replace .debug_info with __debug_info, unlike faerie
* machO requires section relative relocations, and not symbol relative
  relocations. When using symbol relative relocations, the linker
  interprets the relocations as section relative. Thus writing the wrong
  values to the debug sections.

Fixes rust-lang#303

src/backend.rs

@@ -1,4 +1,5 @@
 use std::collections::HashMap;
+use std::convert::TryFrom;
 
 use rustc::session::Session;
 
@@ -122,8 +123,13 @@ impl WriteDebugInfo for ObjectProduct {
         id: SectionId,
         data: Vec<u8>,
     ) -> (object::write::SectionId, object::write::SymbolId) {
+        let name = if self.object.format() == target_lexicon::BinaryFormat::Macho {
+            id.name().replace('.', "__") // machO expects __debug_info instead of .debug_info
+        } else {
+            id.name().to_string()
+        }.into_bytes();
+
         let segment = self.object.segment_name(StandardSegment::Debug).to_vec();
-        let name = id.name().as_bytes().to_vec();
         let section_id = self.object.add_section(segment, name, SectionKind::Debug);
         self.object.section_mut(section_id).set_data(data, 1);
         let symbol_id = self.object.section_symbol(section_id);
@@ -137,10 +143,19 @@ impl WriteDebugInfo for ObjectProduct {
         from: &Self::SectionId,
         reloc: &DebugReloc,
     ) {
-        let symbol = match reloc.name {
-            DebugRelocName::Section(id) => section_map.get(&id).unwrap().1,
+        let (symbol, symbol_offset) = match reloc.name {
+            DebugRelocName::Section(id) => {
+                (section_map.get(&id).unwrap().1, 0)
+            }
             DebugRelocName::Symbol(id) => {
-                self.function_symbol(*symbol_map.get_index(id).unwrap().0)
+                let symbol_id = self.function_symbol(*symbol_map.get_index(id).unwrap().0);
+                let symbol = self.object.symbol(symbol_id);
+
+                // A symbol gets a section assigned when `add_symbol_data` is called.
+                let section = symbol.section.expect("Symbol not defined");
+                let symbol_offset = symbol.value;
+
+                (self.object.section_symbol(section), symbol_offset)
             }
         };
         self.object.add_relocation(from.0, Relocation {
@@ -149,7 +164,7 @@ impl WriteDebugInfo for ObjectProduct {
             kind: RelocationKind::Absolute,
             encoding: RelocationEncoding::Generic,
             size: reloc.size * 8,
-            addend: reloc.addend,
+            addend: i64::try_from(symbol_offset).unwrap() + reloc.addend,
         }).unwrap();
     }
 }

src/driver.rs

@@ -184,10 +184,7 @@ fn run_aot(
 
     let mut module = new_module("some_file".to_string());
 
-    let mut debug = if tcx.sess.opts.debuginfo != DebugInfo::None
-        // macOS debuginfo doesn't work yet (see #303)
-        && !tcx.sess.target.target.options.is_like_osx
-    {
+    let mut debug = if tcx.sess.opts.debuginfo != DebugInfo::None {
         let debug = DebugContext::new(
             tcx,
             module.target_config().pointer_type().bytes() as u8,