fix a bug where max_atomic_width: 0 was lost during ...

the Target <-> JSON roundtrip.

Author: Jorge Aparicio

src/librustc_back/target/mod.rs

@@ -397,7 +397,13 @@ impl Default for TargetOptions {
             allow_asm: true,
             has_elf_tls: false,
             obj_is_bitcode: false,
-            max_atomic_width: 0,
+            /// NOTE this is *not* the real default value. The default value of max_atomic_width is
+            /// actually the pointer width of the target. This default is injected in the
+            /// Target::from_json function. Still, we have to pick some value to put here. We'll pick
+            /// an impossible value: u64::max_value() because using a valid value like 0 or 8 as the
+            /// default would cause the max-atomic-width field to be "lost" (it would get replaced
+            /// by target_pointer_width) during the Target <-> JSON round trip
+            max_atomic_width: u64::max_value(),
         }
     }
 }