change max_atomic_width type from u64 to Option<u64>

to better express the idea that omitting this field defaults this value
to target_pointer_width

src/librustc/session/config.rs

@@ -926,7 +926,7 @@ pub fn default_configuration(sess: &Session) -> ast::CrateConfig {
     let os = &sess.target.target.target_os;
     let env = &sess.target.target.target_env;
     let vendor = &sess.target.target.target_vendor;
-    let max_atomic_width = sess.target.target.options.max_atomic_width;
+    let max_atomic_width = sess.target.target.max_atomic_width();
 
     let fam = if let Some(ref fam) = sess.target.target.options.target_family {
         intern(fam)

src/librustc_back/target/aarch64_apple_ios.rs

@@ -25,7 +25,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             features: "+neon,+fp-armv8,+cyclone".to_string(),
             eliminate_frame_pointer: false,
-            max_atomic_width: 128,
+            max_atomic_width: Some(128),
             .. base
         },
     })

src/librustc_back/target/aarch64_linux_android.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
-    base.max_atomic_width = 128;
+    base.max_atomic_width = Some(128);
     // As documented in http://developer.android.com/ndk/guides/cpu-features.html
     // the neon (ASIMD) and FP must exist on all android aarch64 targets.
     base.features = "+neon,+fp-armv8".to_string();

src/librustc_back/target/aarch64_unknown_linux_gnu.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 128;
+    base.max_atomic_width = Some(128);
     Ok(Target {
         llvm_target: "aarch64-unknown-linux-gnu".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/arm_linux_androideabi.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+vfp3,+d16".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "arm-linux-androideabi".to_string(),

src/librustc_back/target/arm_unknown_linux_gnueabi.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         llvm_target: "arm-unknown-linux-gnueabi".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/arm_unknown_linux_gnueabihf.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         llvm_target: "arm-unknown-linux-gnueabihf".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/arm_unknown_linux_musleabi.rs

@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     // Most of these settings are copied from the arm_unknown_linux_gnueabi
     // target.
     base.features = "+v6".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabi" and not "musleabi" here. LLVM uses it
         // to determine the calling convention and float ABI, and it doesn't

src/librustc_back/target/arm_unknown_linux_musleabihf.rs

@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     // Most of these settings are copied from the arm_unknown_linux_gnueabihf
     // target.
     base.features = "+v6,+vfp2".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and it

src/librustc_back/target/armv7_apple_ios.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "apple".to_string(),
         options: TargetOptions {
             features: "+v7,+vfp3,+neon".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/armv7_linux_androideabi.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+thumb2,+vfp3,+d16".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "armv7-none-linux-android".to_string(),

src/librustc_back/target/armv7_unknown_linux_gnueabihf.rs

@@ -26,7 +26,7 @@ pub fn target() -> TargetResult {
             // Info about features at https://wiki.debian.org/ArmHardFloatPort
             features: "+v7,+vfp3,+d16,+thumb2".to_string(),
             cpu: "generic".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/armv7_unknown_linux_musleabihf.rs

@@ -17,7 +17,7 @@ pub fn target() -> TargetResult {
     // target.
     base.features = "+v7,+vfp3,+neon".to_string();
     base.cpu = "cortex-a8".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and LLVM

src/librustc_back/target/armv7s_apple_ios.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "apple".to_string(),
         options: TargetOptions {
             features: "+v7,+vfp4,+neon".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/asmjs_unknown_emscripten.rs

@@ -21,7 +21,7 @@ pub fn target() -> Result<Target, String> {
         linker_is_gnu: true,
         allow_asm: false,
         obj_is_bitcode: true,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
         .. Default::default()
     };

src/librustc_back/target/i386_apple_ios.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         target_env: "".to_string(),
         target_vendor: "apple".to_string(),
         options: TargetOptions {
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/i686_apple_darwin.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::apple_base::opts();
     base.cpu = "yonah".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_linux_android.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
 
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // http://developer.android.com/ndk/guides/abis.html#x86
     base.cpu = "pentiumpro".to_string();

src/librustc_back/target/i686_pc_windows_gnu.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/librustc_back/target/i686_pc_windows_msvc.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_msvc_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/librustc_back/target/i686_unknown_dragonfly.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::dragonfly_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_freebsd.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::freebsd_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_haiku.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::haiku_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_linux_gnu.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_linux_musl.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_musl_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
     base.pre_link_args.push("-Wl,-melf_i386".to_string());
 

src/librustc_back/target/le32_unknown_nacl.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         exe_suffix: ".pexe".to_string(),
         linker_is_gnu: true,
         allow_asm: false,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         .. Default::default()
     };
     Ok(Target {

src/librustc_back/target/mips64_unknown_linux_gnuabi64.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
             // NOTE(mips64r2) matches C toolchain
             cpu: "mips64r2".to_string(),
             features: "+mips64r2".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mips64el_unknown_linux_gnuabi64.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
             // NOTE(mips64r2) matches C toolchain
             cpu: "mips64r2".to_string(),
             features: "+mips64r2".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mips_unknown_linux_gnu.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mips_unknown_linux_musl.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         }
     })

src/librustc_back/target/mips_unknown_linux_uclibc.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mipsel_unknown_linux_gnu.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mipsel_unknown_linux_musl.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         }
     })

src/librustc_back/target/mipsel_unknown_linux_uclibc.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mod.rs

@@ -353,9 +353,8 @@ pub struct TargetOptions {
     // will 'just work'.
     pub obj_is_bitcode: bool,
 
-    /// Maximum integer size in bits that this target can perform atomic
-    /// operations on.
-    pub max_atomic_width: u64,
+    /// Don't use this field; instead use the `.max_atomic_width()` method.
+    pub max_atomic_width: Option<u64>,
 
     /// Panic strategy: "unwind" or "abort"
     pub panic_strategy: PanicStrategy,
@@ -407,10 +406,7 @@ impl Default for TargetOptions {
             allow_asm: true,
             has_elf_tls: false,
             obj_is_bitcode: false,
-            // NOTE 0 is *not* the real default value of max_atomic_width. The default value is
-            // actually the pointer_width of the target. This default is injected in the
-            // Target::from_json function.
-            max_atomic_width: 0,
+            max_atomic_width: None,
             panic_strategy: PanicStrategy::Unwind,
         }
     }
@@ -431,6 +427,12 @@ impl Target {
         }
     }
 
+    /// Maximum integer size in bits that this target can perform atomic
+    /// operations on.
+    pub fn max_atomic_width(&self) -> u64 {
+        self.options.max_atomic_width.unwrap_or(self.target_pointer_width.parse().unwrap())
+    }
+
     /// Load a target descriptor from a JSON object.
     pub fn from_json(obj: Json) -> TargetResult {
         // While ugly, this code must remain this way to retain
@@ -469,9 +471,6 @@ impl Target {
             options: Default::default(),
         };
 
-        // Default max-atomic-width to target-pointer-width
-        base.options.max_atomic_width = base.target_pointer_width.parse().unwrap();
-
         macro_rules! key {
             ($key_name:ident) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
@@ -484,11 +483,11 @@ impl Target {
                     .map(|o| o.as_boolean()
                          .map(|s| base.options.$key_name = s));
             } );
-            ($key_name:ident, u64) => ( {
+            ($key_name:ident, Option<u64>) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
                 obj.find(&name[..])
                     .map(|o| o.as_u64()
-                         .map(|s| base.options.$key_name = s));
+                         .map(|s| base.options.$key_name = Some(s)));
             } );
             ($key_name:ident, PanicStrategy) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
@@ -562,7 +561,7 @@ impl Target {
         key!(exe_allocation_crate);
         key!(has_elf_tls, bool);
         key!(obj_is_bitcode, bool);
-        key!(max_atomic_width, u64);
+        key!(max_atomic_width, Option<u64>);
         try!(key!(panic_strategy, PanicStrategy));
 
         Ok(base)
@@ -708,10 +707,6 @@ impl ToJson for Target {
         target_option_val!(max_atomic_width);
         target_option_val!(panic_strategy);
 
-        if self.options.max_atomic_width.to_string() != self.target_pointer_width {
-            d.insert("max-atomic-width".to_string(), self.options.max_atomic_width.to_json());
-        }
-
         Json::Object(d)
     }
 }

src/librustc_back/target/powerpc64_unknown_linux_gnu.rs

@@ -14,7 +14,7 @@ pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "ppc64".to_string();
     base.pre_link_args.push("-m64".to_string());
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "powerpc64-unknown-linux-gnu".to_string(),