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

Author: Jorge Aparicio

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
         }
     })