Auto merge of #28282 - DiamondLovesYou:optional-data-layout, r=alexcrichton

NFC.

Author: bors

src/librustc_back/target/mod.rs

@@ -91,7 +91,7 @@ pub struct Target {
 #[derive(Clone, Debug)]
 pub struct TargetOptions {
     /// [Data layout](http://llvm.org/docs/LangRef.html#data-layout) to pass to LLVM.
-    pub data_layout: String,
+    pub data_layout: Option<String>,
     /// Linker to invoke. Defaults to "cc".
     pub linker: String,
     /// Archive utility to use when managing archives. Defaults to "ar".
@@ -186,7 +186,7 @@ impl Default for TargetOptions {
     /// incomplete, and if used for compilation, will certainly not work.
     fn default() -> TargetOptions {
         TargetOptions {
-            data_layout: String::new(),
+            data_layout: None,
             linker: option_env!("CFG_DEFAULT_LINKER").unwrap_or("cc").to_string(),
             ar: option_env!("CFG_DEFAULT_AR").unwrap_or("ar").to_string(),
             pre_link_args: Vec::new(),
@@ -287,6 +287,14 @@ impl Target {
                         )
                     );
             } );
+            ($key_name:ident, optional) => ( {
+                let name = (stringify!($key_name)).replace("_", "-");
+                if let Some(o) = obj.find(&name[..]) {
+                    base.options.$key_name = o
+                        .as_string()
+                        .map(|s| s.to_string() );
+                }
+            } );
         }
 
         key!(cpu);
@@ -300,7 +308,7 @@ impl Target {
         key!(staticlib_prefix);
         key!(staticlib_suffix);
         key!(features);
-        key!(data_layout);
+        key!(data_layout, optional);
         key!(dynamic_linking, bool);
         key!(executables, bool);
         key!(disable_redzone, bool);

src/librustc_trans/trans/context.rs

@@ -229,9 +229,8 @@ unsafe fn create_context_and_module(sess: &Session, mod_name: &str) -> (ContextR
     let mod_name = CString::new(mod_name).unwrap();
     let llmod = llvm::LLVMModuleCreateWithNameInContext(mod_name.as_ptr(), llcx);
 
-    let custom_data_layout = &sess.target.target.options.data_layout[..];
-    if custom_data_layout.len() > 0 {
-        let data_layout = CString::new(custom_data_layout).unwrap();
+    if let Some(ref custom_data_layout) = sess.target.target.options.data_layout {
+        let data_layout = CString::new(&custom_data_layout[..]).unwrap();
         llvm::LLVMSetDataLayout(llmod, data_layout.as_ptr());
     } else {
         let tm = ::back::write::create_target_machine(sess);