document why attributes are set on CallInst

src/librustc/middle/trans/callee.rs

@@ -706,12 +706,15 @@ pub fn trans_call_inner(in_cx: @mut Block,
                 _ => {}
             }
 
-            // Invoke the actual rust fn and update bcx/llresult.
+            // A function pointer is called without the declaration available, so we have to apply
+            // any attributes with ABI implications directly to the call instruction. Right now, the
+            // only attribute we need to worry about is `sret`.
             let mut attrs = ~[];
             if type_of::return_uses_outptr(in_cx.tcx(), ret_ty) {
                 attrs.push((1, StructRetAttribute));
             }
 
+            // The `noalias` attribute on the return value is useful to a function ptr caller.
             match ty::get(ret_ty).sty {
                 // `~` pointer return values never alias because ownership is transferred
                 ty::ty_uniq(*) |
@@ -721,6 +724,7 @@ pub fn trans_call_inner(in_cx: @mut Block,
                 _ => ()
             }
 
+            // Invoke the actual rust fn and update bcx/llresult.
             let (llret, b) = base::invoke(bcx, llfn, llargs, attrs);
             bcx = b;
             llresult = llret;

src/librustc/middle/trans/foreign.rs

@@ -264,6 +264,9 @@ pub fn trans_native_call(bcx: @mut Block,
         }
     };
 
+    // A function pointer is called without the declaration available, so we have to apply
+    // any attributes with ABI implications directly to the call instruction. Right now, the
+    // only attribute we need to worry about is `sret`.
     let attrs;
     if fn_type.sret {
         attrs = &[(1, StructRetAttribute)];