add StackSource and implement it for platform::Stack

src/context.rs

@@ -1,5 +1,5 @@
 use core::prelude::*;
-use platform::Stack;
+use stack::StackSource;
 use arch::Registers;
 use platform;
 
@@ -11,7 +11,7 @@ pub struct Context {
 impl Context {
   #[inline]
   pub unsafe fn new<F>(f: F) -> Context where F: FnOnce() + Send + 'static {
-    let mut stack = Stack::new(4 << 20);
+    let mut stack = platform::StackSource::get_stack(4 << 20);
     let regs = Registers::new(&mut stack, f);
     Context {
       regs: regs,

src/platform.rs

@@ -22,6 +22,15 @@ pub struct Stack {
   valgrind_id: libc::c_uint
 }
 
+pub struct StackSource;
+
+impl stack::StackSource for StackSource {
+  type Output = Stack;
+  fn get_stack(size: usize) -> Stack {
+    Stack::new(size)
+  }
+}
+
 impl stack::Stack for Stack {
   fn top(&mut self) -> *mut u8 {
     unsafe {
@@ -37,7 +46,7 @@ impl stack::Stack for Stack {
 }
 
 impl Stack {
-  pub fn new(size: usize) -> Stack {
+  fn new(size: usize) -> Stack {
     let page_size = env::page_size();
 
     // round the page size up,

src/stack.rs

@@ -2,3 +2,8 @@ pub trait Stack {
   fn top(&mut self) -> *mut u8;
   fn limit(&self) -> *const u8;
 }
+
+pub trait StackSource {
+  type Output: Stack;
+  fn get_stack(size: usize) -> Self::Output;
+}